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LEAF LITTER DECOMPOSITION and MACROINVERTEBRATES in a NEOTROPICAL LOWLAND STREAM, Q. NEGRA, COSTA RICA
DIPLOMARBEIT
zur Erlangung des akademischen Grades Magistra rer. nat.
an der Fakultät für Lebenswissenschaften der Universität Wien
ausgeführt am
Department für Limnologie und Hydrobotanik
verfasst und eingereicht von:
Julia Tschelaut
Wien, August 2005
Danksagung
Ich danke O. Prof. Dr. Fritz Schiemer, für die wissenschaftliche Betreuung der
Arbeit, die große Unterstützung, die Zeit, das Engagement und die konstruktive Kritik
– muchissimas gracias.
Ich danke dem gesamten Team der Tropenstation La Gamba, besonders Dr. Anton Weissenhofer für seine große Unterstützung, die zur Verfügung gestellten Photos
und die Zeit, die er diesem Projekt gewidmet hat.
Meinem Kollegen Christian Pichler danke ich für die gute und produktive
Zusammenarbeit, sowie Astrid Riemerth und Maria Gusenleitner für die
vertiefenden Arbeiten und die zur Verfügung gestellten Daten.
Ich danke weiters Dr. Walter Reckendorfer, der viel zur Durchführung dieses
Projektes beigetragen hat, sowie Dr. Irene Zweimüller, die mir ebenfalls bei
statistischen Fragen geholfen hat.
Bedanken möchte ich mich bei Hubert Kraill, der die chemischen Analysen
durchgeführt hat.
Dr. Wolfgang Wanek, Prof. Dr. Roland Albert, Dr. Peter Weish und Dr. Santiago Gaviria-Melo danke ich für die Unterstützung und das Interesse an der Arbeit sowie
Dr. Wolfram Graf, Dr. Ernst Bauernfeind, Dr. Berthold Janecek, und Prof. Dr. Johann Waringer für die Hilfe bei der Bestimmung des Makrozoobenthos.
Für die schöne gemeinsame Zeit auf der Tropenstation La Gamba danke ich:
las dos Efas, Karim, Christian, Toni, Familie Walder, Nina, Mari, Victor, Eduardo,
Luis, Oli, und Angela - Pura vida!
Ein großes Dankeschön gilt meinen Eltern und Onkels für die Unterstützung und
das langjährige Sponsern meiner bisherigen „Karriere“
Ich danke besonders meinen Freunden – Anke, Kaddl, Lisa und Robsi, für ihre
offenen Ohren, die mein ständiges euphorisches Costa Rica Gequassel aushalten
und Caro, dass das Photogeschäft kein unendliches Labyrinth an Ebenen für mich
geblieben ist.
CONTENTS
page
1. INTRODUCTION 1 2. THE RIVERNETWORK in the „RAINFOREST of the AUSTRIANS“ 4 2.1. Material and Methods 9 2.2. Results 13 2.3. Discussion 41 3. QUEBRADA NEGRA 44 3.1. Material and Methods 44 3.2. Results 46 3.3. Discussion 79 4. MACROINVERTEBRATES and LEAF LITTER DECOMPOSITION in the QUEBRADA NEGRA 81 4.1. Material and Methods 84 4.2. Results 86 4.3. Discussion 96 5. MACROINVERTEBRATE DISTRIBUTION and TROPHIC RELATIONS in a NEOTROPICAL LOWLAND STREAM, QUEBRADA NEGRA, COSTA RICA 99 5.1. Material and Methods 100 5.2. Results 101 5.3. Discussion 109 6. RESEARCH NEEDS 112 LITERATURE CITED 114 PLATES 117
1
1. INTRODUCTION
..............is it not the foreign, the new, the unknown which always stimulates the
science?“ (FÜHREDER 1994)
Studies of tropical streams and rivers have a long history, beginning with early
explorers and naturalists who visited these regions and collected some aquatic
specimens e.g. von HUMBOLDT and BONPLAND (1799-1802), von SPIX and von
MARTIUS (1817-1820), BATES (1848-1859). Later, more extensive collections were
made by professional collectors such as H.S. PARISH (ALEXANDER 1959) and F.
WOYTKOWSKI (WOYTKOWSKI 1978) as well as numerous resident and visiting
scientists. There is a growing interest in the study of neotropical lotic systems and the
number of papers published each year that address tropical stream research has
increased markedly over the last two decades and the advances are clearly evident.
But still, a survey about the percentage of information about what is known of e.g.
taxonomy, life history traits and standing crop biomass, shows that much is still to be
learned about tropical streams and rivers (JACKSON, SWEENEY 1995).
Little is known about the rivers and streams of the Piedras Blancas National Park,
Costa Rica. This protected area provides an unique opportunity to study tropical
freshwater ecology and after participating on a tropical ecology field trip of the
University of Vienna in February 2003 we decided to put our focus of interest on this
topic in order to initiate on-going freshwater ecology studies in this area.
One of the primary objectives of the study was to investigate the rivernetwork within
the Rio Esquinas catchment (Fig.2.3. and 2.4.). Rivers were analyzed with regard to
abiotic parameters such as morphology, hydrology, hydrochemistry, sedimentology
and canopy cover by the riparian vegetation. This work discusses differences
between sites according to geological factors and the seasonal hydrologic
characteristics from streams within the catchment. Research was carried out at nine
study streams and rivers ranging from 1st order streams to 5th order rivers that empty
into the Golfo Dulce at the Pacific Ocean. Furthermore, the morphometric-
hydrological conditions of the Quebrada Negra, a 1st order stream, were investigated
in more detail. Streams display a more or less regular alternation between shallow
2
areas of higher velocity and mixed gravel-cobble substrate (riffles) and deeper areas
of slower velocity and finer substrate (pools). Within the morphometric-hydrological
record of the Q. Negra we identified four different habitat types within the stream run
– riffles, slow shallow areas, pools and cascades.
Analysis of the concentration dynamics and export of solutes from catchments
provides insights into major aspects of ecosystem functions. Chemical composition of
streamwater can reflect and thus reveal geologic features of the catchment. The
chemical regime of the water, particularly nutrient concentrations, influences the
productivity and community structure of the aquatic ecosystem. The relationship
between solute concentration and streamflow can help to explain the interplay among
physical, chemical and biological processes occuring within the catchment.
Stream communities in forested catchments are generally dependent on
allochthonous organic matter as a trophic base. The shading effect of riparian
vegetation effectively limits in situ primary production and inputs of reduced carbon
compounds are therefore of primary importance in the energy budgets of forest
streams (CUMMINS et al.1973, FISHER & LIKENS 1973, CUMMINS 1974). Litter fall
(throughfall) from tree canopies extending over the stream channel constitutes a
direct pathway of import for allochthonous organic matter. Rainforest streams are
shadowed to a great extent and have therefore a small primary production.
Consequently, much of the energy demand by consumers is met from allochthonous
sources. It is largely allochthonous material (leaf litter and detritus) of the forest,
which is brought into the stream and determines its nutrient budget. The
decomposition in tropical lotic systems is much faster than in running waters of higher
latitudes. The leaf litter entered into tropical streams is often determined by the
seasonality of the precipitation, which is an important difference to temperate
streams. Furthermore, the amount of leaf litter entered into streams in the tropics is
twice as much as in temperate regions. Leaves fall directly or are windblown into
streams, become wetted, and commence to leach soluble organic and inorganic
constituents. The rate of leaf breakdown is determined by intrinsic differences among
leaves, a number of environmental variables and the feeding activity of detrivores.
The processing of leaf litter in temperate streams has been the subject of numerous
studies but equivalent tropical ecosystems have received little attention, where many
leaf types are varying in their palatability and tannin levels.
3
Streams are highly heterogeneous environments in which habitat characteristics vary
drastically over small distances. Throughout this study we analysed the relationship
between taxonomical composition and functional organization of stream benthic
communities and some environmental variables in riffle and pool sites of the
Quebrada Negra. Local variations of abiotic factors, such as current velocity,
substratum composition and water depth, shape the distribution of invertebrates.
Distribution patterns of macroinvertebrates in a lotic ecosystem are determined to a
great extend by the substrate. Among the first stream ecologists to investigate the
relationship between benthic distribution patterns and the nature of the substrate
were PERCIVAL & WHITEHEAD (1929). They recognized seven basic substrate types
and found that certain animal species were consistently associated with each. Since
then a lot of ecologists have pointed out the importance of substrate types in
determining stream benthos distributions.
The decomposition and macroinvertebrate colonisation of leaf litter from four different
plant species from the riparian vegetation was investigated using litter bags placed in
the Q. Negra over a 28 day period. The plants were chosen after their frequency and
growth strategy (r- and K- strategists). The objective of the present study was to
determine the difference in decay of this four leaf types but also to determine the
taxonomical composition of the colonising macroinvertebrates.
The second aim of this study was to describe the distribution of stream invertebrates
within the riffle-pool sequences of the Q.Negra, and examin the local variations of
density, richness and functional composition in relation to selected environmental
characteristics like current velocity, water depth and composition of the substratum.
Between-site differences are discussed in relation to physical factors.
This investigation represents the first survey of the benthic invertebrate community
from four different choriotops (habitat types) within the Q. Negra.
2. The river-network in the “Rainforest of the Austrians” Adapted version of WEISSENHOFER & HUBER (2001)
This study took place close to the Biological Field Station La Gamba, located in the Bosque
Esquinas “Regenwald der Österreicher”. This forest belongs to the Piedras Blancas National
Park, Puntarenas, southwest of Costa Rica (8° 42' 46" N, 83° 12' 09" W) and covers 148 km2.
Fig. 2.1. Costa Rica and the Golfo Dulce region with the Corcovado National Park (Parque Nacional Corcovado) and the Piedras Blancas National Park ( Parque Nacional Piedras Blancas = Esquinas forest) (from WEBER & al. 2001)
4
5
GEOGRAPHY – Costa Rica is located between 8° 02' 06" - 11° 13' 12" N latitude and 82° 33'
48" - 85° 57' 57" W longitude between the Caribbean Sea and the Pacific Ocean. The country
boarders to Nicaragua in the north and Panama in the south and is part of the Central
American Isthmus. It has an area of 51 100 km2 with a varied and abrupt topography. The
maximum elevation is 3819 m above mean sea level, and much of the country´s land is
above 500 m.
Four main mountain ranges cross the country from northwest to southeast: (1) the Cordillera
de Guanacaste, (2) the Cordillera de Tilaran, (3) the Cordillera Central and (4) the Cordillera
de Talamanca. The Cordillera de Talamanca extends southwards to Panama and divides the
country into two partitions: the Caribbean and the Pacific slope, both with a distinct climate,
fauna and flora.
To the east the rivers drain into the Caribbean Sea and the western ones drain into the
Pacific Ocean. Rivers on the west coast tend to be short.
In the Esquinas forest the altitude ranges from sea level up to 579 m (Cerro Nicuesa). The
whole region is still tectonically active. Up to ten tremors per day have been measured in the
region, and crustal elevations have been observed.
CLIMATE – On the pacific side of Costa Rica a distinct 'dry' (December - March) and 'rainy'
(May - November) season exists throughout the year. The most heavy rainfalls occur in
September, October and November. February and March are the driest months, sometimes it
does not rain within days. During the drier period of the year some trees drop their leaves
completely and a considerable amount of leaf litter accumulates on the forest floor. High
rainfall starts in April and tends to fall mainly in intense cloudbursts in the afternoon. The
Golfo Dulce region is one of the most humid areas in Costa Rica with more than 6000 mm
precipitation per year. The Esquinas lowland forest and its vicinity are influenced by the rain
gradient caused by the mountains of the Fila Cruces range and the adjacent Talamanca
Mountains. ALLAN (1956) noted the outstandingly high rainfall in the Esquinas region.
Since 1998 meterological data has been recorded at the Field Station La Gamba and
complete data sets for precipitation are available for the years 1999 to 2004. Average annual
precipitation at the field station is about 6200 mm.
The climate is also marked by a high mean annual temperature (27,8°C). The average
minimum temperature is about 23,5°C and the average maximum temperature 32,0 °C. A
constantly high relative humidity, averaging 88,3 % at the station and 97,7 % inside the forest
exists throughout the year.
27,9° 5816
23,220,0
38,533,7
Fig. 2.2. Climatic diagram of the Field Station La Gamba
VEGETATION – Tropical forests are complex ecosystems which cover less than 10 % of the
world’s land surface, and yet contain considerably more than half of the world’s living species
(WILSON 1988). Tropical rainforests are distinguished from all other terrestrial ecosystems by
a very high diversity on many levels (species, habitats, life-forms, etc.).
Costa Rica has 12 major life zones (HOLDRIDGE), based on analyzing combinations of
temperature, rainfall and seasonality. Each zone has a distinctive natural vegetation ranging
from tidal mangrove swamps to subalpine paramó.
The rainforests of the Golfo Dulce region in southeast Costa Rica belong to the most
interesting and species-rich forests in Central America. So far, 2,709 species in 935 genera
of 187 families of vascular plants have been recorded in the rainforests around the Golfo
Dulce (HUBER 1996, HERRERA-MCBRYDE & al. 1997, QUESADA & al. 1997, VAUGHAN 1981,
WEBER & al. 2001, WEISSENHOFER 1996), among them about 700 tree species (QUESADA & al.
1997). The Piedras Blancas National Park consists mainly of narrow ridges and steep slopes
covered with primary forest.
6
7
Within our study site four different characteristic types of vegetation occured.
Primary forest: These are forests which have never been severely influenced by human
activities such as logging, farming or any other large-scale destruction of the original
vegetation.
Secondary forest: These are forests which have suffered severe destruction of the original
vegetation. They are inhabited by subsidiary species according to their stage of succession.
The species composition and the structure of the vegetation are both different and less
diverse.
Riverine vegetation: This forest type is found along small rivers with adjacent flat terraces.
Trees are 35-50 m tall and often have massive trunks with large spreading buttresses.
Conspicuous trees of the canopy layer include Luehea seemanii (Tiliaceae), Sloanea sp.
(Elaeocarpaceae) and Virola spp. (Myristicaceae). The mid canopy layer is relatively open
with trees around 25-35 m tall. Common species include Apeiba tibourbou (Tiliaceae),
Castilla tunu (Moraceae) and Spondias mombin (Anacardiaceae). A prominent species of the
understory is Guatteria chiriquiensis (Annonaceae) and Tetrathylacium macrophyllum
(Flacourtiaceae). The well-represented shrub stratum is dominated by Carludovica drudei
(Cyclanthaceae), Calathea spp. (Marantaceae), Costus spp. (Costaceae), Dieffenbachia spp.
(Araceae) Heliconia spp. (Heliconiaceae) and Acalypha diversifolia (Euphorbiaceae). The
ground layer is bare except for Selaginella spp. and tree seedlings. The water level can rise
dramatically during heavy rainfalls and erase all plants along its way.
Mangrove forest: Mangrove forests accompany the sheltered seashores and estuaries of
rivers, where tidal inundations of salt water from the sea occur.
Mangroves are floristically poor, thus representing the opposite extreme of tropical forests
with their rich species diversity. Species like Rhizophora mangle, Rhizophora racemosa
(Rhizophoraceae) and Pelliciera rhizophorae (Theaceae) are common.
8
STUDY SITE – In 1993 the society “Regenwald der Österreicher” established the Biological
Field Station La Gamba on the edge of the Piedras Blancas National Park. The station is
situated next to primary forest and offers 20 students and scientists accommodation and
working space. The location is an excellent base to carry out scientific work on a tropical
sujet. Since then a lot of Austrian scientists but also international students and scientific
teams came to work on different tropical biology topics. But also tourists are welcome to
enjoy this special research atmosphere right in between the tropical rainforest. The study was conducted during the 'dry' season in February till April and the 'rainy' season
in August till September 2004.
A net of rivers, small streams and drainage channels pass through the national park and its
surroundings, which all flow into Rio Esquinas (esquinas means corner and the river is
named after its several meanders). He forms the natural boarder of the Bosque Esquinas in
the North and West of the national park and drains into the Pacific Ocean. Mangrove swamps
next to the mouth of the Rio Esquinas are existing to a great extent. The riverbanks of the two
main rivers passing through the La Gamba valley, the Rio Bonito and the Rio Esquinas, are
covered by farm land and secondary forest at different stages of regrowth. Due to logging till
the last century nearly no primary forest is left over in the lowland, except small spots along
the coast and deep inside the park. However, the upslope areas within the study catchment
show almost no signs of anthropogenic disturbance. Most of the smaller streams lie within the
rain forest.
Most of Costa Rica´s forested area was cleared in the last century. 25 % of Costa Rica is
somehow protected and national parks are established throughout the country. The Viennese
musician professor Michael SCHNITZLER initialized the protection of the Esquinas area and the
project “Regenwald der Österreicher. Due to donations of people from Austria, Canada, USA
and Costa Rican institutions, the protection started to grow and in 1991 the Piedras Blancas
National Park was designated. Meanwhile the Ministry for Environment and Energy of Costa
Rica (MINAE) is responsible for the protection of the Esquinas area. More land is still bought
to be converted into the national park.
9
2.1. MATERIAL AND METHODS
To get an overview of the running waters in the Piedras Blancas National Park and data to
compare we did research on nine study rivers within this area ranging from 1st order streams
to 5th order rivers: Quebrada Mari, Quebrada Negra, Quebrada Gamba, Quebrada Bolsa,
Quebrada Chorro, Quebrada Sardinal, Rio Oro, Rio Bonito, Rio Esquinas.
At each river we chose an easily accessible sampling site where one or two transects were
established by stretching a cord perpendicular to the current. In this cross sections we
measured the stream depth and the current velocity at regular distances of 0,2 – 0,7 m. The
velocity was measured with an Ottflügel, Type C2, in 40 % water depth above streambottom.
Recording to this detailed hydrological and morphological data, it was possible to calculate
the flow (water volume per second) and build up a depth and a current velocity profile.
Furthermore stream width, stream bed width and slope angle of the bank were assessed.
Water temperature and oxygen content were measured with a WTW Oximeter 330. A WTW
pH-meter was used to record the pH-value.
Sediment size was estimated visually. Therefore we used five standard particle size ranges
(< 2 mm, 2 - 6,3 mm, 6,3 - 20 mm, 20 - 63 mm, > 63 mm) and noted the proportion of each
range in the stream bed sediment.
The shadowing of the river by the riverine vegetation was estimated visually aswell and
assigned to one of the following values: 0 %, 5 %, 25 %, 50 %, 75 %, 100 %. At the
Quebrada Sardinal we recorded the riparian vegetation on a species level.
Between February 2004 and February 2005 water samples were taken at least once of each
site and analyzed for conductivity, pH, Alk., Cl-, SO42-, Si-SiO4, P-PO4, P-s, P-t, N-NO3. N-
NO2, N-NH4, N-sKj, N-tKj, Na+, K+, Ca2+, Mg2+ at the laboratory in Austria as soon as
possible. Water samples, which were taken on the 26.02.04, 03.04.04, 27.04.04, 12.02.05,
13.02.05 and 15.02.05 were assigned to the 'dry' season and the samples from 24.06., 03.08.
and 20.09.2004 are counting for the 'rainy' season.
10
Analysis of data
The statistical analysis of data was performed with the software package SPSS. The PCA
(principal components analysis) is a multivariate procedure which rotates that data such that
maximum variabilities are projected onto the axis. Cluster analysis classifies a set of
observations into two or more groups based on combinations of interval variables. This was
performed on the basis of water chemistry data from the 'dry' and 'rainy' season for all studied
streams.
Fig.2.3. Rio Esquinas catchment with nine sampling sites (Q.Negra, Q.Mari, Q.Chorro, Q.G amba, Q.Sardinal, Q.Bolsa, Rio Bonito, Rio Oro, Rio Esquinas)
11
Fig. 2.4. Rio Esquinas catchment, Rio Bonito catchment and Quebrada Negra catchment within the Piedras Blancas National Park and its vicinty
12
2.2. Results The nine streams studied (Fig. 2.3.) are all tributaries of the Rio Esquinas that lie within
the Piedras Blancas National Park and its vicinity. Every stream or river was conducted
twice – once during the 'dry' and once during the 'rainy' season – except of Quebrada
Mari, Quebrada Chorro and Rio Esquinas.
Quebrada Mari
The 'Quebrada Mari' is an unnamed 1st order stream, which is one out of 3 headwaters
of the Rio Esquinas. The stream has its source (~1100 m above sea level) in the
mountains of Fila Cruces and has a length of 3,6 km. Sampling took place on the 24th
February 2004 and chemical parameters like temperature, conductivity, pH-value and
oxygen content were measured three times along the stream course. The first sampling
site was a few meters up the stream at the bridge leading to San Miguel. Riparian
vegetation is dense and canopy cover is about 25 % at the sampling site 1. Large
boulders and rocks are dominating the stream bed. Rocks with a diameter of ~1 m are
common. The stream is 2 - 2,5 m wide and the mean depth is approximately 0,2 m. Flow
is about 150 ls-1.
13
Tab.:2.1. Data of three sampling sites at the Q.Mari Quebrada Mari date 24.02.2004 24.02.2004 24.02.2004 position - - - sampling site bridge 2 waterfall time 10.00 am 11.00 am 1.00 pm sea level [m] 450 500 580 temperature [°C] 22,1 22,3 22,4 pH-value 8,07 8,06 8,11 O2 [%/mgl-1] 133 / 10,8 98 / 7,9 103 / 8,1 stream width* [m] 2 – 2,5 - - average depth* [m] 0,20 - - average current velocity* [ms-1] 0,40 - - flow* [ls-1] 150 - - shading [%] 25 - - * estimated
Quebrada Negra
The Quebrada Negra is a tropical low land stream with a low altitudinal gradient and has
its source in the primary forest (~180 m above sea level) of the Esquinas rainforest. On
the entire way (~2,7 km) till the stream flows into Quebrada Gamba (~20 m above sea
level) he discharges primary and secondary rainforest and the La Gamba valley. The
stream flows close behind the Field Station La Gamba and the sampling site was about
14
1,3 km from the stream origin. For a detailed study site description an easily accessible
100 m sector next to the Field Station was chosen. Substratum is gravel in riffles with
leaf accumulations and silt in pools. Water temperature is about 25°C. A comparable
study site was chosen 1 km up the stream with large boulders intermixed with smaller
cobble substrate. For a full report and further details see chapter 3.
Tab.2.2. Data of three sampling sites within the Q. Negra Quebrada Negra date 20.02.2004 20.02.2004 20.02.2004position N 08°42,054' N 08°41,806' W 083°12,085' W 083°12,305' sampling site field station Lodge waterfall time 9.00 am 11.00 am 12.00 am sea level [m] 80 100 160 temperature [°C] 24,7°C 24,9°C 25,2°C pH-value 7,73 8,1 O2[%/mgl-1] 92,5 / 7,6 98 / 8,05 10350,00%
Quebrada Chorro
15
The Quebrada Chorro is a stream with a length of 5,3 km and empties into the Rio
Bonito. The source is at an elevation of 220 m above sea level. Sampling took place on
the 22nd of February and on the 19th of August close to the waterfall (~ 200 m
downstream). At the study site the Quebrada Chorro is a 2nd order stream and 50 % of
the river is shaded. Stream width (3,8 m / 5,3 m), mean depth (0,09 m / 0,15 m) and
average current velocity (0,33 ms-1 / 0,51 ms-1) show a distinct increase in August. Flow
is four times as high (410 ls-1) in the 'rainy' season as in the 'dry' season (108 ls-1). The
stream bottom consists of gravel and cobbles.
Tabl.:2.3. Data of two sampling sites at the Q. Chorro Quebrada Chorro date 22.02.2004 19.08.2004 position N 08° 42,425’
W 083° 10,552’N 08° 42,425’ W 083° 10,552’
sampling site waterfall waterfall time 10.00 am 12.00 am sea level [m] 80 80 temperature [°C] 26,3 - pH-value 7,97 - O2 [% / mgl-1] 8,16 / 102,0 - stream width [m] 3,8 5,3 average depth [m] 0,09 0,15 average current velocity [ms-1] 0,33 0,51 flow [ls-1] 108 410 sediment [mm]
Q 25 Q 50 Q 75
- - -
5,3 18,1 61,6
shading [%] 50 50
16
Quebrada Gamba
The Quebrada Gamba has a length of 7,7 km and the stream empties into the Rio
Bonito. The source is at an elevation of 260 m above sea level. At the sampling site, a
few meters downstream from the bridge leading to La Gamba, it is a 2nd order river.
Stream width and average current velocity nearly do not differ from 'dry' season (2nd of
April) to 'rainy' season (1st of August), but the average depth (0,32 m / 0,49 m) increases
as well as the flow (397 ls-1 / 650 ls-1). Canopy cover is at both times of sampling 50 %.
Gravel is the dominating substratum. Tab.:2.4. Data of two samplings at the Q. Gamba Quebrada Gamba date 02.04.2004 01.08.2004 position N 08° 42,072’
W 083° 11,530’ N 08° 42,072’ W 083° 11,530’
sampling site bridge bridge time 9.30 am 11.00 am sea level [m] 20 20 stream width [m] 6,9 6,8 average depth [m] 0,32 0,49 average current velocity [ms-1] 0,19 0,19 flow [ls-1] 397 650
17
sediment [mm] Q 25 Q 50 Q 75
- - -
7,6 24,0 75,9
shading [%] 25 25
Quebrada Bolsa
The Quebrada Bolsa has its source at 320 m above sea level and after 6,5 km it flows
into the Quebrada Gamba. The cross section was established near the bridge leading to
km 37 at the Interamericana on the 2nd of April and on the 1st of August. Stream width
ranges from 9,6 m to 11 m. The mean depth is between 0,128 m and 0,163 m and the
average current velocity between 0,289 ms-1 and 0,391 ms-1. The detected flow is 408 ls-
1 and 682 ls-1. Whereas half of the river is shaded in the 'dry' season, canopy cover
increases to 75 % in the 'rainy' season. The stream bottom is dominated by gravel and
cobbles.
18
Tab.:2.5. Data of two samplings at the Q. Bolsa Quebrada Bolsa date 02.04.2004 01.08.2004 position N 08° 42,452’
W 083° 11,137’ N 08° 42,452’ W 083° 11,137’
sampling site bridge bridge time 9.00 am 12.00 am sea level [m] 20 20 stream width [m] 9,6 11,0 average depth [m] 0,13 0,16 average current velocity [ms-1] 0,29 0,39 flow [ls-1] 408 682 sediment [mm]
Q 25 Q 50 Q 75
- - -
9,1 28,9 91,2
shading [%] 50 75
Quebrada Sardinal
The Quebrada Sardinal – a 2nd order stream - is a tributary of the Rio Bonito and has a
length of 7,3 km. The sampling site (N 08° 43,407', W 083° 12,773', 30 m above sea
level) was 250 m upstream before he flows into the Rio Bonito. Riparian vegetation is
dense and canopy cover was in February about 5 % and during the 'rainy' season 25 %.
19
Substratum is dominated by gravel with some stones (diameter: 0,06 m). The stream is
during the 'dry' season about 6 m wide, but gets larger in August. Mean depth is
approximately 0,2 m. The current velocity in February is low – flow is about 200 ls-1, but
reaches during the 'rainy' season 0,75 ms-1 and flow gets up to 1700 ls-1. Rock surfaces
are densely covered with periphyton. Tab.:2.6. Data of three samplings at the Q. Sardinal Quebrada Sardinal date 21.02.2004 21.02.2004 20.08.2004 position N 08° 43,407’ N 08° 43,407’ N 08° 43,407’ W 083° 12,773’ W 083° 12,773’ W 083° 12,773’sampling site transect 1 transect 2 transect 1 time 15.00 16.00 16.00 sea level [m] 30 30 30 temperature [°C] 27,6 27,6 - pH-value 7,63 7,63 - O2[%/mgl-1] 100,6 / 7,86 100,6 / 7,86 - stream width [m] 6,0 5,8 10,0 average depth [m] 0,16 0,32 0,23 average current velocity [ms-1] 0,28 0,12 0,75 flow [ls-1] 260 173 1765 sediment [mm]
Q 25 Q 50 Q 75
- - -
- - -
3,8 12,0 38,1
shading [%] 5 5 25 Tab.:2.7. Riparian vegetation at the Rio Sardinal with type of growth, frequency(++ often, + present, - absent), height [m] and distance to the riverbank [m] of recorded plants Rio Sardinal
taxon type of growth frequencyheight
[m] distance to the riverbank [m]
MONOCOTYLEDONS Cyclanthaceae Cyclanthus bipartitus giant herb + - - Carludovica drudei giant herb + - - Heliconiaceae Heliconia latispatha herb ++ - 0 Marantaceae Calathea lutea giant herb ++ - - Poaceae Dendrocalamus giganteous + - - Zingiberaceae Hedychium coronarium herb + - -
20
DICOTYLEDONS Anacardiaceae Anacardium excelsum tree + 9 - Spondias mombin tree ++ 32 0 Annonaceae Guatteria amplifolia tree + 12 - Bombacaceae Ceiba pentandra canopy tree + 35 - Cucurbitaceae Gurania macoyana liana + - - Euphorbiaceae Acalypha diversifolia shrub/tree ++ - - Alchornea costaricensis tree + 17 - Fabaceae - Faboideae Lonchocarpus sp. tree + 10 - Calopogonium sp. vine + - - Lauraceae Ocotea sp. tree ++ 20 0 Marcgraviaceae
Souroubea sp. hemiepiphytic
shrub + - - Melastomataceae
Topobea maurofernandeziana hemiepiphytic
tree + - - Sterculiaceae Theobroma cacao tree ++ - - Tiliaceae Luehea seemannii canopy tree ++ 25 0 Apeiba tibourbou canopy tree ++ - - Trichospermum grewiifolium canopy tree ++ - - The abundant riparian vegetation maintains mostly typical species which occur along the
streams and rivers in this region of Costa Rica.
Trees of the canopy layer include Luehea seemannii, Apeiba tibourbou, Trichospermum
grewiifolium (Tiliaceae), Ceiba pentandra (Bombacaceae), and Spondias mombin
(Anacardiaceae) which all reach a height of about 30 m. Common species of the mid
tree layer are Anacardium excelsum (Anacardiaceae), Guatteria amplifolia
(Annonaceae), Alchornea costaricensis (Euphorbiaceae), Lonchocarpus sp. (Fabaceae
– Faboideae), Ocotea sp. (Lauraceae) and Theobroma cacao (Sterculiaceae). The
shrub stratum contains species like Cyclanthus bipartitus, Carludovica drudei
(Cyclanthaceae), Calathea lutea (Marantaceae) and Acalypha diversifolia
(Euphorbiacea)
21
Rio Oro
Rio Oro is at the study site a 3rd order stream, which flows after 6,2 km into Rio Bonito
next to Villa Briceno – km 37 at the Interamericana. The sampling site (N 08°43,004', W
083°10,033', 20 m above sea level) was at the bridge, close before Villa Briceno – km
37. Canopy cover is low – about 5 % and the substrate is dominated by gravel. The
stream width is about 14 m in February and 19 m in August and the depth varies
between 0,08 m and 0,33 m. Mean current velocity ranges from 0,23 ms-1 in the 'dry'
season to 0,29 ms-1 in the 'rainy' season. The flow comes in February to 187 ls-1 and in
August to 1622 ls-1. Tab.:2.8. Data of two samplings at the Rio Oro Rio Oro date 23.02.2004 19.08.2004 position N 08° 43,004' N 08° 43,004' W 083° 10,033' W 083° 10,033'sampling site bridge bridge time 8.45 am 10.00 am sea level [m] 20 20 temperature [°C] 26,8°C -
22
pH-value 7,75 - O2 [%/mgl-1] 109,2 / 8,64 - stream width [m] 14,5 19 average depth [m] -0,08 -0,33 average current velocity [ms-1] 0,23 0,29 flow [ls-1] 187 1622 sediment [mm]
Q 25 Q 50 Q 75
- - -
1,5 6,5 28,5
shading [%] 5 5
Rio Bonito
The Rio Bonito (20,1 km) is one of the two largest rivers within the Piedras Blancas
National Park. The river discharges from dense primary forest (200 m above sea level)
where its canopy cover is about 100 %. At the study site, upstream of the village La
Gamba, it is a 3rd order river with low canopy cover – about 5 %. The Rio Bonito passes
through the anthropogen influenced valley of La Gamba until it drains as a 4th order river
into the Rio Esquinas. Two cross sections with a distance of 10 m were build up at the
study site. Transect 1 shows a pool area, while transect 2 is characterised by a constant
current velocity and depth. The stream width is 3,8 and 7,2 m in February and 6,3 and
23
8,6 m in August and the depth is 0,34 m and 0,12 m during the 'dry' season and 0,37 m
and 0,21 min the 'rainy' season. Mean current velocity ranges from 0,24 and 0,60 ms-1 in
the 'dry' season to 0,52 ms-1 and 0,82 ms-1 in the 'rainy' season. The flow comes in
February to ~540 ls-1 and in August to 1560 ls-1. Tab.:2.9. Data of four samplings at the Rio Bonito Rio Bonito date 02.03.2004 02.03.2004 14.08.2004 14.08.2004 position N 08°43,868' N 08°43,868' N 08°43,868' N 08°43,868' W 83°17,723' W 83°17,723' W 83°17,723' W 83°17,723'sampling site transect 1 transect 2 transect 1 transect 2 time 10.00 am 11.00 am 11.00 am 12.00 am sea level [m] 30 30 30 30 temperature [°C] 28,2 - - - pH-value 8,3 - - - O2 [% / mgl-1] 101 / 8,0 - - - stream width [m] 3,8 7,2 6,3 8,6 average depth [m] 0,34 0,12 0,37 0,21 average current velocity [ms-1] 0,24 0,60 0,52 0,82 sediment [mm]
Q 25 Q 50 Q 75
- - -
- - -
- - -
6,3 18,3 53,3
flow [ls-1] 545 523 1555 1565 shading [%] 5 5 5 5
24
Rio Esquinas
Rio Esquinas is a 5th order river and with a lenght of 42 km the largest one within the
Piedras Blancas National Park. He forms the natural border of the Bosque Esquinas in
the North and West of the national park and drains into the Pacific Ocean. Mangroves
occur in the tidal estuaries next to the mouth of the Rio Esquinas to a great extent.
Mangroves are floristically poor, thus representing the opposite extreme of tropical
forests with their rich species diversity. Species like Rhizophora mangle, Rhizophora
racemosa (Rhizophoraceae) and Pelliciera rhizophorae (Theaceae) are common.
The sampling site (N 08°43,868', W 83°17,723' , 0 m above sea level) was about 4 km
upstream of its mouth into the Pacific Ocean. Canopy cover is 0 % and sediment is
dominated by sand and mud. The stream width is about 30 m and average stream depth
is 1,5 m. The maximum depth reaches 4 m. The estimated current velocity is ~0,6 m and
resulting flow about 20 000 ls-1. Tab.2.10. Data of four samplings at the Rio Esquinas Rio Esquinas date 25.02.2004 position N 08°43,868' W 83°17,723'sampling site mouth time 13.00 am sea level [m] 0
25
temperature [°C] 29,4 pH-value 7,78 O2 [% / mgl-1] 104,4 / 7,94 stream width* [m] 30 average depth* [m] 1,5 average current velocity* [ms-1] 0,6 flow* [ls-1] 20 000 shading [%] 0 * estimated
river length [km]0 5 10 15 20 25 30 35 40 45
altit
ude
[m]
0
20
40
60
80
100
120
140
160
180
200altitude
Gam
baBo
nito
Esqu
inas
Neg
ra
flow
[ls-1
]0
2500
5000
7500
10000
12500
15000
17500
20000
22500
flow
Fig. 2.5. River lenght [km] versus altitude [m] and flow [ls-1] during the 'dry' season of the Q.Negra flowing into Q.Gamba, Rio Bonito and Rio Esquinas.
The altitudinal gradient from the source of the Q.Negra to the mouth of Rio Esquinas,
which is 45 km downstream at the Pacific Ocean, is shown in Fig.2.5. The Q. Negra has
its source at an elevation of 180 m above sea level. After 2,7 km km Q. Negra flows into
the Q. Gamba at an altitude of less than 70 m. 1,2 km downstream the Q. Gamba drains
into Rio Bonito, which flows after 3,1 km into Rio Esquinas. During the 'dry' season
Q.Negra has a flow of 22,5 ls-1, Q.Gamba of 397 ls-1, Rio Bonito of 545 ls-1 and Rio
Esquinas of 20000 ls-1 before he empties into the Golfo Dulce.
26
Chorro - DS
0 2 4 6 8 10 12 14 16
bank
leve
l [m
]
-0,5
0,0
0,5
1,0
Chorro - RS
stream width [m]0 2 4 6 8 10 12 14 16
bank
leve
l [m
]
-0,5
0,0
0,5
1,0
1,60
1,60
Fig.:2.6. Cross section of Quebrada Chorro with stream width [m] and depth [m] during 'dry' [DS] and
'rainy' [RS] season
27
Sardinal - cross section 1 - DS
0 2 4 6 8 10 12 14
bank
leve
l [m
]
-1,0
-0,5
0,0
0,5
1,0
Sardinal - cross section 2 - DS
0 2 4 6 8 10 12 14
bank
leve
l [m
]
-1,0
-0,5
0,0
0,5
1,0
Sardinal - cross section 1 - RS
stream width [m]0 2 4 6 8 10 12 14
bank
leve
l [m
]
-1,0
-0,5
0,0
0,5
1,0
1,20 1,40
2,00
1,35 1,60
Fig.: 2.7. Cross sections of Quebrada Sardinal with stream width [m] and depth [m] during 'dry' [DS] and 'rainy' [RS] season
28
Oro - DS
0 2 4 6 8 10 12 14 16 18 20
bank
leve
l [m
]
-0,5
0,0
0,5
1,0
Oro - RS
stream width [m]0 2 4 6 8 10 12 14 16 18 20
bank
leve
l [m
]
-1,0
-0,5
0,0
0,5
1,0
2,00
2,00
1,40
1,50
Fig.: 2.8. Cross transect of Rio Oro with stream width [m] and depth [m] during 'dry' [DS] and 'rainy' [RS] season A comparison of selected cross sections from Quebrada Chorro, Quebrada Sardinal and
Rio Oro from the 'dry' and 'rainy' season show the increase in stream width and water
depth during the 'rainy' season. Distinct seasonal differences can be seen in Fig.2.9.,
Fig.2.10. and Fig.2.11. Data of cross sections from Quebrada Gamba, Quebrada Bolsa,
Rio Bonito and Rio Esquinas are provided in the Appendix.
The two figures of the cross section at Quebrada Chorro show the increasing stream
width from 3,8 m during the 'dry' season to 5,3 m in the 'rainy' season. Maximum depth
increases from 0,14 m in the 'dry' season to 0,23 m in the 'rainy' season
The cross transect of Quebrada Sardinal within the riffle section was constructed in both
seasons. Stream width (5,6 / 10,5 m) and maximum depth (0,25 / 0,31 m) increases
markedly aswell as the flow (260 / 1765 ls-1) from the 'dry' to the 'rainy' season. Study
site two, which was only analyzed in the 'dry' season, shows a pool section with a
maximum depth of 0,67 m. The flow is 175 ls-1 and the stream width 5,8 m.
29
Rio Oro gets striking larger and deeper during the 'rainy' season. Streamwidth ranges
from 14,4 m during the 'dry' season to 18,9 m in the 'rainy' season. Maximum depth is
0,12 m in February and 0,57 m in August.
riverNegra Chorro Bolsa Gamba Sardinal Oro Bonito
sedi
men
t siz
e [m
m]
0
20
40
60
80
100Q75 Q50 Q25
Fig.: 2.9. Comparison of the sediment size [mm] from the nine study rivers during the 'dry' and the 'rainy' season. Arrangement of the rivers according to their stream order [1st – 5th] Sediment size was only recorded at seven study sites in the 'rainy' season. Sediment
size differs markedly between the study sites. Rio Oro shows the smallest sediment size
whereas Quebrada Bolsa has the largest substrate.
river
Negra Mari Chorro GambaSardinal Bolsa Bonito Oro Esquinas
wid
th [m
]
0
5
10
15
20
25
30
35
40
45
rainy seasondry season
Fig.:2.10. Comparison of the river width [m] from the nine study rivers during the 'dry' and the 'rainy' season. Arrangement of the rivers according to their stream order [1st – 5th]
30
The streamwidth of Q.Negra, Q.Chorro, Q.Sardinal, Rio Oro and Rio Bonito markedly
gets larger from the 'dry' season to the 'rainy' season. Q.Bolsa and Q.Gamba are wider
during the 'dry' season, but get deeper in the 'rainy' season as the transect figures show.
At Q.Mari and Rio Esquinas it was just once, during the 'dry' season, possible to
estimate the stream width. River width ranges from 2,3 m (Q.Mari) to 42 m at the
Esquinas study site.
riverNegra Mari Chorro GambaSardinal Bolsa Bonito Oro Esquinas
dept
h [m
]
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
1,8
2,0
dry seasonrainy season
Fig.:2.11. Comparison of the average depth [m] with standard deviation from the nine study rivers during the 'dry' and the 'rainy' season. Arrangement of the rivers according to their stream order [1st – 5th] All seven rivers where data is available from both seasons get deeper during the 'rainy'
season, just Q.Mari and Rio Esquinas were only analyzed once in the 'dry' season.
Standard deviation are markedly smaller at cross transects, which were constructed at
riffles to the ones in pools.
31
riverNegra Mari Chorro GambaSardinal Bolsa Bonito Oro Esquinas
curre
nt v
eloc
ity [m
s-1]
0,0
0,2
0,4
0,6
0,8
1,0
1,2
dry seasonrainy season
Fig. 2.12. Comparison of the average current velocity [ms-1] with standard deviation from the nine study rivers during the 'dry' and the 'rainy' season. Arrangement of the rivers according to their stream order [1st – 5th] Current velocity increases at every study site during the 'rainy' season. Data of Q. Mari
and Rio Esquinas is only available for the 'dry' season. In the 'dry' season average water
current velocity is slow, ranging from 0,187 ms-1 (Q. Gamba) to 0,5 ms-1 (Rio Esquinas).
In the 'rainy' season higher speeds were recorded. Whereas Q. Gamba does not show a
distinct difference, average current velocity is increasing intensively in Quebrada
Sardinal (0,285 / 0,749 ms-1). Standard deviation is higher at cross sections within pools.
32
river
Negra Mari Chorro GambaSardinal Bolsa Bonito Oro Esquinas
flow
[m3 s-1
]
0
1
2
10rainy seasondry season
Fig.: 2.13. Comparison of the flow [m3s-1] from the nine study rivers during the 'dry' and the 'rainy' season. Arrangement of the rivers according to their stream order [1st – 5th] Flow increases at all study rivers, but for Q. Mari and Rio Esquinas no data is available
for the 'rainy' season. The rivers show large differences in flow. Quebrada Negra is the
smallest stream with a baseflow of 0,031 m3s-1. Flow of Rio Esquinas is about 300 times
as high (10m3s-1). Streamflow was highly seasonal.
Furthermore all rivers show large fluctuations between 'dry' and 'rainy' season. Water
flow is increasing in the late 'rainy' season (September, October). Therefore differences
in flow are depending on sampling date. Values between 0,094 m3s-1 (Q. Negra) and
1,776 m3s-1 (Q.Sardinal) were recorded. Rio Oro shows the largest fluctuation (0,187 /
1,622 m3s-1).
33
Tab.:2.11. Hydrochemical characteristics of 9 sampled rivers within the Piedras Blancas National Park
river date Lf pH Alk Cl- SO42- Si-SiO4 P-PO4 P-s P-t N-NO3 N-NO2
N-NH4
N-sKj
N-tKj Na+ K+ Ca2+ Mg2+
µS/cm; 25°C
mg/l
mg/l
mg/l
mg/l
µg/l
µg/l
µg/l
µg/l
µg/l
µg/l
µg/l
µg/l
mg/l
mg/l
mg/l
mg/l
Rio Bonito 26.02.04 155 8 104,9 0,5 1,4 21,5
13 14 19 47 0 6 47 55 5,0 0,6 16,7 7,0Rio Oro 26.02.04 372 8,3 141,6 1,9 12 12,2 14 16 22 166 0 1 65 92 7,8 1,3 44,4 2,6Esquinas 26.02.04 299 8,2 167,2 5 9,4 13,2 16 17 40 92 1 0 48 79 9,6 1,1
44,3 4,7
Quebrada Mari 26.02.04 260 8,3 126,9 0,2 2,6 4,4 3 4 11 285 0 0 19 24 1,0 0 34,8 2,9Quebrada Negra 26.02.04 200 8,1 128,1 0,4 1,3 26,2 48 49 50 64 0 0 28 39 6,9 0,8 24,0 5,6Rio Sardinal 26.02.04 132 8,1 101,3 0,4 1,0 22,0 19 21 21 173 0 31 39 39 3,9 0,6 18,3 5,5Quebrada Bolsa 03.04.04 145 8,4 91,5 0,6 1,5 23,4 31 32 34 120 0 20 60 83 6,2 0,7
14,8 4,4
Quebrada Gamba 03.04.04 154 8,4 102,5 0,5 1,1 23,2 20 21 25 62 0 3 44 52 6,2 0 17,3 5,0Quebrada Negra 03.04.04 175 8,4 112,3 0,5 1,4 23,8 26 28 32 95 0 16 49 70 6,1 0,8 20,4 4,8Quebr. Bolsa 27.04.04 158 8,3 102,5 0,5 1,6 23,5 28 40 40 118 1 16 19 24 6,5 0,7 17,0 4,8Quebrada Gamba 27.04.04 161 8,4 108,6 0,5 1,1 23,8 28 39 41 86 1 4 18 32 6,3 0,7 18,1 5,0Quebrada Negra 27.04.04 185 8,4 117,2 0,4 1,4 25,0 45 45 46 66 1 9 18 39 6,4 0,8 20,8 4,9Quebrada Negra 24.06.04 197 7,9 129,4 0,5 1,1 29,7 40 42 46 37 0 0 16 33 7,9 0,4 26,9 6,4Quebrada Negra 03.08.04 196 8,5 129,4 0,6 1,1 30,5 46 46 49 69 1 4 54 67 7,3 0,7
26,5 6,4
Quebrada Bolsa 20.09.04 157 8,4 106,2 0,5 1,2 25,9 76 77 87 21 4 34 134 154
7,1 0 18,8 5,5Rio Bonito 20.09.04 150 8,4 100,1 0,4 0,9 22,2 22 23 25 34 1 6 44 51 4,6 0 16,4 7,3Quebrada Chorro 20.09.04 128 8,2 81,8 0,4 0,6 27,9 42 44 45 9 1 25 32 64 5,8 0,6
14,2 4,0
Quebrada Gamba 20.09.04 160 8,3 106,2 0,4 0,7 27,0 55 58 76 23 1 69 100
167
6,8 0 18,4 5,6Quebrada Negra 20.09.04 177 8,5 114,7 0,4 0,9 13,3 47 47 53 143 1 48 77 88 6,3 0,6 22,6 5,4Rio Oro 20.09.04 314 8,4 178,2 0,7 14,2 26,9 36 36 75 269 0 13 101 120 7,3 0,9 52,4 2,4Quebrada Sardinal 20.09.04 126 8,2 83 0,3 0,7 22,5 19 20 22 25 1 12 34 39 3,7 0 12,3 6,1Quebrada Negra 12.02.05 205 8,4 125,7 1,4 1,3 26,8 39 39 50 36 0,3 27 44 83 7,5 0,8 26,2 6,0Quebrada Negra 13.02.05 204 8,2 125,7 1,2 1,3 26,7 31 31 42 110 3,1 4 27 86 7,3 0,9 26,0 6,0Quebrada Negra 15.02.05* 152 8,1 92,7 1 1,2 22,3 31 32 45 86 1 4 69 109 5,7 0,8 19,0 4,4Quebrada Negra 15.02.05** 168 8,2 103,7 0,6 1,1 22,0 36 38 56 82 1 4 64 138 5,7 0,8 22,5 4,5Rio Esquinas 17.02.05 278 8,3 156,2 1,3 12,9 7,4 8 8 18 195 0 25 55 79 7,8 0,8 46,0 3,5Quebrada Gamba 17.02.05 169 8,3 102,5 1,1 1,3 23,7 29 30 34 23 1,2 32 36 47 6,9 0,6 19,9 5,6Rio Oro 17.02.05 367 8,4 197,7 1,6 22,9 12,5 1 2 68 78 1,6 21 43 147 9,1 1,1 65,2 2,8
34
river date Na+ K+ Ca2+ Mg2+ Alk Cl- SO4
2- Ionenbilanz skat san an% mval
mval
mval
mval
mval
mval
mval
diffkat%
mval
mval
Rio Bonito
26.02.04 0,2 0 0,8 0,6 1,7 0 0 -7,3 1,6 1,8 107,3Rio Oro 26.02.04 0,3 0 2,2 0,2 2,3 0,1 0,3 6,2 2,8 2,6 93,8Esquinas 26.02.04 0,4 0 2,2 0,4 2,7 0,1 0,2 -1,2 3,0 3,1 101,2Quebrada Mari 26.02.04 0 0 1,7 0,2 2,1 0 0 -6,1 2,0 2,1 106,1Quebrada Negra
26.02.04 0,3 0 1,2 0,5 2,1 0 0 -8,3 2,0 2,1 108,3
Rio Sardinal 26.02.04 0,2 0 0,9 0,5 1,7 0 0 -8,9 1,6 1,7 108,9Quebrada Bolsa 03.04.04 0,3 0 0,7 0,4 1,5 0 0 -11,4 1,4 1,5 111,4Quebrada Gamba 03.04.04 0,3 0 0,9 0,4 1,7 0 0 -10,9 1,5 1,7 110,9Quebrada Negra
03.04.04 0,3 0 1,0 0,4 1,8 0 0 -10,5 1,7 1,9 110,5
Quebr. Bolsa 27.04.04 0,3 0 0,8 0,4 1,7 0 0 -11,7 1,5 1,7 111,7Quebrada Gamba 27.04.04 0,3 0 0,9 0,4 1,8 0 0 -12,9 1,6 1,8 112,9Quebrada Negra 27.04.04 0,3 0 1,0 0,4 1,9 0 0 -12,7 1,7 2,0 112,7Quebrada Negra 24.06.04 0,3 0 1,3 0,5 2,1 0 0 3,0 2,2 2,2 97,0Quebrada Negra 03.08.04 0,3 0 1,1 0,4 2,1 0 0 0,9 2,2 2,2 99,1Quebrada Bolsa
20.09.04 0,3 0 0,9 0,5 1,7 0 0 -4,8 1,7 1,8 104,8
Rio Bonito 20.09.04 0,2 0 0,8 0,6 1,6 0 0 -3,3 1,6 1,7 103,3Quebrada Chorro 20.09.04 0,3 0 0,7 0,3 1,3 0 0 -4,3 1,3 1,4 104,3Quebrada Gamba 20.09.04 0,3 0 0,9 0,5 1,7 0 0 -5,4 1,7 1,8 105,4Quebrada Negra
20.09.04 0,3 0 1,1 0,4 1,9 0 0 -2,6 1,9 1,9 102,6
Rio Oro 20.09.04 0,3 0 2,2 0,2 2,9 0 0,3 -2,5 3,2 3,2 102,5Quebrada Sardinal
20.09.04 0,2 0 0,6 0,5 1,4 0 0 -8,3 1,3 1,4 108,3
Quebrada Negra 12.02.05 0,3 0 1,3 0,5 2,1 0 0 0,9 2,1 2,1 99,1Quebrada Negra 13.02.05 0,3 0 1,3 0,5 2,1 0 0 0,6 2,1 2,1 99,4Quebrada Negra 15.02.05* 0,2 0 0,9 0,4 1,5 0 0 0,6 1,6 1,6 99,4Quebrada Negra
15.02.05** 0,2 0 1,1 0,4 1,7 0 0 1,1 1,8 1,7 98,9
Rio Esquinas 17.02.05 0,3 0 2,3 0,3 2,6 0 0,3 2,8 2,9 2,9 97,2Quebrada Gamba 17.02.05 0,3 0 1,0 0,5 1,7 0 0 1,7 1,8 1,7 98,3Rio Oro 17.02.05 0,4 0 3,3 0,2 3,2 0 0,5 3,9 3,9 3,8 96,1(* 5.00 pm, ** 7.30 pm)
35
A summery description of the chemical characteristics of the sampling sites is provided
in Table 2.11. The Q. Mari (260 µScm-1), the Rio Oro ( 372 / 314 µScm-1) and the Rio
Esquinas (299 µScm-1) show in comparison to the Q. Negra, Chorro, Gamba, Bolsa,
Sardinal and Bonito a distinctly higher conductivity. Seasonal differences are slight.
The pH – value of all studies rivers is about 8 in the 'dry' season aswell as in the 'rainy'
season.
Rio Oro and Rio Esquinas have the highest alkalinity. The differences do not show a
clear trend between 'dry' and 'rainy' season.
Rio Oro and Rio Esquinas show distinctly higher values of chlorid. The differences
between the two seasons in general, except at Rio Oro, are slight.
The Rio Oro (12,0 / 14,2 mgl-1) and the Rio Esquinas (9,4 mgl-1) have high
concentrations of SO42-. The values in the Q. Negra, Mari, Chorro, Bolsa, Sardinal and
Rio Bonito are between 0,6 and 2,6 mgl-1. The are almost no differences between 'dry'
and 'rainy' season.
During the 'dry' season the S-SiO4 concentration of Q. Mari, Rio Oro and Rio Esquinas
is distinctly lower than in the Q. Negra, Gamba, Bolsa, Sardinal and Rio Bonito. Clear
seasonal differences are only apparent in the samples of Rio Oro.
The Q. Mari has a very low concentration of P-PO4. During the 'rainy' season all streams
have higher values of phosphorus concentration than in the 'dry' season. The Q. Mari
shows a very low value of soluble phosporus. Q. Negra, Chorro, Gamba and Bolsa have
the highest concentrations. Most of the streams show higher values in the 'rainy' season.
Q. Gamba (76 µgl-1), Q.Bolsa (87 µgl-1) and Rio Oro (75 µgl-1) show the highest values
of total phosphorus concentration during the 'rainy' season.
Q. Mari (285 µgl-1) has the highest concentration of N-NO3 during the 'dry' season and
the Rio Oro (269 µgl-1) during the 'rainy' season.
N-NO2 concentrations are higher at all study streams during the 'rainy' season. Seasonal
differences are obvious. Q. Bolsa has the highest concentration (4 µgl-1) of all rivers
during the 'rainy' season. Q. Gamba (69 µgl-1) has the highest concentration of N-NH4
during the 'rainy' season. No clear trend between the two seasons and between the
study rivers is perceptible.
In the 'dry' season Quebrada Mari has the lowest and Quebrada Oro the highest values
of N-sKj. In the 'rainy' season Q. Negra, Gamba, Bolsa and Oro show a distinct increase
in the concentration of N-sKj. In the 'dry' season Quebrada Mari has the lowest and
36
Quebrada Oro the highest values. In the 'rainy' season Q. Negra, Gamba, Bolsa and
Oro show a distinct increase in the concentration of N-tKj.
The Q. Mari (1,0 mgl-1) has the lowest, the Rio Esquinas (9,6 mgl-1) the highest content
of sodium in the 'rainy' season. Seasonal differences are not present.
Kalium concentrations are in general higher during the 'dry' season. Rio Oro and Rio
Esquinas are the only two rivers which have a Kalium concentration higher than 1 mgl-1
during the 'dry' season.
The content of calcium in the Q. Negra, Chorro, Gamba, Sardinal, Bolsa and the Rio
Bonito is between 14,2 and 25,3 mgl-1. Mari (34,8 mgl-1), Oro (44,4 / 52,4 mgl-1) and
Esquinas (44,3 mgl-1) show clearly higher concentrations. Streams do not show a
distinct seasonal difference.
The Q. Negra, Chorro, Gamba, Sardinal, Bolsa, Rio Bonito and Rio Esquinas show
values between 4,0 mgl-1 and 7,3 mgl-1 of magnesium. The Q. Mari (2,9 mgl-1) and the
Q. Oro (2,6 / 2,4 mgl-1) have about half of the amount of magnesium as the other
mentioned rivers. The differences between 'dry' and 'rainy' season are slight.
river
mva
l
0
1
2
3
4
CaMgNaAlkSO4
Negra Mari Chorro Gamba Sardinal Bolsa Bonito Oro Esquinas
Fig.:2.14. Ion balances based on concentrations of Ca2+, Mg2+, Na+, alkalinity and SO42- [mval] for the nine
study streams Differences in solute concentrations among the nine streams seem largely a function of
area specific runoff.
37
34,64FAC 1
geochemical paramters
-2 -1 0 1 2 3 4
nutri
ents
FAC
227
,07
-3
-2
-1
0
1
2
3BonitoOroEsquinasMariNegraSardinalBolsaGambaChorro
low high
low
high
Fig.:2.15. Ordination biplot resulting from a PCA (principal component analysis) according to the hydrochemical characteristics of the nine study streams during the 'dry' and 'rainy' season
The results of the PCA (principal component analysis) show the distribution of the study
streams in relation to the first two axis. The PCA describes 61,64 % of the total
physicochemical variability during the 'dry' and 'rainy' season of the streams. “Axis 1”
accounted for 34,64 % of the variance. “Axis 2” explains about 27,07 %. In Fig. 2.15. the
study sites are lined up along axis 1. Full black signs are standing for the sampling site
during the 'dry' season wheras the white signs stand for the 'rainy' season. The principal
components are the nutrients like nitrogen and phosphor and the geochemical
parameters. Axis 1 can be interpreted as the axis for the nutrients and Axis 2 stands for
the geochemical parameters
38
C A S E 0 5 10 15 20 25 Label Num +---------+---------+---------+---------+---------+ Gamba3 11 Negra3 12
1 = 26.02.04 2 = 03.04.04 3 = 27.04.04 4 = 24. 06.04 5 = 03.08.04 6 = 20.09.04 7 = 12.02.05 8 = 13.02.05 9 = 15.02.05, 17:00 10 = 15.02.05; 19:30 11 = 17.02.05
Gamba11 27 Negra5 14 Negra7 22 Bolsa2 7 Negra2 9 Gamba2 8 Bolsa3 10 Chorro6 17 Negra9 24 Negra10 25 Negra8 23 Negra1 5 Negra4 13 Bonito6 16 Sardinal6 21 Bonito1 1 Sardina1 6 Negra6 19 Bolsa6 15
Gamba6 18 Mari1 4
Oro1 2 Esquina11 26 Esquina1 3
Oro6 20 Oro11 28 Fig.:2.16.The dendrogram is showing relationships between the nine rivers based on chemical parameters. Dendrogram using Average Linkage (Between Groups), Rescaled Distance Cluster Combine
The river dendrogram (Fig.2.8.) reveals two large river clusters. Cluster one is the
largest and consists of six rivers. Cluster two consists of the Rio Oro and the Rio
Esquinas. The Quebrada Mari cannot be clearly assigned to one of the two clusters.
The clusters can be assigned to two different catchments according to their geochemical
parameters. The first is the subcatchment of Rio Bonito and the second one are the
rivers, which drain from Fila Cruces (Rio Oro, Rio Esquinas)
39
Tab.:2.12. Results of a discriminant function analysis with Eigenvalue, % of Variance, Cumulative %, Canonical Correlation, Chi-square and Significance (p < 0,05) Function Eigenvalue % of Variance Cumulative % Canonical Correlation
1 12,705 100 100 0,963 Test of function(s) Wilks-Lambda Chi-square df Significance
1 0,073 39,267 22 0,013 A discriminant function analysis according to the hydrochemical parameters of the rivers
shows a significant (p<0,05) difference of the streams between the 'dry' and 'rainy'
season.
40
41
2.3. DISCUSSION
The physical and chemical data reported from some selected streams of the Rio
Esquinas catchment provide a limited picture of the variable conditions existing at the
sampling sites. However, the data show the differences between the sites and their
changing conditions during the 'dry' and 'rainy' season. High water temperatures (>
25°C, except of Q.Mari with 22,1°C) is a permanent feature of the rivers in the
Piedras Blancas National Park and its surroundings, as it is of many other rivers in
Central America.
The results of the morphometric-hydrological measurements show the expected
trends. The two first order streams, Quebrada Negra and Quebrada Mari, have the
lowest values of stream width and flow. In general, stream width and flow gets larger
with increasing stream order. Mean current velocities and mean depths do not show
these tendencies. This might be due to randomly chosen study sites. The cross
sections were sometimes constructed within pool areas and sometimes within riffle
sections, therefore these two parameters are not really comparable within the study
sites.
No trends from 1st to 3rd order river are clearly visible within the sediment size.
Sediment size was just visually estimated once during the 'rainy' season, therefore
the records at the sampling sites only show a small picture and are not really
comparable. Sediment size is not only correlating with the streamorder; it is
dependent upon different physical parameters (current velocity, incline) of the stream.
Mean current velocities and mean depths increase in the 'rainy' season as well as the
recorded flow and are also dependend on the sampling time and sampling site. The
differences in mean current velocities between 'dry' and 'rainy' season are larger
within riffle sections, whereas differences of mean depths are larger within pool
sections. Standard deviation of both parameters is higher in the 'rainy' season which
means a higher heterogeneity.
42
The streams in this seasonal ('dry' and 'rainy' season) environment exhibit a high
annual variation in discharge and turbidity. In the 'rainy' season frequent (almost
daily) storms can cause bankfull conditions and in some cases flooding. Results of
the recorded flow show in all rivers a distinct increase from the 'dry' to the 'rainy'
season. The study catchments are characterized by strong sesaonal variations in
runoff. In the 'dry' season, streamflow exhibits a continous recession and is highly
predictable. In the 'rainy' season there is both a predictable component (the increase
in baseflow) as well as an unpredictable component involving the timing and
magnitude of individual storms. The differences in flow become more apparent when
samples where taken in the late 'rainy' season. The rivers and streams are
characterized by extreme short-term variability in flow. Most Central American rivers
are short in length and do not sustain flood condition for long periods between
successive downpours.
Tropical rivers in general have low concentrations of chemical constituents compared
with temperate rivers. This is because tropical rivers are for the most part
precipitation-dominated, but local geology can also be important. Geological
processes are probably largely responsible for the observed physical and chemical
differences between the studied rivers. Quebrada Mari, one of the headstreams of
Rio Esquinas, Rio Oro and Rio Esquinas have its source in the Fila Cruces while all
the others arise from the Fila Gamba and Fila Golfito. Geology plays a greater role
than land use in determining the nutrient concentrations of unpolluted waters. The
chemical data of the water samples indicate the same source of Quebrada Mari and
Rio Oro. Conductivity, calcium concentration and sulfate concentration is higher, and
the phosphorus concentration is lower than in the six other studied rivers, which have
their source in the Golfo Dulce area. The catchment area of Q.Mari and Rio Oro
seems to exist predominately from calcium sulphate (gypsum) – therefore high
concentrations of CaSO4. Differences in solute concentrations among the six streams
seem largely a function of area specific runoff.
Nitrate values typically are elevated by runoff of agricultural fertilizer and phosphorus
values by sewage effluent. Quebrada Bolsa, which flows close to the village La
Gamba and the Quebrada Negra next to the Esquinas Rainforest Lodge, show high
values of total phosphorus. The lowest amount of total phosphorus was recorded in
the Quebrada Mari, which is surrounded by an anthropogenic unaffected region.
43
The reported concentrations of nitrate, phosphate, total dissolved nitrogen and total
dissolved phosphorus, exhibit a clear relationship with land use. Streams like
Quebrada Gamba, Quebrada Bolsa, Rio Bonito and Rio Esquinas are draining
agricultural land, which has a strong influence on the river chemistry. These rivers
are next to plantations (cacao, oilpalms and other land use) and have higher nutrient
concentrations than those draining forested land. Lowlands are more heavily farmed
and settled than upland regions.
Streams within the Piedras Blancas National Park and its vicinity show significant
differences in their chemistry between the 'dry' and 'rainy' season. If the water
samples of Q.Negra, Q.Bolsa and Q.Gamba from the 27.04.2005 are assigned to the
'rainy' season, the differences become clearer.
Sites in pasture streams like Rio Bonito, Rio Oro, Quebrada Sardinal and Rio
Esquinas were less shaded than those of the forest stream.
Diverse assemblages of macroconsumers (i.e. fish and shrimps) characterize the
streams and rivers of Bosque Esquinas.
44
3. Quebrada Negra
3.1. MATERIAL AND METHODS
For a detailed study site description of the Quebrada Negra, a 100 m stream sector
next to the Biological Field Station was chosen. Transects were established at
regular intervals (5 m apart) by stretching a cord perpendicular to the current. In this
perpendicular stripes water depth and stream current velocity were measured every
0,2 m. The velocity was assessed with an Ottflügel, Type C2, in 40 % water depth
above streambottom. Flow and water volume were calculated to build up a depth and
current velocity profile. Furthermore stream width, stream bed width and slope angle
of the bank were measured. The 100 m sector was analyzed twice – once during the
'dry' season from 14th -18th February 2004 and once in the 'rainy' season from 6th - 8th
August 2004. A comparable study site was chosen 1 km upstream, next to the
Esquinas Rainforest Lodge, and three cross sections were constructed. Abiotic
parameters were recorded as mentioned above.
In the studied section of the Q. Negra four different habitat types (choriotopes) were
identified. Choriotopes are classified by a certain type of structure and differ in their
current velocity, depth and substrate. Four choriotop types are recognized in the
stream: riffles, shallow and slow sites, pools and cascades. Their position within the
100 m was recorded.
To start the hydrological measurement a water level gauge was installed. The water
level was recorded daily at 10 am, together with the precipitation and the minimum
and maximum temperature of the previous day. Precipitation was monitored in a rain
gauge in an open area at the La Gamba Field Station. A detailed short term
chronology of the water level fluctuations and precipitation during rainfall was
recorded on August 14th to show the effects of high rainfall. We estimated peak flows
(when wading was impossible) as the product of the cross-sectional area and a
single estimate of velocity.
45
Water temperature and oxygen content were measured with an WTW Oximeter 330.
A WTW pH-meter was used to record the pH-value. These abiotic parameters were
recorded three times within the stream course of the Q.Negra - see chapter 2.
Between February 2004 and February 2005 water samples were taken ten times and
analyzed for conductivity, pH, Alk., Cl-, SO42-, Si-SiO4, P-PO4, P-s, P-t, N-NO3. N-
NO2, N-NH4, N-sKj, N-tKj, Na+, K+, Ca2+, Mg2+ at the laboratory in Austria as soon as
possible.
The riparian vegetation was recorded to describe the ecotone, an intermediate zone
between the forest and the aquatic site, more detailed. We chose two sites of 10 m
width within the 100 m transect and another one at the comparable site for an
assessement of site specific trees, shrubs, herbs and the ground layer together with
their height, crown diameter and their distance to the bank.
To quantify the leaf litter input of the riverine vegetation 20 samples within the 100 m
section were taken at random. The leaves from an area of 0,108 m2 were dried and
weighted. This examination has been carried out twice in the 'dry' season (on the 5th
March and after heavy rain on the 29th March) and once in the 'rainy' season on the
8th August. At the same time we counted the number of leaf accumulations (> 0,025
m2) within the 100 m section.
Analysis of data
The statistical analysis of data was performed with the software package SPSS. The
PCA (principal components analysis) is a multivariate procedure which rotates that
data such that maximum variabilities are projected onto the axis. This was performed
on the basis of water chemistry data from the 'dry' and 'rainy' season.
3.2. RESULTS
cross section - 0 m - DS
0 2 4 6 8
bank
leve
l [m
]
-0,5
0,0
0,5
1,0
cross section - 0 m - RS
0 2 4 6 8
-0,5
0,0
0,5
1,0
1,68 1,57
cross section - 5 m - DS
0 2 4 6 8
bank
leve
l [m
]
-0,5
0,0
0,5
1,0
cross section - 5 m - RS
0 2 4 6 8-0,5
0,0
0,5
1,0
1,421,58
cross section - 10 m - DS
0 2 4 6 8
bank
leve
l [m
]
-0,5
0,0
0,5
1,0
cross section - 10 m - RS
0 2 4 6 8-0,5
0,0
0,5
1,0
1,50 1,40
cross section - 15 m - DS
stream width [m]0 2 4 6 8
bank
leve
l [m
]
-0,5
0,0
0,5
1,0
cross section - 15 m - RS
stream width [m]0 2 4 6 8
-0,5
0,0
0,5
1,0
2,26 2,20
46
cross section - 45 m - DS
0 2 4 6 8
bank
leve
l [m
]
-0,5
0,0
0,5
1,0
cross section - 45 m - RS
0 2 4 6 8-0,5
0,0
0,5
1,0
1,73 1,90 2,00 1,80
cross section - 50 m- DS
0 2 4 6 8
bank
leve
l [m
]
-0,5
0,0
0,5
1,0
cross section - 50 m - RS
0 2 4 6 8-0,5
0,0
0,5
1,0
1,50 1,67 1,70 1,90
cross section - 55 m - DS
stream width [m]0 2 4 6 8
bank
leve
l [m
]
-0,5
0,0
0,5
1,0
cross section - 55 m - RS
stream width [m]0 2 4 6 8
-0,5
0,0
0,5
1,0
1,78 3,80 1,75 3,60
47
cross section - 85 m - DS
0 2 4 6 8
bank
leve
l [m
]
-0,5
0,0
0,5
1,0
cross section - 85 m - RS
0 2 4 6 8-0,5
0,0
0,5
1,0
1,68 1,71 1,91 1,77
cross section - 90 m - RS
0 2 4 6 8
Y D
ata
-0,5
0,0
0,5
1,0
cross section - 90 m - DS
0 2 4 6 8
bank
leve
l [m
]
-0,5
0,0
0,5
1,0
1,79 1,78 2,05 1,70
cross section - 95 m - DS
0 2 4 6 8
bank
leve
l [m
]
-0,5
0,0
0,5
1,0
cross section - 95 m - RS
0 2 4 6 8-0,5
0,0
0,5
1,0
2,00 1,572,00 1,61
cross section - 100 m - DS
stream width [m]0 2 4 6 8
bank
leve
l [m
]
-0,5
0,0
0,5
1,0
cross section - 100 m - RS
stream width [m]0 2 4 6 8
-0,5
0,0
0,5
1,0
2,25 1,40 2,20 1,35
Fig. 3.1. Selected cross sections of riffles and pools (0 - 15 m, 45 - 55 m, 85 - 100 m) within the 100 m sector, showing stream bed width [m] and depth [m]
48
49
This comparison of the selected cross sections (Fig. 3.1.) within the 100 m sector of
the Q.Negra show the distinct increase of stream width and water depth from the 'dry'
to the 'rainy' season and its heterogeneity with alternating riffles and pools. The
course of the stream in both seasons and all the single transects within the 100 m
section can be seen in Figure 3.2 and 3.3. The position of the field station and the
type of riparian vegetation are indicated.
A series of figures compares the width, depth, current velocity and flow data of the
Q.Negra from the 'dry' and 'rainy' season. Average stream width (Fig.3.4.) shows a
distinct difference between the two seasons. The variance within the 100 m section is
higher during the 'dry' season than in the 'rainy' season. Mean depth (Fig. 3.5.) is
increasing from 0,089 m on the 13.02.2004 to 0,141 m on the 06.08.2004. Average
current velocity was 0,207 ms-1 in February and 0,297 ms-1 in August. This is
accompanied by a distinct increase in mean flow (Fig. 3.7.). On the 06.08.2004 (18,5
ls-1) mean flow is five times higher as in the 'dry' season on the 13.02.2004 (94,2 ls-1).
secondary forest
botanical garden
0 m
50 m
100 m
0 m 30 m
Fig.3.2. Course of the Q.Negra during the 'dry' season within the 100 m section with bank level [m] and water depth [m]. Position of the sampling points, type of riparian vegetation and the Field station are indicated.
50
secondary forest
botanical garden
0 m
50 m
100 m
0 m 30 m
Fig 3.3. Course of the Q.Negra during the 'rainy' season within the 100 m section with bank level [m] and water depth [m]. Position of the sampling points, type of riparian vegetation and Field station are indicated
51
wid
th [m
]
0
1
2
3
4
5
dry season13.02.2004
rainy season06.08.2004
Fig. 3.4. Stream width [m] of the Q. Negra in the 'dry' and 'rainy' season
dept
h [m
]
0,0
0,1
0,2
0,3
0,4
0,5
0,6
dry season13.02.2004
rainy season06.08.2004
Fig. 3.5. Stream depth [m] of Q. Negra during the 'dry' and 'rainy' season
52
curre
nt v
eloc
ity [m
s-1]
0,0
0,2
0,4
0,6
0,8
1,0
1,2
dry season13.02.2004
rainy season06.08.2004
Fig. 3.6. Current velocity [ms-1] of the Q.Negra in the 'dry' and 'rainy' season
flow
[ls-1
]
0
20
40
60
80
100
120
140
160
180
dry season13.02.2004
rainy season06.08.2004
Fig. 3.7. Flow [ls-1] of the Q.Negra during the 'dry' and 'rainy' season
53
longitudinal transect [m]
0 10 20 30 40 50 60 70 80 90 100
dept
h [m
]
0,0
0,1
0,2
0,3
0,4
0,5
0,6 riffle pool riffle pool rifflepoolriffle
longitudinal transect [m]
0 10 20 30 40 50 60 70 80 90 100
dept
h [m
]
0,0
0,1
0,2
0,3
0,4
0,5
0,6
riffle pool riffle pool riffleriffle pool
Fig. 3.8. Mean depth [m] and pool - riffle - alternation of the Q.Negra during the 'dry' and 'rainy' season within the longitudinal 100 m section and standard deviation
54
The two graphs(Fig.3.8.) show the natural depth profile of the 100 m section. The
longitudinal transect is characterized by four riffle and three pool sequences.
At the beginning of the 'rainy' season the Q. Negra started to erode the riverbank and
one of the houses and four trees of the botanical garden were endangered. Due to a
bank regulation at meter 20 till 35 the morphological regime in the 'rainy' season
differs from the one during the 'dry' season (Fig. 3.9).
longitudinal transect [m]0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100
dept
h [m
]
0,0
0,1
0,2
0,3
0,4
0,5dry seasonrainy season
bank regulation
Fig. 3.9. Mean depth [m] of the Q.Negra during the 'dry' and 'rainy' season within the longitudinal 100 m section and standard deviation. Marked bank regulation site from the 'rainy' season.
55
longitudinal transect [m]
0 10 20 30 40 50 60 70 80 90 100
curr
ent v
eloc
ity [m
s-1]
0,0
0,2
0,4
0,6
0,8
1,0
1,2riffle pool riffle pool rifflepoolriffle
longitudinal transect [m]
0 10 20 30 40 50 60 70 80 90 100
curr
ent v
eloc
ity [
ms-1
]
0,0
0,2
0,4
0,6
0,8
1,0
1,2
riffle pool riffle pool riffleriffle pool
Fig. 3.10. Mean current velocity [ms-1] during the 'dry' and 'rainy' season within the 100 m sector and standard deviation
56
The two graphs (Fig. 3.13.) show the natural current velocity profile of the 100 m
section. The longitudinal transect is characterized by four riffle and three pool
sequences. Mean current velocities are higher in the 'rainy' season. – in pools speed
reaches 0,2 ms-1 and in riffles 0,6 ms-1. The highest recorded current velocity is 1,06
ms-1. During the 'dry' season current velocities in pools are about 0,1 ms-1 and 0,4
ms-1 in riffle sections. The highest recorded current velocity is 0,80 ms-1.
Due to a bank regulation at meter 20 till 35 the morphological regime in the 'rainy'
season differs from the one during the 'dry' season (Fig. 3.11.).
longitudinal transect [m]0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100
curre
nt v
eloc
ity [m
s-1]
0,0
0,2
0,4
0,6
0,8
1,0
dry seasonrainy season
bank regulation
Fig. 3.11. Mean current velocity [ms-1] of the Q.Negra in the 'dry' and 'rainy' season within the 100 m section and standard deviation. Marked bank regulation site from the 'rainy' season.
57
longitudinal transect [m]0 10 20 30 40 50 60 70 80 90 100
dept
h [m
]
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,0
0,1
0,2
0,3
0,4
0,5
0,6
mean current velocity DS mean depth DS
curre
nt v
eloc
ity [m
s-1 ]
Fig. 3.12. Mean current velocity [ms-1] and mean depth [m] in the 'dry' season (DS) within the longitudinal 100 m section This graph shows now both – mean depth and mean current velocity during the 'dry'
season. It corresponds to the natural regime of a not regulated stream. Whereas
current velocities decrease at pool sequences, riffle sequences are characterized by
higher current velocities.
longitudinal transect [m]0 10 20 30 40 50 60 70 80 90 100
dept
h [m
]
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7average current velocity RSaverage depth RS
curre
nt v
eloc
ity [m
s-1]
Fig. 3.13. Mean current velocity [ms-1] and mean depth [m] in the 'rainy' season (RS) within the longitudinal 100 m section of the Q.Negra
58
This figure (3.13.) shows now both – mean water depth and the mean current velocity
during the 'rainy' season of the Q.Negra. Differences of mean depth and mean
current velocity between the two seasons is not only due to the natural dynamics but
also caused by a regulation of the riverbank, which was carried out between the 20
and 35 m cross transect.
Sites where high velocities were recorded, show low average depths. Contrary low
velocities occured at deep sections. A correlation of mean depth versus mean current
velocity from the 'dry' and 'rainy' season is shown in figure 3.14.
mean current velocity [ms-1]
0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35
mea
n de
pth
[m]
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
rainy seasonregression line RSdry seasonregression line DS
Fig. 3.14. The mean depth [m] plotted against the mean current velocity [ms-1] in the 'dry' and 'rainy' season. The lines are the regression for each sampling date.
59
longitudinal transect [m]0 10 20 30 40 50 60 70 80 90 100
flow
[ls-1
]
0
20
40
60
80
100
120
140
160
180
dry seasonrainy season
Fig. 3.15. Flow [ls-1] during the 'dry' (DS) and 'rainy' season (RS) within the longitudinal 100 m section of the Q.Negra Flow during the 'dry' season is quite steady with small deviations within the 100 m
longitudinal transect. Flow ranges from about 30 to 40 ls-1. Not much water seems to
disappear into the hyporheic zone. During the 'rainy' season flow gets a way larger
and ranges from about 90 to 100 ls-1. One large fluctuation to 150 ls-1 at meter 5
might be due to a measurement mistake.
60
date1.3. 1.4. 1.5. 1.6. 1.7. 1.8. 1.9. 1.10. 1.11. 1.12. 1.1. 1.2. 1.3. 1.4. 1.5. 1.6. 1.7.
wat
er le
vel [
m]
0,00
0,05
0,10
0,15
0,20
0,25
prec
ipita
tion
[m]
0,00
0,05
0,10
0,15
0,20
0,25
water levelprecipitation
2004 2005
Fig. 3.16. Water level of the Q.Negra and precipitation at the tropical field station La Gamba from 23rd February 2004 to 1st July 2005. Water level on 23.02.2004 was defined as a baseline of 0 to provide a measure of
range and fluctuation in water levels over the observation period. Water level cero
shows the baseflow of 0,031m3s-1. Within the sampling time it is the lowest observed
water level. The highest water level which lasted for several hours was 0,19 m. Water
levels rise and drop quickly. Sudden changes in water level were the rule throughout
the months we observed the water level of Q.Negra in 2004. High water levels
occured even in the 'dry' season after a rain, but the highest levels were noted in
months of heavy and continuous rain when the soil was soaked and runoff heavy.
Frequent afternoon and early evening rains with more than 20 mm precipitation
caused about an average raise of 1,25 cm in water level, which returned to near
normal during the night. The average decrease of the water level on days with no
precipitation is about 0,42 cm. Distinct fluctuations in water level between the two
seasons can be seen in figure 3.16.
61
time 1:00 2:00 3:00 4:00 5:00
wat
er le
vel [
cm]
0
10
20
30
40
50
60
70
80
water level
prec
ipita
tion
[mm
]
0
5
10
15
20
precipitation
Fig. 3.17. Short term fluctuation and detailed recording of the waterlevel and precipitation from 13:00 till 17:00 on the14.08.2004 at the Q.Negra The highest recorded water level from the Q. Negra was 65 cm during a heavy rain
on the 14th of August, but this lasted only for a few minutes. Two peaks of the rising
water level are shown in Figure 3.17. Rain started at 1.02 pm and the water level
started to rise slowly. The first water level peak of 50 cm at 2.30 pm was followed by
a drop to 21 cm 15 minutes later. Heavy rainfall started again at 3.15 pm and the
water level followed to the highest peak of 65 cm at 3.50 pm. Turbidity generally
increased during flooding but cleared quite quickly when rain stopped.
62
water level [m]0,00 0,05 0,10 0,15 0,20
time
[d]
0
50
100
150
200
250
300
350
Fig 3.18. Water level [m] of the Q.Negra at a certain amount of days [d]
This graph shows on how many days per year (366 days) a certain water level was
reached. Data is available from 23rd February 2004 till 22nd February 2005. A water
level of 0,01 m at the defined water gauge was reached 121 times, wheras a water
level of more than 0,10 m was recorded only 36 times. The highest assessed water
level was 0,19 m.
63
water level [m]0,00 0,02 0,04 0,06 0,08 0,10 0,12
flow
[ls-1
]
0
20
40
60
80
100
120
140
rainy season 06.08.2004
dry season 13.02.2004
y = 20,816e18,477x
R2 = 0,98
mean flow rainy season
mean flow dry season
Fig. 3.19. Measured flow [ls-1] of five different water levels [m]. Accounted mean flow [ls-1] of the Q. Negra during the 'dry' and 'rainy' season (according to mean water level of three month) and mean flow [ls-1] of 06.08 and 13.02.04, when the 100 m sector was constructed.
Flow for five different water levels was calculated based on current velocity and depth
profiles at the 55 m cross transect within the 100 m sector. At water levels above
0,10 m wading was impossible and it was not able to measure the current velocity.
Flow was calculated for five different water levels, where it was possible to measure
the current velocity. At a water level above 0,10 m we were not able to measure the
velocity anymore. To calculate the mean flow within the two seasons we took the
average water level of three months of each season. Streamflow was highly
seasonal. Average water level in the 'dry' season (23.02 - 29.02.2004 / 01.12.04 -
23.02.2005) is 0,41 cm and 2,72 cm in the 'rainy' (01.08 - 30.10.2004) season. This
includes average flows of 22,5 and 58,9 m3s-1. The relative magnitude of high flow
episodes generallly corresponded to measured precipitation.
64
mean depth [m]0,04 0,06 0,08 0,10 0,12 0,14 0,16 0,18 0,20 0,22
mea
n cu
rrent
vel
ocity
[ms-1
]
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
BADEAB
Fig. 3.20. Classified choriotop types (A - riffles, B - shallow sites with low current velocity, D - pools, E - cascades) according to mean depth [m] and mean current velocity [ms-1] within the Q.Negra The Q. Negra displays a more or less regular alternation between shallow areas of
higher velocity and mixed gravel-cobble substrate (riffles) and deeper areas of slower
velocity and finer substrate (pools). The four classified choriotop types within the Q.
Negra are based on their different morphological and hydrological properties. At the
most general level of resolution, we divide channel units into fast- and slow-water
categories that approximately correspond to the commonly used terms 'riffle' (A) and
'pool' (D). Within the slow-water categories, we also identified shallow habitats (B),
where waterdepth was not more than 5 cm. At the comparable study site to the 100
m section, we identified another choriotop type (E). Cascades are marked by a steep
incline and rocks with a diameter of 20-50 cm. Water depth is in general shallow and
mean current velocity is 0,18 ms-1. In the 'rainy' season choriotop B was gone and
therefore a fifth choriotop type (AB) was classified. AB could not either be assigned to
A or D. AB is not as deep as pools (D), not as shallow as B and has a lower current
velocity than riffles (A).
65
66
Tab. 3.1. Hydrochemical characteristics of the Q.Negra at ten sampling times between February 2004 and February 2005
26.0
2.04
03.0
4.04
27.0
4.04
24.0
6.04
03.0
8.04
20.0
9.04
12.0
2.05
13.0
2.05
15.0
2.05
*
15.0
2.05
**
Lf [µScm-1; 25°C] 200 175 185 197 196 177 205 204 152 168 pH 8,1 8,4 8,4 7,9 8,5 8,5 8,4 8,2 8,1 8,2 Alk. [mgl-1] 128,1 112,3 117,2 129,4 129,4 114,7 125,7 125,7 92,7 103,7 Cl- [mgl-1] 0,4 0,5 0,4 0,5 0,6 0,4 1,4 1,2 1 0,6 SO4
2- [mgl-1] 1,3 1,4 1,4 1,1 1,1 0,9 1,3 1,3 1,2 1,1
Si-SiO4 [mgl-1] 26,2 23,8 25 29,7 30,5 13,3 26,8 26,7 22,3 22 P-PO4 [µgl-1] 48 26 45 40 46 47 39 31 31 36 P-s [µgl-1] 49 28 45 42 46 47 39 31 32 38 P-t [µgl-1] 50 32 46 46 49 53 50 42 45 56 N-NO3 [µgl-1] 64 95 66 37 69 143 36 110 86 82 N-NO2 [µgl-1] 0 0 1 0 1 1 0,3 3,1 1 1 N-NH4 [µgl-1] 0 16 9 0 4 48 27 4 4 4 N-sKj [µgl-1] 28 49 18 16 54 77 44 27 69 64 N-tKj [µgl-1] 39 70 39 33 67 88 83 86 109 138 Na+ [mgl-1] 6,9 6,1 6,4 7,9 7,3 6,3 7,5 7,3 5,7 5,7 K+ [mgl-1] 0,8 0,8 0,8 0,4 0,7 0,6 0,8 0,9 0,8 0,8 Ca2+ [mgl-1] 24 20,4 20,8 26,9 26,5 22,6 26,2 26 19 22,5 Mg2+ [mgl-1] 5,6 4,8 4,9 6,4 6,4 5,4 6 6 4,4 4,5 Na+ [mval] 0,3 0,3 0,3 0,3 0,3 0,3 0,3 0,3 0,2 0,2 K+ [mval] 0 0 0 0 0 0 0 0 0 0 Ca2+ [mval] 1,2 1,0 1,0 1,3 1,1 1,1 1,3 1,3 0,9 1,1 Mg2+ [mval] 0,5 0,4 0,4 0,5 0,4 0,4 0,5 0,5 0,4 0,4 A [mval] 2,1 1,8 1,9 2,1 2,1 1,9 2,1 2,1 1,5 1,7 Cl- [mval] 0 0 0 0 0 0 0 0 0 0 SO4
2- [mval] 0 0 0 0 0 0 0 0 0 0
san [mval] 2,0 1,7 1,7 2,2 2,2 1,9 2,1 2,1 1,6 1,8 skat [mval] 2,1 1,9 2,0 2,2 2,2 1,9 2,1 2,1 1,6 1,7 an% 108,3 110,5 112,7 97,0 99,1 102,6 99,1 99,4 99,4 98,9 Ion balance [diffkat%] -8,3 -10,5 -12,7 3,0 0,9 -2,6 0,9 0,6 0,6 1,1 (* 5 pm, ** 7.30 pm)
A summery description of the chemical characteristics of the Q.Negra is provided in
Table 3.1. Conductivity ranges between 175 and 200 µScm-1 and was relatively
constant seasonally such as the pH-value and the chlorid concentration. The pH
ranges from 7,9 to 8 and the chlorid concentration shows values between 0,4 mgl-1
and 1,4 mgl-1.
Sulfide concentrations showed a distinct seasonal pattern. Concentrations are higher
during the 'dry' than in the 'rainy' season; in February and April values range from 1,1
to 1,4 mgl-1 whereas values during the 'rainy' season range from 0,9 to 1,1 mgl-1.
67
The stream has its lowest phosporus (26 µgl-1), soluble phosphorus (28 µgl-1) and
total phosphorus (32 µgl-1) concentration on the 3rd April 2004 and shows no distinct
seasonal differences such as the concentration of S-SiO4, where values are ranging
from 13,3 mgl-1 in September to 30,5 mgl-1 in August.
No clear trend between the 'dry' and 'rainy' season is also perceptible according to
the nitrate, nitrit and ammonium concentrations. Nitrit concentrations are low. Values
of 0 µgl-1 were measured three times (20.02./03.04./24.06.2004) and concentrations
of 1 µgl-1 were measured five times within the sampling period. The highest recorded
nitrit concentration of 3,1 µgl-1 was on the 13th February 2005. No ammonium was
measured on the 20th February and 24th June 2004. The highest ammonium
concentration was 48 µgl-1 in September 2005.
Concentrations of N-sKj, N-tKj show no seasonal pattern. On the 24th June, the
stream has its lowest (16 µgl-1) concentration of N-sKj. The highest value of 77 µgl-1
occurs in September. On the 24th June, Quebrada Negra has the lowest (33 µgl-1)
concentration of N-tKj. The highest value of 138 µgl-1 occurs on the 15th February
2005 after rain.
Sodium concentrations range between 6,1 mgl-1 in April 2004 and 7,9 mgl-1 in June
2004. Seasonal differences are slight.
Potassium and calcium concentrations showed a distinct seasonal pattern. The
potassium content is in general higher during the 'dry' season than in the 'rainy'
season. Samples in the 'dry' season have a value of 0,8 mgl-1. The lowest content of
potassium (0,4 mgl-1) was measured in June 2004. During the 'rainy' season the
content of calcium is higher than in the 'dry' season. The lowest content of calcium
(20,4 mgl-1) was measured in April 2004.
Magnesium concentrations range between 4,4 mgl-1 in February 2005 and 6,4 mgl-1
in June and August 2004. Seasonal differences are not perceptible.
34,67FAC 1
geochemical parameters
-2 -1 0 1 2
phos
phor
usFA
C 2
25,1
5
-3
-2
-1
0
1
2
dry seasonrainy season
low high
low
high
Fig. 3.21. Ordination biplot resulting from a PCA (principal component analysis) of the chemical analyses of ten water samples during the 'dry' and 'rainy' season in the Q.Negra
The results of the PCA (principal component analysis) show the distribution pattern of
the sampling dates in relation to the first two axis. The PCA describes 59,82 % of the
total physicochemical variability during the 'dry' and 'rainy' season of the Q.Negra.
“Axis 1” accounts for 34,67 % of the variance. “Axis 2” explains 25,15 %. In Figure
3.21. the sampling dates are lined up along axis 1. Full black signs stand for the 'dry'
season and the white signs for the 'rainy' season. The principal components are the
geochemical parameters (Na+, K+, Ca2+, Mg2+) – axis 1 and the phosphorus
concentration – axis 2. Except of two sampling dates within the 'dry' season, it seems
that watersamples from the 'rainy' season have higher concentration of geochemical
parameters.
68
69
Tab. 3.2. Riparian vegetation at the Q. Negra with type of growth, frequency (++ often, + present, - absent), height [m] and distance to the riverbank [m] of recorded plants.
type of growth frequency height [m] distance to the riverbank [m]
taxon 45-55 m /85-95 m / Lodge MONOCOTYLEDONS Araceae Dieffenbachia oerstedii herb - / - / + - / - / - - / - / - Homalomena wendlandii herb + / - / + - / - / - - / - / - Philodendron sp. herb + / - / - - / - / - - / - / - Spathiphyllum silvicola herb + / - / - - / - / - - / - / - Arecaceae Bactris glandulosa palm - / - / + - / - / - - / - / - Prestoea decurrens palm - / - / + - / - / - - / - / - Welfia regia palm - / - / + - / - / 15 - / - / - Costaceae Costus pulverulentus herb + / + / - 0,6 / - / - - / - / - Cyclanthaceae Asplundia pittieri herb - / - / + - / - / - - / - / - Carludovica drudei giant herb + / + / + - / - / - - / - / - Cyclanthus bipartitus giant herb - / - / + - / - / - - / - / - Heliconiaceae Heliconia danielsiana herb - / - / + - / - / - - / - / - Heliconia imbricata herb - / - / + - / - / - - / - / - Heliconia latispatha herb ++ / + / - 2 / - / - - / - / - Heliconia trichocarpa herb - / - / + - / - / - - / - / - Heliconia sp. herb ++ / + / - - / - / - - / - / - Marantaceae Calathea lutea giant herb - / + / - - / - / - - / - / - Calathea crotalifera giant herb - / - / ++ - / - / - - / - / - DICOTYLEDONS Annonaceae Guatteria chiriquiensis tree + / + / + 8 / 25 / - 6 / 3 / - Anacardiaceae Spondias mombin tree - / + / - - / 27 / - - / - / - Boraginaceae Cordia cymosa tree - / - / + - / - / 28 - / - / 14 Cecropiaceae Cecropia obtusifolia tree - / - / ++ - / - / 22 - / - / - Cecropia insignis tree - / - / + - / - / 10 - / - / - Convolvulaceae Dicranostyles sp. liana + / - / - - / - / - - / - / - Dilleniaceae Doliocarpus hispidus liana + /- / - - / - / - - / - / - Elaeocarpaceae Sloanea sp. tree - / - / + - / - / 6 - / - / - Euphorbiaceae Acalypha diversifolia shrub/tree ++ / + / + 6 / - / 6 0 / 0 / 0 Hyeronima alchorneoides tree + / - / - 20 / - / - - / - / - Fabaceae-Faboideae Lonchocarpus sp. shrub/tree - / + / + - / 22 / 12 - / - / - Mucuna sp. liana + / - / - - / - / - - / - / - Fabaceae-Caesalpinioideae Bauhinia bahiachalensis liana + / - / - - / - / - - / - / - Flacourtiaceae
70
Tetrathylacium macrophyllum tree - / - / + - / - / 15 - / - / - Gesneriaceae Episcia lilacina herb + / - / - - / - / - - / - / - Lecythidaceae Grias cauliflora tree - / - / + - / - / - - / - / - Lythraceae Lagerstroemia c.f. tree + / - / - 13 / - / - 5 / - / - Melastomataceae Clidemia dentata shrub + / - / - - / - / - - / - / -
Blakea gracilis epiphytic
shrub/tree - / - / + - / - / - - / - / - Meliaceae Carapa guianensis tree/treelet - / - / + - / - / 25 - / - / - Moraceae Artocarpus altilis tree - / + / - - / 25 / - - / 0,5 / - Castilla tunu tree - / + / + - / - / 18 - / - / - Ficus tonduzii tree - / - / + - / - / - - / - / - Myristicaceae Virola koschnyi tree + / + / - 3 / 10 / - - / 1 / - Virola guatemalensis tree - / - / + - / - / 14 - / - / - Piperaceae Peperomia saintpauliella herb - / - / + - / - / - - / - / - Piper auritum shrub/tree ++ / + / - 2 / - / - 0 / - / - Rubiaceae Pentagonia wendlandii shrub/treelet - / - / + - / - / - - / - / - Pentagonia tinajita treelet - / - / + - / - / - - / - / - Psychotria elata shrub + / - / - 4 / - / - - / - / - Psychotria solitudinum shrub + / - / - 3 / - / - - / - / - Simira maxonii tree + / - / - - / - / - - / - / - Sapindaceae Cupania livida shrub/tree - / + / - - / 17 / - - / - / - Sterculiaceae Theobroma cacao tree ++ / + / - 1,5 / 7 / - - / - / - Tiliaceae Apeiba tibourbou tree - / - / + - / - / - - / - / - Luehea seemannii canopy tree ++ / + / + 25 / 23 / - 1,5 / 1 / - Trichospermum grewiifolium canopy tree ++ / - / - 20 / - / - - / - / - Urticaceae Myriocarpa longipes shrub/tree + / - / ++ 3 / - / - 0 / - / - Urera elata c.f. shrub/tree + / - / - - / - / - - / - / - Violaceae Rinorea dasyadena c.f. tree - / - / + - / - / - - / - / - PTEROPHYTES Oleandraceae Nephrolepis sp. fern + / + / - - / - / - 4 / 0 / - Cyatheaceae Alsophila firma treefern - / - / + - / - / 5 - / - / - LYCOPHYTES Selaginellaceae Selaginella sp. ground layer + / + / + - / - / - 2 / 2 / 1
71
The abundant riparian vegetation at the Quebrada Negra maintains mostly typical
species which occur along the streams and rivers in this region of Costa Rica. The
riverine vegetation was recorded three times along the streamrun. Twice within the
100 m sector (at the 45 - 55 m and 85 – 95 m transect) and once at the Lodge site.
Trees of the canopy layer include Guatteria chiriquiensis (Annonaceae), Spondias
mombin (Anacardiaceae), Cordia cymosa (Boraginaceae), Cecropia obtusifolia
(Cecropiaceae), Sloanea medusula (Elaeocarpaceae), Hyeronima alchorneoides
(Euphorbiaceae), Lonchocarpus sp. (Fabaceae – Faboideae), Carapa guianensis
(Meliaceae), Artocarpus altilis, Castilla tunu (Moraceae), Cupania livida
(Sapindaceae), Apeiba tibourbou, Luehea seemannii, Trichospermum grewiifolium
(Tiliaceae).
Common species of the mid tree layer are Welfia regia (Arecaceae), Cecropia
insignis (Cecropiaceae), Tetrathylacium macrophyllum (Flacourtiaceae),
Lagerstroemia c.f. (Lythraceae), Virola koschnyi and Virola guatemalensis
(Myristicaceae) and Theobroma cacao (Sterculiaceae).
The shrub stratum contains species like Dieffenbachia oerstedii, Homalomena
wendlandii, Philodendron sp., Spathiphyllum silvicola (Araceae), Costus
pulverulentus (Costaceae), Carludovica drudei, Cyclanthus bipartitus
(Cyclanthaceae), Heliconiaceae, Calathea lutea (Marantaceae), Piper auritum
(Piperaceae), Pentagonia wendlandii, Psychotria elata (Rubiaceae), Myriocarpa
longipes, Urera elata (Urticaceae) and Nephrolepis sp. (Oleandraceae).
The ground layer is bare except for Episcia lilacina (Gesneriaceae) and Selaginella
sp.
Fig. 3.22. Riparian vegetation of the Q.Negra between 45 m and 55 m of the 100 m section
72
Fig. 3.23. Riparian vegetation of the Q.Negra between 85 m and 95 m of the 100 m section
73
Fig. 3.24. Riparian vegetation of the Q.Negra at the comparable study site next to the Esquinas Lodge
74
75
Tab. 3.3. Number of leaf accumulations (within the stream) of three sampling dates (twice in the 'dry' season and once in the 'rainy' season) within each 5 m transect of the 100 m section at the Q.Negra amount of leaf accumulations transect [m] 05.03.2004 29.03.2004 08.08.20040-5 m 1 1 3 5-10 m 3 3 3 10-15 m 4 2 2 15-20 m 5 1 3 20-25 m 2 2 0 25-30 m 2 1 2 30-35 m 6 2 2 35-40 m 8 1 1 40-45 m 3 1 3 45-50 m 2 2 2 50-55 m 1 0 1 55-60 m 1 2 0 60-65 m 1 0 1 65-70 m 1 1 2 70-75 m 1 0 0 75-80 m 0 0 1 80-85 m 0 1 1 85-90 m 1 0 0 90-95 m 4 2 2 95-100 m 2 1 0 sum 48 23 29
The highest number of leaf accumulations (48) within the 100 m sector of the Q.
Negra were found during the 'dry' season on the 05.03.3004. The debris record of the
29.03.2004 after a flood event shows a distinct decrease of leaf accumulations, only
23 debris dams were present. During the 'rainy' season on the 08.08.2004 only 29
debris dams were found.
longitudinal transect [m]
0-15
5-20
10-2
5
15-3
0
20-3
5
25-4
0
30-4
5
35-5
0
40-5
5
45-6
0
50-6
5
55-7
0
60-7
5
65-8
0
70-8
5
75-9
0
80-9
5
85-1
00
debr
is [g
m-2
]
0
50
100
150
200
250
debris DS 05.03.2004debris DS 29.03.2004debris RS 08.08.2004
dept
h [m
]
0,04
0,06
0,08
0,10
0,12
0,14
0,16
0,18
0,20
0,22
depth DSdepth RS
Fig. 3.25. Amount of debris [gm-2] of three pooled transects within the 100 m sector – Q.Negra The mean amount of debris and mean water depth of the Q.Negra within three
pooled transects is shown in Figure 3.25. Debris is higher in the 'dry' season than in
the 'rainy' season. Values of more than 100 g per squaremeter were often recorded
on the 5th of March. The highest value in the 'rainy' season was 17,8 g. Moreover the
graph shows a distinct decrease in debris on the 29th of march, the day after flooding.
In comparison to the 5th of march at most of the sites debris is lower. At riffle sections
more leaf litter was found than in pool sections.
76
dry
wei
ght [
gm-2
]
0
50
100
150
200
250
dry season 05.03.2004
dry season29.03.2004
rainy season08.08.2004
Fig. 3.26. Mean amount [gm-2] of collected leaf litter on three sampling dates (05.03. / 29.03. / 08.08.04) within the 100 m transect during 'dry' and 'rainy' season The graph shows the huge difference between the two seasons. More leaf litter was
found during the 'dry' season in the Q. Negra. Reasons are the higher leaf fall during
the 'dry' season and during the 'rainy' season most of the leaf litter gets transported
downstream due to the floodpulses and the higher flow. After a flood event on the
29th of March leaf litter was transported downstream, but still more debris than in the
'rainy' season was found. In August just 4,61 gm-2 were recorded.
77
depth [m]0,04 0,06 0,08 0,10 0,12 0,14 0,16
dry
wei
ght [
gm-2
]
0
50
100
150
200
250
Fig. 3.27. The leaf litter dry weight [gm-2] plotted against the stream depth [m] in the 'dry' season (05.03.2004). The line is the regression.
The mean amount of debris within three pooled transects plotted against the stream
depth is shown in Figure 3.27. Within the 100 m sector of the Q.Negra the dry weight
of the debris during the 'dry' season on the 05.03.2004 correlates with the stream
depth. Clear differences between riffles and pools according to their amount of debris
dams are visible. At riffle sections more leaf litter was found than in pool sections,
because at shallow sites more leaves are accumulating in front of the stones of the
Q.Negra.
78
79
3.3. DISCUSSION The natural dynamics of the stream are expressed in the alternating riffle and pool
sections. The 100 m transect of the Q. Negra represents the expected trend of high
heterogeneity. Current velocity, stream depth, stream bed width and sediment size
fluctuate within the longitudinal 100 m section. Geomorphic changes between the
'dry' and 'rainy' season of the 100 m logitudinal transect are slight (Fig. 3.9.). Riffle –
pool – alternation of the 'dry' season corrsponds to the 'rainy' season, except
between the 20 - 35 m transect, which is caused by a regulation of the riverbank.
In the year 2004 'rainy' season started at the end of march. This was two to three
weeks earlier in comparison to the recorded precipitation data of former years at the
Field Station La Gamba. Streams in this seasonal ('dry' and 'rainy' season)
environment exhibit a high annual variation in discharge and turbidity. In the 'rainy'
season, frequent (almost daily) storms can cause bankfull conditions and in some
cases flooding. Water level fluctuations are following the precipitation. The 'dry' and
'rainy' season can be distinguished at first sight, but the difference in mean water
level is slight. The seasonal differences in baseflow discharge, however, show a
distinct increase from the 'dry' to the 'rainy' period. The mean flow in the 'rainy' period
is about 60 ls-1 and therefore 3 times as high as in the 'dry' season.
Like most Central American rivers (BUSSING 1993), which do not sustain flood
conditions for long periods between successive downpours, the short term
fluctuations of the Q. Negra are very typical and play a more important role than the
long term fluctuations. The water level rises quickly after the beginning of rain
because of the soil. Water discharges above ground and not much water disappears
in the hyporheique interstitial, which is shown by a comparison of flow values at
different cross transects (Fig. 3.15.). Flow during flood conditions could not be
measured but was estimated for 16000 ls-1 at a water level of 110 cm. Water levels
drop very quickly, which can be explained by the high resilience of the rainforest.
Precipitation and discharge have enormous effects on various morphological and
hydrological parameters of the stream. The results of the morphometric-hydrological
measurements at the Q. Negra show a clear picture of a natural, dynamic and
unregulated streamcourse. The four classified habitat types in the 'dry' season, which
differ in their current velocities and mean water depths also show the heterogeneous
80
run of the stream. In the 'rainy' season a fifth habitat type (choriotop AB) was
recorded.
The chemical data reported from our study stream consists of ten spot-
measurements (three in the 'rainy' and seven in the 'dry' season), indicating the
variable conditions during the seasons.
Tropical rivers in general have low concentrations of chemical constituents compared
with temperate rivers. This is because tropical rivers are for the most part
precipitation-dominated. Local geology can also be important. Geology plays a great
role together with land use in determining the nutrient concentrations of unpolluted
waters. The chemical data of the water samples from the Q. Negra indicate its source
in the Fila Gamba, Golfo Dulce area.
Magnesium and Calcium originate almost entirely from the weathering of rocks,
particularly magnesium-silicate minerals. Conductivity, magnesium, calcium and
sulfate concentration is lower and the phosphorus concentration is higher than in
other rivers from the region, which are sourced from the Fila Cruces. The Quebrada
Negra, located next to the Esquinas Rainforest Lodge, shows high values of total
phosphorus. Phosphorus values are typically elevated by sewage water.
The riparian vegetation of the Quebrada Negra is dense and consists of typical
species which occur along the streams and rivers in this region of Costa Rica. Higher
leaf fall during the 'dry' season is responsible for the higher leaf litter input in the
stream, and during the 'rainy' season most of the leaf litter gets transported
downstream due to the floodpulses and the higher flow.
4. Macroinvertebrates and leaf litter decomposition in the Quebrada Negra
The decomposition and macroinvertebrate colonisation of exposed leaf litter was
investigated using litter bags placed over a 28 day period in the Quebrada Negra.
Leaf decay rates and macroinvertebrate densities on leaf packs of Cecropia
obtusifolia (Cecropiaceae), Acalypha diversifolia (Euphorbiaceae), Tetrathylacium
macrophyllum (Flacourtiaceae) and Sloanea medusula (Elaeocarpaceae) were
compared. The abundance and taxonomic composition of invertebrates colonizing
the leaves were recorded. Anatomical cuts of the leaf material show the varying
nature of the leaf structure and support the results of the differences in decay (in
prep. GUSENLEITNER & RIEMERTH 2005).
The survey of the riverine vegetation (see chapter 3) provided a basis for the
selection of the four plant species. The trees were selected according to life history
strategies and abundance. All four chosen plant species are common and typical for
the riparian vegetation in this area of Costa Rica.
Tab. 4.1. Plant species of the riparian vegetation and their life history strategy used for the litter bag experiment taxon life history strategy Cecropia obtusifolia (Cecropiaceae) r Acalypha diversifolia (Euphorbiaceae) features of r and K Tetrathylacium macrophyllum (Flacourtiaceae) features of r and K Sloanea medusula (Elaeocarpaceae) K ACALYPHA DIVERSIFOLIA (Euphorbiaceae) – Plate 1a Acalypha diversifolia is a monoecious shrub or small tree up to 5(-15)m tall, which is
widely distributed in tropical America. It is a very common species of the Central
American lowlands in evergreen and deciduous forests from southern Mexico to
Peru, Bolivia and Brazil and one of the most abundant shrubs of the region. The plant
exhibits r and K features; light demanding, often present in the understory along
streamruns, anemophily.
A. diversifolia (Jacq). Leaves chartaceous, narrowly elliptic to ovate-elliptic or oblong-
lanceolate, 6-20 cm long, 2-8 cm wide, base obtuse to cuneate, margin dentate,
81
sparsely pubescent above, pubescent along the veins beneath; inflorescences
axillary, spicate, 2-10cm long, mostly bisexual, with a few female flowers at base;
male flowers minute, calyx cupular, petals absent, stamens (4-)8(-16); female flowers
minute, 2-3, subtended by a foliaceous, 3-4 mm long bract, petals absent, ovary (2-3)
locular; fruits small capsules with 3 bivalved cocci, ca. 2,5 mm in diameter.
CECROPIA OBTUSIFOLIA (Cecropiaceae) – Plate 1c Common name (Costa Rica): guarumo (BURGER 1977, HOLDRIDGE et al. 1997)
Cecropia obtusifolia is a shortliving pionieer species (r- strategist) with prominent stilt
roots. Trees are up to 23 (20) m tall and reach a DBH of 25,3 cm. They are common
species on wet sites in clearings and secondary forest, from southern Mexico to
Ecuador. Cecropia obtusifolia is a light demanding species, grows fast and often
appears at early successions as a monoculture.
Leaves are deeply lobed, lobes usually 11-13, stipules fully amplexicaul, 5-12 cm
long; staminate inflorescences pedunculate clusters of 12-18 spikes, these 8-22 cm
long, subsessile or up to 5 mm stipitate, pistillate inflorescences pedunculate clusters
of usually 4 spikes, these 18-50 cm long, sessile or shortly stipitate, accrescent in
fruit; fruits small achenes, 2 mm long, 1,2 mm wide, usually flattened.
Cecropia obtusifolia lives in close association with the heavily stinging Azteca ants.
These animals inhabit the hollow stems of the trees, which are subdivided by thin
transverse walls at each node (Zizka 1990). Due to the ants no epiphytic growth on
Cecropia is possible. Large trunks can be used as water gutter.
SLOANEA MEDUSULA (Elaeocarpaceae) – Plate 1b Sloanea medusula is a large tree with long-lived leaves of primary rainforests from
Guatemala to Colombia. Trees are up to 40 m tall; leaves alternate, coriaceous,
glabrate to puberulent, margin undulate to irregular; inflorescences 4-18 cm long;
flowers 10-20 mm in diameter, usually pinkish; fruits 4-valved capsules, to 4,5 cm
long, subglobose, seed 1. Sloanea medusula is a K- strategist; rare, tall, canopy
cover, hardwood, fruits are dispersed by birds.
82
TETRATHYLACIUM MACROPHYLLUM (Flacourtiaceae) – Plate 1d The genus Tetrathylacium belongs to the pan-tropical family Flacourtiaceae. Two
species are known: T. macrophyllum POEPP. & ENDL. (Synonyms: T. Costaricense
STANDL., T. Nutans Sleum., T. pacificum STANDL., Edmondstonia pacifica Seem.) and
T. Johanseni Standl., which both occur on the south pacific part of Costa Rica. In
contrast to T. macrophyllum, T. Johanseni has no myrmecophytic traits.
T. macrophyllum is a treelet growing mainly in the forest understorey. It is found
preferentially on steep slopes near rivers and creeks in primary forest (JANZEN 1983).
The average height is 8 m but it may reach a maximum height of 15-20 meters.
leaves oblong, entire to serrate with caducous stipules; inflorescences usually
axillary, pendent with numerous spike-like secondary axes; flowers reddish-purple or
maroon, spaced along spikes; fruits subglobose, redpurple, up to 2,5 cm in diameter.
The distribution area of T. macrophyllum is mostly in primary and secondary lowland
rain forest, from Costa Rica to Amazonian Peru and Brazil. The altitudinal distribution
ranges from 0 up to 1500 m. The plant exhibits r and K features; often present in the
understory along streamruns or at gaps, rapid growth, seeds are dispersed by birds.
83
4.1. MATERIAL AND METHODS Preparation of litter bags
Litter bags were made out of nylon mesh (10 × 10 mm) and cotton binding. Leaves
were collected from trees in the riparian vegetation at the Quebrada Negra. Only old
leaves, prior to abscission, were used. Small stones were added to each litter bag to
ensure that contents were in contact with the stream bed. 16 litter bags of each
species (8 g of leaves) were tied on strings and placed in the stream parallel to the
current. The experimental sites were located in riffles of moderate depth and velocity
(approximately 0,08 m deep and 0,2 ms-1).
Collection and processing of litter bags
The plants were exposed over a 28 day period in the stream. Two bags of each
species were collected after 4, 8, 14, 21 and 28 days by placing a net (200 µm) under
each so as not to lose colonizing invertebrates. After cutting the attachment string the
litter bags were transferred to a plastic tube and transported to the laboratory.
Invertebrates were washed from the leaves, sorted and preserved in 70 % ethanol.
Macroinvertebrates were later counted and determined to family level. Insects were
assigned to functional feeding groups following MERRITT and CUMMINS (1996).
Leaves were dried to constant weight at 70°C and weighed. The experiment ran from
28.02.2004 to 27.03.2004. The study came to a premature close owing to a high
water and loss of the remaining litter bags.
Anatomical leaf cuts were made of Acalypha diversifolia (Euphorbiaceae), Cecropia
obtusifolia (Cecropiaceae), Sloanea medusula (Elaeocarpaceae) and Tetrathylacium
macrophyllum (Flacourtiaceae) (GUSENLEITNER & RIEMERTH 2005).
Analysis of data
The loss of leaf mass over time is approximately log-linear, although some data have
been interpreted as linear or as consisting of two or more distinct stages. WEBSTER
and BENFIELD (1986) argue that a simple exponential model provides a general and
utilitarian description of the breakdown process. The exponential model Wt = Wo e –kt
(where Wt is the amount of leaf litter remaining after time, t, of the initial amount Wo,
and k is the processing coefficient) was used to calculate processing coefficients.
The exponent k (in units days-1), which is the slope of the plot of loge of leaf mass
84
versus time, provides a single measure of breakdown rate. This model assumes that
there is a constant fractional loss of material present at any given time.
The statistical analysis of data was performed with the software packages SPSS and
CANOCO. The influence of plant taxon and time of exposition within the stream on
the macroinvertebrate colonization was tested by a univariate analysis of variance. A
one way ANOVA also tested if there is a signifikant difference of the
macroinvertebrate colonization within the four plant species. Only species, who
appeared in more than three samples were used in the analysis.
The DCA (detrended correspondence analysis) is a procedure of analysis of
gradients. This was performed on the basis of the macroinvertebrate distribution in
context with the time of exposition of the leaf material. The results are represented as
a Biplot.
85
4.2. RESULTS
exposure time [d]0 5 10 15 20 25 30
dry
wei
ght [
g]
0
1
2
3
4
5
6 Acalypha diversifolia (Euphorbiaceae)Cecropia obtusifolia (Cecropiaceae)Tetrathylacium macrophyllum (Flacourtiaceae)Sloanea medusula (Elaeocarpaceae)
Fig. 4.1. Single values and mean loss of weight from four different types of leaf packs (Acalypha diversifolia, Cecropia obtusifolia, Tetrathylacium macrophyllum, Sloanea medusula) exposed in the Q. Negra for 28 days.
exposure time [d]0 5 10 15 20 25 30
dry
wei
ght [
%]
0
20
40
60
80
100
Acalypha diversifolia (Euphorbiaceae) Cecropia obtusifolia (Cecropiaceae) Tetrathylacium macrophyllum (Flacourtiaceae) Sloanea medusula (Elaeocarpaceae)
Fig. 4.2. Single values of percentage loss [%] and mean percentage [%] loss of weight from four different types of leaf packs (Acalypha diversifolia, Cecropia obtusifolia, Tetrathylacium macrophyllum, Sloanea medusula) exposed in the Q. Negra for 28 days.
86
The differences of the four plant species in decay and loss of weight within the
exposure time are shown in Figure 4.1. and Figure 4.2. Weight loss from leaf packs
was affected dramatically by litter type. Sloanea medusula (Elaeocarpaceae), the K-
strategist does not show the rapid loss of weight as the between r- and K-strategist,
Tetrathylacium macrophyllum (Flacourtiaceae). By day 28, Tetrathylacium
macrophyllum packs had lost 72,28 % of their initial weight, Acalypha diversifolia had
lost 66,93 %, Cecropia obtusifolia had lost 54,66 %, but leaves of Sloanea medusula
had lost only 20,13 % on day 24. The weight loss of Tetrathylacium macrophyllum
(e.g. 47,24 % loss after 14 days) was consistently faster than that of the Sloanea
medusula (e.g. 13,60 % loss after 17 days). Tetrathylacium was after 28 days of
exposure almost grazed down to the leaf nervation.
Tab. 4.2. Values of daily litter decomposition rate (k) from four different types of leaf packs (Acalypha diversifolia, Cecropia obtusifolia, Tetrathylacium macrophyllum, Sloanea medusula) exposed in the Q. Negra for 28 days, obtained from the slopes of regression equations, between log Wt and t taxon W0 Wt t k Acalypha diversifolia (Euphorbiaceae) 3,16 1,045 28 0,0395198Cecropia obtusifolia (Cecropiaceae) 3,295 1,71 28 0,0234255Tetrathylacium macrophyllum (Flacourtiaceae) 3,175 0,88 28 0,0458265Sloanea medusula (Elaeocarpaceae) 4,67 3,73 24 0,0093646 The relationship between the leaf litter remaining (Wt) of each plant species and their
values for daily litter decomposition rates (k) is presented in Table 4.2.
According to the “hierarchy of species along a processing continuum” of PETERSEN
and CUMMINS (1974) and the known decomposition rates, the four exposed plant
species can be either put into the “fast” or “slow” group. Acalypha diversifolia (0,04),
Cecropia obtusifolia (0,0234), and Tetrathylacium macrophyllum (0,0458) can be
assigned to the “fast” group. Sloanea medusula (0,0094) can be placed into the slow
group.
87
Anatomical cuts
Sloanea medusula (Elaeocarpaceae) – K-strategist
88
Tetrathylacium macrophyllum (Flacourtiaceae) - between r- and K-strategist
89
The histological analysis (GUSENLEITNER & RIEMERTH 2005) of the exposed plant
species show the varying nature of the leaf structure. Sloanea medusula
(Elaeocarpaceae) has a larger middle rib than Tetrathylacium macrophyllum
(Flacourtiaceae); the leafblade is almost the same.
The red coloured cells are the lignified cells and the blue coloured cells are cellulose
– the non-lignified cell walls. Sloanea medusula is higher lignified and has a higher
content of tannins than Tetrathylacium macrophyllum, which makes T. macrophyllum
more palatable for the macroinvertebrates. These anatomical cuts support the results
of the differences in decay of the two plant species.
Macroinvertebrate colonization
exposure time [d]0 5 10 15 20 25 30
mea
n nu
mbe
r of m
acro
inve
rtebr
ates
0
50
100
150
200
Acalypha diversifolia (Euphorbiaceae)Cecropia obtusifolia (Cecropiaceae)Tetrathylacium macrophyllum (Flacourtiaceae) Sloanea medusula (Elaeocarpaceae)
Fig. 4.3. Mean number of macroinvertebrates colonizing four different types of leaf packs (Acalypha diversifolia, Cecropia obtusifolia, Tetrathylacium macrophyllum, Sloanea medusula) within exposure time [d]
90
A clear trend to colonize the leaves from day one of the exposure within the stream is
evident. Macroinvertebrate colonisation first increases on all four plant species but
abundance at Acalypha diversifolia, Tetrathylacium macrophyllum and Cecropia
obtusifolia declined at the end of the study, when most of the plant material is broken
down and little of the leaves – just the lignified, not palatable parts - remained in the
bags. The number of species present in the litter bags increased steadily in the first
14 days of the study (Fig. 4.3) at all plant species except at Sloanea medusula. By
day 28 species richness has declined. The K-strategist Sloanea medusula does not
show the rapid increase of colonization like the 3 other plants.
A total of 3339 individuals of macroinvertebrate morphospecies were collected. The
invertebrate community was dominated by insect groups. 13 orders and about 39
families colonised the litter bags during the course of study (see Tab. 4.3.) The
assemblage was dominated by Chironomidae (41,39 %), mostly Orthocladiinae
(23,06 %) and Ephemeroptera (39,41 % of the total fauna), comprising Baetidae
(6,83 %), Caenidae(0,6 %), Leptohyphidae (27,73 %) and Leptophlebiidae (4,25 %).
Trichoptera (8,12 %), mostly Leptoceridae (2,73 %) and Hydropsychidae (2,40 %)
were also abundant, while Plecoptera (Perlidae) constituted only 0,03 % of total
colonizers. Diptera (44,65 %) were also quite abundant. Odonata, Coleoptera and
Turbelaria were the only other order that made up more than 1 % of the total
assemblage. Numerically the groups of Hydrachnellae, Mollusca and Hydra were of
little importance. Colonizer densities (total individuals and abundance of major taxa)
were highest on Cecropia obtusifolia and Tetrathylacium macrophyllum leaves and
lowest on Sloanea medusula.
91
Tab. 4.3. Total number and percent composition [%] of the litter bag macroinvertebrate fauna and FFG (Fi-filterer, Pr-predator, Co – collector-gatherer, Sh-shredder, Pi-piercer, Sc-scraper)
taxon total percent composition Functional
Feeding Group EPHEMEROPTERA 1316 39,41 Baetidae 228 6,83 Co Baetidae_sp1. 113 3,38 Co Baetidae_sp2. 113 3,38 Co Baetidae_sp3. 2 0,06 Co Caenidae 20 0,60 Co Caenidae sp. 20 0,60 Co Leptohyphidae 926 27,73 Co Leptohyphes_sp. 10 0,30 Co Leptohyphidae_sp1. 503 15,06 Co Leptohyphidae_sp2. 103 3,08 Co Leptohyphidae_sp3. 249 7,46 Co Leptohyphidae_sp4. 8 0,24 Co Leptohyphidae_sp5. 43 1,29 Co Leptohyphidae_sp6. 10 0,30 Co Leptophlebiidae 142 4,25 Co Leptophlebiidae_sp1. 129 3,86 Co Leptophlebiidae_sp2. 4 0,12 Co Leptophlebiidae_sp3. 2 0,06 Co Leptophlebiidae_sp4. 7 0,21 Co TRICHOPTERA 271 8,12 Hydropsychidae sp. 80 2,40 Co-Fi Hydroptilidae 12 0,36 Pi Oxyethira_sp. 3 0,09 Pi Hydroptila_sp. 9 0,27 Pi Leptoceridae 91 2,73 Sh Atanatolica_sp. 91 2,73 Sh Philopotamidae 65 1,95 Co-Fi Wormaldia_sp. 65 1,95 Co-Fi Calamoceratidae 18 0,54 Sh Phylloicus_sp. 18 0,54 Sh new-unknown 5 0,15 PLECOPTERA 1 0,03 Perlidae 1 0,03 Pr Anacroneuria_sp. 1 0,03 Pr DIPTERA 1491 44,65 Chironomidae 1382 41,39 Chironomini_sp. 77 2,31 Co Orthocladiinae_sp. 770 23,06 Co Tanypodinae_sp. 452 13,54 Pr Tanytarsini_sp. 83 2,49 Co-Fi Ceratopogonidae 18 0,54 Pr Dixidae 4 0,12 Co Athericidae 7 0,21 Pr
92
Simuliidae 50 1,50 Co-Fi Stratiomyidae 1 0,03 Co Thaumaleidae 21 0,63 Sc Culicidae 1 0,03 Co Tipulidae 7 0,21 Sh COLEOPTERA 39 1,17 Elmidae 21 0,63 Co Elmidae_sp. 21 0,63 Co Hydrophilidae 8 0,24 Pr Hydrophilidae_sp. 8 0,24 Pr Dytiscidae 4 0,12 Pr Dytiscidae_sp. 3 0,09 Pr Dytiscidae_sp2. 1 0,03 Pr Hydroscaphidae 2 0,06 Sc Hydroscaphidae_sp. 2 0,06 Sc Lampyridae 2 0,06 Lampyridae_sp. 2 0,06 Ptilodactylidae 1 0,03 Sh Anchytarsus_sp. 1 0,03 Sh Staphylinidae 1 0,03 Pr Staphylinidae_sp. 1 0,03 Pr HETEROPTERA 29 0,87 Naucoridae 25 0,75 Pr Limnocoris_sp. 25 0,75 Pr Hebridae 1 0,03 Pr Hebrus major 1 0,03 Pr Veliidae 3 0,09 Pr Microvelia_sp. 1 0,03 Pr Rhagovelia_sp. 2 0,06 Pr ODONATA 61 1,83 Anisoptera 8 0,24 Pr Zygoptera 53 1,59 Pr HYDRACHNELLAE 12 0,36 TURBELARIA 41 1,23 MOLLUSCA 4 0,12 Co-Fi HYDRA 1 0,03 REST 73 2,19 Total individuals 3339
93
Detrended correspondence analysis
DCA Axis 122,5
time axis
0 1 2 3
DC
A A
xis
29,
2
0
1
2
4
44
48
814
14
14
12124
28
284
4
4 4
8 88 814
14
14
17
17
21
22121
21
24
28
28
28
28
Fig. 4.4 Ordination biplot resulting from a DCA of the sampling scores (exposure times) of four types of leaf packs placed for a 28 day period in the Q. Negra. (blue = Acalypha diversifolia, red = Cecropia obtusifolia, black = Sloanea medusula, green = Tetrathylacium macrophyllum)
DCA Axis 122,5
time axis
-2 -1 0 1 2 3 4 5
DC
A A
xis
29,
2
-3
-2
-1
0
1
2
3
4
CoPr
Co
Co
Fi
FiFi
Fi
CoCo
Co
Co
Fi
CoPr
ShCo
CoPr
Co
Pr
Sc
ShCo
Co
Pr
CoPr
Sh
CoCo
Pr
PrCo Pi
Fig. 4.5. Ordination biplot resulting from a DCA of the fauna collected from four types of leaf packs placed in the Q. Negra. The macroinvertebrates were assigned to functional feeding groups. (Fi-filterer, Pr-predator, Co – collector-gatherer, Sh-shredder, Pi-piercer, Sc-scraper)
94
A detrended correspondence analysis (Fig. 4.4. and 4.5.) shows the patterns of the
sample scores and the macroinvertebrate colonization. Axis 1 can be interpreted as
the time axis. The macroinvertebrates were assigned to feeding groups after MERRITT
and CUMMINS (1996) and the seperation into functional feeding groups revealed
considerable differences between the colonisation dynamics of filterers and
shredders. Colonisation of the plants by collectors and filterers was rapid in the first 8
days but when the leaf material gets broken down to a certain grade, no more
substrate is left over for the filterers. On the other hand the leaf material is then
palatable for the shredders and the shredders appear. In contrast to the filterer and
shredder group, collector species existed throughout the study.
Tab. 4.4. One way ANOVA. The influence of exposition time [d] and leaf material (taxon) on the macroinvertebrate colonization. species day taxon taxon*dayEphemeroptera 0,0498 n.s n.s Trichoptera 0,0299 n.s n.s Plecoptera n.s n.s n.s Diptera n.s n.s n.s Chironomidae 0,0014 n.s n.s Coleoptera n.s n.s n.s Odonata 0,0239 0,0357 n.s Heteroptera n.s n.s n.s The statistical analysis (one way ANOVA - analysis of variance) shows no significant
influence of the plant taxon on the abundance of macroinvertebrate colonisation (p >
0,05). The major macroinvertebrate colonizers like Ephemeroptera, Trichoptera,
Chironomidae and Odonata showed a significant difference in colonisation within
time (p < 0,05). All other species of colonizers showed no significant differences in
colonising the plants within time. Most of the variation in colonizer densities and
species composition was explained by the time of exponation. The effect of time had
more influence on the abundant species on the leaves than the leaf type itself.
95
4.3. DISCUSSION The results reported herin indicate that leaves of plants with different life history
strategies are broken down with varying rates of processing in the stream.
Invertebrate consumers contribute to the decay of organic matter in temperate
streams (KAUSHIK & HYNES 1971, PETERSEN & CUMMINS 1974, WALLACE & WEBSTER
1996). Studies have shown that abundance of shredding invertebrates and decay
rates of leaves are positively related (WALLACE, WEBSTER & CUFFNEY 1982, BENFIELD
& WEBSTER 1985, MALMQVIST 1993).
Species-specific differences in leaf breakdown rates are well documented (PETERSEN
& CUMMINS 1974, PADGETT 1976). PETERSEN and CUMMINS (1974) found a “hierarchy
of species along a processing continuum” and seperated leaf species into three
categories, according to the rate of the breakdown process in the stream. Species
with a processing coefficient, k, of greater than 0,01 day-1 were placed in the “fast”
group. According to this category Sloanea medusula has a slow breakdown rate,
wheras Tetrathylacium macrophyllum leaves lost weight considerably faster. This
matched our initial assumptions about their relative palatibility to macroinvertebrates,
and this was reflected in higher colonizer densities on Tetrathylacium macrophyllum.
The high rates of leaf litter decomposition for Acalypha diversifolia, Cecropia
obtusifolia and Tetrathylacium macrophyllum, in comparison to Sloanea medusula,
may be attributed to the differences in leaf structure, i.e. thickness and presence or
absence of cuticularized cell walls as well as the differences in their chemical
composition, for instance, tannin content, of the four species. Sloanea medusula
leaves are thicker than the leaves of the three other plants. These factors may
explain the relatively slow weight loss of Sloanea medusula observed in this study.
The relationship between leaf lifetime and the type of defense is documented in a
study (COLEY 1985, 1988) for Panamanian trees, which supports the hypothesis that
immobile defenses are more common in longer lived leaves. There are significant
positive relationships between leaf lifetime and all the fiber and condensed tannin
measures.
Colonizer densities on leaf packs increased with time, a trend seen in most studies of
litter breakdown in streams (DUDGEON 1982, WEBSTER & BENFIELD 1986, BENSTEAD
1996). Indeed, time and leaf type explained most of the variation in
macroinvertebrate densities on leaf packs in this study (DUDGEON & WU 1999).
96
A marked feature of the litter bag community was its dominance by insects. The
number of Ephemeroptera, Diptera (mainly Chironomidae), Trichoptera, Coleoptera
and Odonata species was particularly high. By contrast, the Plecoptera were almost
not represented. Faunal composition of the riverbed (chapter 5) was quite similar to
the one reported in this study, both for taxonomical and functional composition.
COVICH (1988) states that the taxonomic composition of neotropical stream
communities is characterised by high endemicity of certain groups coupled with a
paucity of species in others.
A DCA indicated differences in the composition of the macroinvertebrate feeding
group assemblage within time of the leaf exposition. Two of the major groups
(filterers and shredders) showed very different colonization patterns. The response
from the filterer community was initially rapid, but shredders did not appear before
the leaf material was exposed for a certain time. Filterers may need the leaf material
as a substrate and shredders rely on the leaf material, which is more palatable after a
certain time of exposure, as a food resource. In contrast to the filterer and shredder
group, collector species existed throughout the study. The balance of the
macroinvertebrate colonization e.g. immigration and emigration of filterers and
shredders, was therefore strongly dependent on the mass of leaf litter remaining.
The colonisation of the collectors seems to be a more passive process than that of
filterers or shredders and independent of the amount of leaf litter remaining in the
bags. Predatory invertebrates showed also a rapid colonisation of the litter bags. By
the end of the study period the number of predators had declined, possibly as a
result of the decreasing amounts of leaf litter present in the bags. This decrease may
indicate the importance of leaf litter as a microhabitat for predators in the studies
stream.
Potential detrivores present in the stream, such as fish and crab species, are not
recorded in the litter bags because of the mesh size. These groups were assumed to
have partial access to the contents of the litter bags, and their contribution to litter
processing in the stream studied is not known but may have been significant
(WOOTON & OEMKE 1992).
If stream macroinvertebrates are using leaf litter as a source of food, we would
expect shredders to comprise of a significant proportion of leaf-pack colonizers.
However, this functional feeding group constituted only 3,5 % of animals from the
collected colonizer assemblage during the present study. Elsewhere in a Costa Rican
97
stream (BENSTEAD 1996) two species of trichopteran shredders comprised of over 30
% of litter bag colonists (and 98 % of all shredders). The lack of shredders is also
reported in studies about Costa Rican lowland streams (PRINGLE & RAMIREZ 1998).
PRINGLE et al. (1993) suggests due to the lack of insect shredders the hypothesis that
in neotropical streams the decomposition of plant material in fine particulate organic
matter is operated either by macroconsumers, such as crustaceans and fish, or by
enhanced microbial activity. The litter processing could have been a result of the
combined feeding activities of caddis flies, mayflies and chironomids plus microbial
action and the physical fragmentation of leaves.
DUDGEON (1982) concluded that the rapid processing rate in subtropical Hong Kong
streams was largely due to high ambient temperature, and it seems likely that high
temperature, in comparison to temperate streams, was an important contributing
factor in this study. High water temperature would give rise to rapid microbial
conditioning of leaf litter, with a potential, subsequent increase in the rate of
consumption by the shredder community.
Despite the paucity of shredders among leaf colonists, the findings of this
investigation imply strongly that the responses of stream macroinvertebrates to leaf
litter occur because of its role as a source of food.
In any stream where allochthonous inputs consist of leaves of various types, some
will be more palatable than others. The palatable litter will serve mainly as a food
source and support high densities of macroinvertebrates, while lower densities of
animals will be assosiated with less palatable litter which is used mainly as a
substrate. In tropical streams, where many leaf types of varying palatibility and
diverse defensive compounds are present (including a greater proportion of species
with high levels of condensed tannins: STOUT 1989), the patch-specific response of
faunal densities to changes in the total amounts of this mixture can be expected to be
rather weak, and macroinvertebrate abundance is unlikely to correlate closely with
litter biomass (DUDGEON 1999).
98
5. Macroinvertebrate distribution and trophic relations in a neotropical lowland stream, Q.Negra, Costa Rica
The Q. Negra is characterized by a wide diversity of habitats with different current
velocity, depth and substrate. The morphometric-hydrologic conditions, the
hydrochemical characteristics and the riparian vegetation have been analysed.
Detailed observations on physical-chemical characteristics of the Q. Negra are found
in chapter 3. Within this framework the macroinvertebrate community has been
described. The main objective was to examine the composition and trophic structure
of the benthic community. This investigation represents the first survey of the benthic
invertebrate community from four different habitat types within the Q. Negra. Four
basic choriotops (habitat types) have been distinguished in the stream: riffles, shallow
sites with low current velocity, pools and cascades.
99
5.1. MATERIAL AND METHODS The sampling was conducted in March and April 2004 under conditions of low
discharge. Different sampling methods (Surber sampler, kick sampler) were used
depending on the physical characteristics of the sites.
The benthic invertebrate composition of the stream bottom in every choriotop was
assessed by using a standard Surber sampler (mesh size: 200 µm) and a qualitative
kick sampler (250 µm). Two random Surber samples and one kick sample were
taken from each site (choriotop). Samples were collected by disturbing the substrate
manually to a depth of about five cm for approximately five minutes.
Riffle environments (Choriotop A) were selected in relatively shallow turbulent areas
(0,1 m deep) where the water flow was high (usually 0,47 ms-1). Choreotop B were
sites with shallow areas and low water flow. Pools (Choriotop D), 0,24 m deep and
with low flow (0,15 ms-1) were sampled. Cascades (Choriotop E) were only
conducted with the kick sampling net. Quantitative sampling was not possible
because of the large size of boulders which dominated its substrate.
Invertebrate samples were sorted under a binocular and placed in 70 % ethanol. In
the laboratory, macroinvertebrates from each sample were hand sorted, identified to
taxonomic units and counted. Identification of the taxa was performed according to
MERRITT & CUMMINS (1996) and aquatic insects were identified in most cases on the
level of families. Invertebrates were assigned to functional feeding groups (FFG),
according to MERRITT & CUMMINS (1996): collector-gatherers (Co), filterers (F),
predators (P), scrapers (Sc) piercers (Pi) and shredders (Sh).
Analysis of data The statistical analysis of data was performed with the software package SPSS.
A one way ANOVA (analysis of variance) tested if there is a signifikant difference of
the macroinvertebrate colonisation within the four habitat types.
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5.2. RESULTS Habitat types Tab. 5.1. Four different choriotop types within the Q.Negra, distinguished by the mean values of physical characteristics such as velocity and depth
choriotop type mean velocity [ms-1] mean depth [m] A riffles 0,47 0,10 B shallow / slow velocity 0,21 0,05 D pools 0,06 0,24 E cascades 0,15 0,10
Riffle environments (Choriotop A) are characterized by relatively shallow turbulent
areas (0,1 m deep) and a high current velocity (0,47 ms-1). Choriotop B were sites
with shallow areas and low water flow. Pools (Choriotop D) have a mean depth of
0,24 m and a mean current velocity of 0,15 ms-1. Cascades (Choriotop E) are
characterized by a high mean current diversity, low mean depth and large size of
boulders which dominated its substrate. A general comparison of the community
composition and density of the benthic community between the four choriotop types
can be made according to their velocity and depth differences.
Fig. 5.1. Pool and riffle site within the 100 m sector of the Q.Negra
101
Distribution of the macroinvertebrate fauna in the Q.Negra Tab. 5.2. Mean number, standard deviation and percent composition [%] of the macroinvertebrate fauna within four different choriotop types (A-riffles, B-shallow sites, low velocity, D-pools, E-cascades) and FFG (Fi-filterer, Pr-predator, Co – collector-gatherer, Sh-shredder, Pi-piercer, Sc-scraper) A B D E total individuals = 218 total individuals = 26 total individuals = 120 total individuals = 447 n = 3 n = 3 n = 5 n = 2
taxon
mea
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D
perc
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on [%
]
mea
n nu
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r ± S
D
perc
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]
mea
n nu
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r ± S
D
perc
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on [%
]
mea
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Func
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G
roup
EPHEMEROPTERA 27,00 ± 7,21 36,67 1,00 ± 1,00 11,49 7,60 ± 7,13 31,93 71,50 ± 6,36 31,36 Baetidae 2,00 ± 2,00 2,72 0,33 ± 0,58 3,83 0,20 ± 0,45 0,84 12,50 ± 0,71 5,48 Co Baetidae_sp1. 2,00 ± 2,00 2,72 0,00 0,00 0,20 ± 0,45 0,84 0,00 0,00 Co Baetidae_sp2. 0,00 0,00 0,33 ± 0,58 3,83 0,00 0,00 0,50 ± 0,71 0,22 Co Baetidae_sp3. 0,00 0,00 0,00 0,00 0,00 0,00 12,00 ± 1,41 5,26 Co Caenidae 0,00 0,00 0,00 0,00 2,6 ± 2,70 10,92 0,00 0,00 Co Caenidae_sp. 0,00 0,00 0,00 0,00 2,6 ± 2,70 10,92 0,00 0,00 Co Leptohyphidae 18,33 ± 3,79 24,90 0,67 ± 1,15 7,66 4,00 ± 4,95 16,81 43,00 ± 7,07 18,86 Co Leptohyphes_sp. 2,67 ± 1,53 3,62 0,00 0,00 0,00 0,00 7,50 ± 4,95 3,29 Co Lepthyphidae_sp1. 15,67 ± 2,52 21,28 0,33 ± 0,58 3,83 0,20 ± 0,45 0,84 24,50 ± 2,12 10,75 Co Leptohyphidae_sp2. 0,00 0,00 0,00 0,00 1,00 ± 1,73 4,20 0,00 0,00 Co Leptohyphidae_sp4. 0,00 0,00 0,00 0,00 0,20 ± 0,45 0,84 0,00 0,00 Co Leptohyphidae_sp5. 0,00 0,00 0,33 ± 0,58 3,83 2,60 ± 4,77 10,92 11,00 ± 0,00 4,82 Co Leptophlebiidae 6,67 ± 3,79 9,05 0,00 0,00 0,80 ± 0,84 3,36 16,00 ± 0,00 7,02 Co Leptophlebiidae_sp1. 5,67 ± 4,04 7,70 0,00 0,00 0,40 ± 0,55 1,68 11,00 ± 1,41 4,82 Co Leptophlebiidae_sp2. 0,00 0,00 0,00 0,00 0,20 ± 0,45 0,84 0,00 0,00 Co Leptophlebiidae_sp3. 0,00 0,00 0,00 0,00 0,20 ± 0,45 0,84 0,00 0,00 Co Leptophlebiidae_sp4. 0,00 0,00 0,00 0,00 0,20 ± 0,45 0,84 0,00 0,00 Co Leptophlebiidae_sp5. 0,67 ± 0,58 0,91 0,00 0,00 0,60 ± 0,89 2,52 4,50 ± 0,71 1,97 Co Leptophlebiidae_sp6. 0,33 ± 0,58 0,45 0,00 0,00 0,20 ± 0,45 0,84 0,50 ± 0,71 0,22 Co TRICHOPTERA 6,33 ± 1,53 8,60 1,00 ± 1,00 11,49 0,20 ± 0,45 0,84 41,5 ± 13,44 18,20 Glossosomatidae_sp 0,33 ± 0,58 0,45 0,00 0,00 0,00 0,00 0,00 0,00 Sc Hydropsychidae_sp 2,00 ± 1,00 2,72 1,00 ± 1,00 11,49 0,00 0,00 18,50 ± 2,12 8,11 Co-Fi Hydroptilidae 0,33 ± 0,58 0,45 0,00 0,00 0,00 0,00 2,00 ± 0,00 0,88 Pi Hydroptila_sp. 0,33 ± 0,58 0,45 0,00 0,00 0,00 0,00 2,00 ± 0,00 0,88 Pi Philopotamidae 3,33 ± 1,53 4,53 0,00 0,00 0,00 0,00 14,00 ± 9,90 6,14 Co-Fi Wormaldia_sp. 3,33 ± 1,53 4,53 0,00 0,00 0,00 0,00 14,00 ± 9,90 6,14 Co-Fi Polycentropodidae_sp 0,00 0,00 0,00 0,00 0,20 ± 0,45 0,84 0,00 0,00 Co-Fi Rhyacophilidae_sp 0,33 ± 0,58 0,45 0,00 0,00 0,00 0,00 0,00 0,00 Pr new 0,00 0,00 0,00 0,00 0,00 0,00 7,00 ± 5,66 3,07 PLECOPTERA 0,67 ± 1,15 0,91 0,00 0,00 0,00 0,00 1,00 ± 1,41 0,44 Perlidae 0,67 ± 1,15 0,91 0,00 0,00 0,00 0,00 1,00 ± 1,41 0,44 Pr Anacroneuria_sp. 0,67 ± 1,15 0,91 0,00 0,00 0,00 0,00 1,00 ± 1,41 0,44 Pr DIPTERA 9,30 ± 4,51 12,63 2,70 ± 3,79 31,03 8,60 ± 0,89 36,13 30,50 ± 0,71 13,38 Chironomidae 5,33 ± 1,15 7,24 1,00 ± 1,73 11,49 8,20 ± 6,98 34,45 22,50 ± 2,12 9,87 Chironomini_sp. 0,33 ± 0,58 0,45 0,00 0,00 1,20 ± 0,84 5,04 0,00 0,00 Co Orthocladiinae_sp. 3,00 ± 1,00 4,07 0,33 ± 0,58 3,83 0,20 ± 0,45 0,84 4,50 ± 3,54 1,97 Co Tanypodinae_sp. 2,00 ± 0,00 2,72 0,00 0,00 5,40 ± 8,38 22,69 15,00 ± 2,83 6,58 Pr Tanytarsini_sp. 0,00 0,00 0,67 ± 1,15 7,66 1,40 ± 2,07 5,88 3,00 ± 2,83 1,32 Co-Fi Ceratopogonidae 0,00 0,00 0,00 0,00 0,40 ± 0,89 1,68 1,00 ± 1,41 0,44 Pr Dixidae 0,00 0,00 0,00 0,00 0,00 0,00 0,50 ± 0,71 0,22 Co Empididae 0,00 0,00 0,00 0,00 0,00 0,00 0,50 ± 0,71 0,22 Pr Athericidae 0,00 0,00 0,00 0,00 0,00 0,00 1,50 ± 0,71 0,66 Pr Simuliidae 1,00 ± 0,00 1,36 0,00 0,00 0,00 0,00 2,00 ± 1,41 0,88 Co-Fi Thaumaleidae 0,00 0,00 0,00 0,00 0,00 0,00 0,50 ± 0,71 0,22 Sc Tipulidae 3,00 ± 3,61 4,07 1,67 ± 2,08 19,16 0,00 0,00 2,00 ± 1,41 0,88 Sh
102
MEGALOPTERA 0,33 ± 0,58 0,45 0,00 0,00 0,00 0,00 2,00 ± 2,83 0,88 Corydalidae 0,33 ± 0,58 0,45 0,00 0,00 0,00 0,00 2,00 ± 2,83 0,88 Pr COLEOPTERA 18,00 ± 16,52 24,45 1,00 ± 0,00 11,49 2,80 ± 3,11 11,76 58,50 ± 0,71 25,66 Elmidae 11,00 ± 9,54 14,94 0,67 ± 0,58 7,66 1,80 ± 1,64 7,56 27,50 ± 6,36 12,06 Co Elmidae_sp. 11,00 ± 9,54 14,94 0,67 ± 0,58 7,66 1,80 ± 1,64 7,56 27,50 ± 6,36 12,06 Co Hydrophilidae 0,00 0,00 0,33 ± 0,58 3,83 0,00 0,00 0,00 0,00 Pr Hydrophilidae_sp. 0,00 0,00 0,33 ± 0,58 3,83 0,00 0,00 0,00 0,00 Pr Psephenidae 7,00 ± 7,00 9,51 0,00 0,00 1,00 ± 1,73 4,20 10,00 ± 1,41 4,39 Sc Psephenops_sp. 7,00 ± 7,00 9,51 0,00 0,00 1,00 ± 1,73 4,20 10,00 ± 1,41 4,39 Sc Ptilodactylidae 0,00 0,00 0,00 0,00 0,00 0,00 19,50 ± 2,12 8,55 Sh Anchytarsus_sp. 0,00 0,00 0,00 0,00 0,00 0,00 19,50 ± 2,13 8,55 Sh Staphylinidae 0,00 0,00 0,00 0,00 0,00 0,00 1,00 ± 1,41 0,44 Pr Staphylinidae_sp. 0,00 0,00 0,00 0,00 0,00 0,00 1,00 ± 1,41 0,44 Pr Dryopidae 0,00 0,00 0,00 0,00 0,00 0,00 0,50 ± 0,71 0,22 Sh Dryopidae_sp. 0,00 0,00 0,00 0,00 0,00 0,00 0,50 ± 0,71 0,22 Sh HETEROPTERA 0,33 ± 0,58 0,45 1,67 ± 1,15 19,16 0,40 ± 0,89 1,68 0,50 ± 0,71 0,22 Naucoridae 0,33 ± 0,58 0,45 1,33 ± 1,53 15,33 0,40 ± 0,89 1,68 0,00 0,00 Pr Limnocoris_sp. 0,33 ± 0,58 0,45 1,33 ± 1,53 15,33 0,40 ± 0,89 1,68 0,00 0,00 Pr Veliidae 0,00 0,00 0,33 ± 0,58 3,83 0,00 0,00 0,50 ± 0,71 0,22 Pr Microvelia_sp. 0,00 0,00 0,33 ± 0,58 3,83 0,00 0,00 0,50 ± 0,71 0,22 Pr ODONATA 3,67 ± 1,53 4,98 0,33 ± 0,58 3,83 0,80 ± 1,30 3,36 7,00 ± 2,83 3,07 Anisoptera 0,33 ± 0,58 0,45 0,33 ± 0,58 3,83 0,40 ± 0,55 1,68 3,50 ± 4,95 1,54 Pr Zygoptera 3,33 ± 2,08 4,53 0,00 0,00 0,40 ± 0,89 1,68 3,50 ± 2,12 1,54 Pr LEPIDOPTERA 1,33 ± 1,15 1,81 0,00 0,00 0,00 0,00 0,50 ± 0,71 0,22 Sh ARACHNIDAE 0,00 0,00 0,00 0,00 0,20 ± 0,45 0,84 0,00 0,00 HYDRACHNELLAE 3,33 ± 4,04 4,53 1,00 ± 1,00 11,49 1,20 ± 1,64 5,04 10,50 ± 0,71 4,61 TURBELARIA 1,33 ± 1,53 1,81 0,00 0,00 0,20 ± 0,45 0,84 0,00 0,00 CRUSTACEAE 0,00 0,00 0,00 0,00 0,00 0,00 1,00 ± 0,00 0,44 REST 2,00 ± 2,00 2,72 0,00 0,00 1,80 ± 3,03 7,56 3,50 ± 0,71 1,54
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riffle
percent composition [%]0 5 10 15 20 25 30
Leptohyphidae
Leptophlebiidae
Baetidae
Caenidae
rheo
philie
pool
percent composition [%]0 5 10 15 20 25 30
Fig. 5.2. Percent composition [%] of Ephemeroptera at riffles and pools, Q.Negra. Ecological arrangement according to their preference for lotic habitats
riffle
percent composition [%]0 1 2 3 4 5
Philopotamidae
Hydropsychidae
Glossosomatidae
Hydroptilidae
Rhyacophilidae
Polycentropod
rheo
philie
pool
percent composition [%]0 1 2 3 4 5
Fig. 5.3. Percent composition [%] of Trichoptera at riffles and pools, Q.Negra. Ecological arrangement according to their preference for lotic habitats
riffle
percent composition [%]0 2 4 6 8 10 12 14 16
Elmidae
Psephenidae
pool
percent composition [%]0 2 4 6 8 10 12 14 16
Fig. 5.4. Percent composition [%] of Coleoptera at riffles and pools, Q.Negra. Ecological arrangement according to their preference for lotic habitats
104
riffle
percent composition [%]0 5 10 15 20 25
Orthocladiinae
Tanypodinae
Chironomini
Tanytarsini
pool
percent composition [%]0 5 10 15 20 25
Fig. 5.5. Percent composition [%] of Chironomidae at riffles and pools, Q.Negra. Ecological arrangement according to their preference for lotic habitats
50 taxa were identified (Tab. 5.2.). Identification keys of stream insects and
invertebrates in Central America on species level are not available. In this study,
aquatic insects represent the dominant component of stream benthos.
At riffle sites (A) Ephemeroptera was the most represented insect order (36,7 %),
and within this group the most abundant family was that of Leptohyphidae (24,9 %).
Coleoptera and Diptera were the following major groups of invertebrates collected in
riffles. Members of Elmidae were the dominant Coleoptera. Chironomidae was the
main family, followed by Tipulidae and Simuliidae of Diptera. Trichoptera was
represented by two caseless filterer families: Hydropsychidae and Philopotamidae.
Odonata was represented by Zygoptera and Anisoptera.
At shallow sites with low current velocity (B) Diptera (31 %) was the most
represented insect order and within this group Tipulidae (19,16 %) were the most
abundant family followed by Chironomidae (11,49 %). Heteroptera (19,16 %),
Ephemeroptera (11,49 %) and Trichoptera (11,49 %) were the following major
groups of invertebrates.
In pools (D) Diptera (36,1 %) was the most represented insect order and within this
group the most abundant family was that of Chironomidae (34,5 %). Ephemeroptera
(31,9 %) was the following major group of invertebrates. Leptohyphidae (16,8 %) was
the main family, followed by Caenidae (10,9 %). Caenidae are not found at the other
sampling sites.
At cascade sampling sites (E) Ephemeroptera, Coleoptera and Trichoptera
collectively contributed about two-thirds of the total number of the taxa found.
Ephemeroptera was the most represented insect order in the samples, and within
this group the most abundant family was that of the Leptohyphidae (18,9 %) followed
105
by Leptophlebiidae (7,0 %). Coleoptera was the second major group of invertebrates
collected in the stream, members of Elmidae and Ptilodactylidae wer dominant.
Ptilodactylidae were not found at the other sampling stations.
Some species like Plecoptera (Anacroneuria sp.) are only found within riffle and
cascade sites. To point out the different macroinvertebrate distribution within the
stream, the composition of the streamfauna at riffle and pool sites is shown in Figure
5.2., 5.3., 5.4. and 5.5. The percent composition of Ephemeroptera, Trichoptera,
Coleoptera and Chironomidae within riffle and pools can be seen. Note the different
scale of the x-axis.
Tab. 5.3. One-way ANOVA of invertebrate abundance within four different choriotop types (A-riffles, B-shallow sites, low velocity, D-pools, E-cascades) Statistical comparisons (ANOVA, Tamhane-T2) between choriotops in number of invertebrates.
Taxon Signifikanz Tamhane Ephemeroptera <0,01 AE; DE Baetidae <0,01 AE; BE Caenidae n.s. - Leptohyphidae <0,01 AB; AD Leptophlebiidae <0,01 BE; DE Trichoptera <0,01 - Glossosomatidae n.s. - Hydropsychidae <0,01 - Hydroptilidae <0,01 BD; BE Philopotamidae <0,01 BD Polycentropodidae n.s. - Rhyacophilidae n.s. - new <0,01 AB; AD; BD Plecoptera n.s. - Diptera <0,05 - Chironomidae <0,05 BE Megaloptera n.s. - Coleoptera <0,01 BE; DE Lepidoptera n.s. - Heteroptera n.s. - Odonata n.s. -
The statistical comparison (one way ANOVA, Tamhane-T2) between
macroinvertebrate abundance and different habitat types of the Q. Negra show a
clear picture of different population densities. There is a significant difference of the
abundance of certain taxa of the invertebrate community within the four different
habitat types of the Q.Negra. The major macroinvertebrates like Ephemeroptera,
106
Trichoptera, Trichoptera, Diptera (Chironomidae) and Coleoptera showed a
significant difference in colonisation within different habitat types (p < 0,05). All other
major taxa such as Megaloptera, Plecoptera, Odonata and Heteroptera of colonizers
showed no significant differences within the choriotop types.
Trophic relationships
The analysis of the trophic structure of stream communities is a tool to understand
the relationships between the macroinvertebrate community and the different organic
matter inputs into the river (CUMMINS 1973).
We examined the trophic structure at the sampling sites using the system of
functional feeding groups (FFG) of CUMMINS (1973) to place each of the species in its
trophic class. The Quebrada Negra invertebrate fauna is in terms of species numbers
mostly made up of collector-gatherers (53,3 %), followed by predators (16,5 %),
filterers (14,5 %), shredders (8,5 %), grazer-scrapers (6,5 %) and piercer (0,7 %).
The most representative family of the collector-gatherers groups are Ephemeroptera
and some Chironomids that are collectors. Trichoptera such as Hydropsychidae
contribute to the filterers. Shredders were poorly represented in the samples with
clear differences between the stations, individuals of Ptilodactylidae and Tipulidae
contribute to this group. Shredders like Leptoceridae and Calamoceratidae were
found within the study “Macroinvertebrates and leaf litter decomposition in the
Q.Negra” – see chapter 4.
A diagram of the major trophic pathways in the Q. Negra is given in Fig. 5.6. and 5.7.
The block diagram is representing the trophic interactions of the typical community of
the Q. Negra and is based on the food habitats of each species. Many species utilize
food from two or more trophic levels. Fish species would almost certainly occupy
several of the functional feeding groups and would probably exert their greatest
impact through predation.
The abundance of macroinvertebrates provides abundant food for many other
predators, including a diverse fish assemblage. Food resources of invertebrate
consumers include periphyton and other surface layer complexes, macrophytes,
detritus and other animals.
The consumption of autumn-shed leaves in woodland streams by various
invertebrates is the most extensively investigated trophic pathway involving CPOM. It
107
is largely allochthonous material (leaf litter and detritus) of the forest, which is
brought into the stream and determines its nutrient budget. Invertebrates that feed on
decaying leaves (shredders) in our study stream include crane fly larvae (Tipulidae)
and several families of trichopterans (Leptoceridae, Calamoceratidae) and
Coleoptera (Ptilodactylidae). Caddisflies in the superfamiliy Hydropsychoidea (which
includes the Philopotamidae, Polycentropodidae and Hydropsychidae) spin silken
nets in a variety of designs. Most are passive filter feeders, constructing nets in
exposed locations. Larvae of black flies (Simuliidae) are highly specialized
suspension feeders. Rainforest streams are shadowed to a great extent and have
therefore a small primary production. Due to the low periphyton production in the
Q.Negra, scrapers and grazers constitute only 6,5 % of the macroinvertebrate
community. Predation, used here to refer to the consumption of animal prey, is a
widespread and potentially important process affecting the biota of running waters.
The classification of the invertebrate consumers of streams into feeding guilds has
demonstrated great utility for description and analysis. Characteristics of a particular
stream or river, including its size, hydrology and the vegetation of the surrounding
landscape significantly influence which pathways predominate.
The trophic level above (the herbivores) controls events in the trophic level below
(the periphyton). In contrast there is evidence that food availability determines
herbivore abundance and distribution. Whether bottom-up or top-down control
prevails, may depend on time, place and environmental circumstances, and they may
not be mutually exclusive.
108
5.3. DISCUSSION
The main objective of the work was to determine the composition and trophic
structure of the macroinvertebrate communties in the Q. Negra. The results show the
effects of current velocity, depth and substrate on the benthic macroinvertebrate
community.
Faunal composition of the riverbed was similar to the one reported in studies about
Costa Rican lowland streams (PRINGLE & RAMIREZ 1998), both for taxonomical and
functional composition. Stream invertebrate assemblages varied within the course of
the Q. Negra. Both invertebrate density and taxonomical richness increased with the
increasing current velocity. Substratum influences invertebrate abundance and
taxonomical richness, with boulders and cobbles richer than sandy habitats.
We found that the number and taxa diversity of stream benthos greatly varied among
different sites within the Q.Negra. In general all the dominant species are gatherer-
collectors or filter-feederer with their associated predators.
As previous stated, current velocity was an important factor in shaping benthic
communities, both in structural and functional composition: higher velocities were
associated with a richer and more abundant invertebrtae assemblage. It is likely that
current is related to water oxygenation and also plays a key role in the functional
feeding of some groups, such a filterers. Cascades also have a turbulent water
regime, although velocity is not as high as at riffle sites. Moreover, it is well
established that micro-flow dynamics play a key role in the small scale distribution of
benthic communities (STATZNER and HOLM 1982).
Macroinvertebrate shredders were nearly absent in the samples while collector-
gatherer and filterer (both feeding on fine particulate matter) were dominant. Not
even taxa of shredders (Calamoceratidae, Leptoceridae) reported in chapter 4
(Macroinvertebrtaes and leaf litter decomposition) were abundant. PRINGLE et al.
(1993) suggests due to the lack of insect shredders the hypothesis that in neotropical
streams the decomposition of plant material in fine particulate organic matter is
operated either by macroconsumers, such as crustaceans and fish, or by enhanced
microbial activity. In this study crustaceans were not recorded and are not shown in
the trophic pathway diagrams.
109
Fig. 5.6. Major trophic pathways of the lotic food web, Q.Negra, Costa Rica
110
Fig.5.7. Major trophic pathways of the lotic food web, Q.Negra, Costa Rica and percent composition [%] of major invertebrates
111
6. Research needs The single survey of the rivers within the Piedras Blancas National Park is a good
initial effort but provides only a coarsely resolved basis for integration of research
results. Further studies of the rivers and streams in the Rainforest of the Austrians
are clearly “warranted”. It remains to be seen therefore, whether the findings reported
herin will apply to other streams, or even to the same streams in a year where
variation in the magnitude of rains alters the duration or intensity of spates.
Further objects of research could be testing ecotone concepts or the river continuum
concept (RCC) in tropical lotic systems according to their global applicability.
Is the RCC applicable to tropical streams? This and other questions can only be
answered by future research.
The seasonal climate ('dry' and 'rainy' season) provides ideal conditions for testing
the food web theory and niche partitioning, in neotropical lotic systems.
Understanding the ecological energetics will require detailed research. We expect
that there are differences in the habitat structure between the 'dry' and 'rainy' season.
This requires a more detailed description of the stream. Further interesting aspects
are biogeochemical aspects of rainforest-river systems, nutrient retention and nutrient
spiralling. Clearly, more information on the requirements and interactions of stream
organism is necessary to understand whether top down effects play an important role
in tropical streams.
Within the Piedras Blancas National Park the freshwater invertebrate fauna is largely
unsurveyed and undescribed. This protected area in the south of Costa Rica offers
an unique opportunity to study the tropical lotic invertebrate community. Identifying
the relative role of seasonal changes in physico-chemical factors and availability and
nature of food resources in explaining temporal pattern in functional organisation of
macroinvertebrate communities might be a particularly interesting line of
investigation.
Understanding community structure and function and their determinants is one of the
main objectives of ecology. In Costa Rica, except for some studies on aquatic insects
(PRINGLE et.al. 1998) benthic macroinvertebrate fauna is poorly known. Results from
such studies can be used to predict taxa distribution at local small-scale and identify
factors that can influence micro-distribution patterns in the lotic system of this area. It
is clear that many questions remain to be resolved regarding the way in which
112
physical, chemical and seasonal factors control benthic macroinvertebrate
community organization. In particular, there is scope for further studies of the
seasonal changes that occur in lotic communities and how they relate to seasonal
cycles in environmental variables.
In this regard, we consider the present study an initial step.
113
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Papua New Guinea Perspectives in Tropical Limnology 239-254
116
c b
a
d
a Acalypha diversifolia (Euphorbiacae)b Sloanea medusula (Elaeocarpacea)c Cecropia obtusifolia (Cecropiaceae)d Tetrathylacium macrophyllum (Flacourtiacae) Plate 1
EPHEMEROPTERA
Baetidae Caenidae
Leptophlebiidae Leptohyphidae
PLECOPTERA
Plate 2Perlidae
TRICHOPTERA
Calamoceratidae Hydropsychidae
Hydroptilidae Leptoceridae
Philopotamidae Plate 3
DIPTERA
Ceratopogonidae Dixidae
SimuliidaeRhagionidae
Stratiomyidae Tipulidae
Plate 4
Culicidae Thaumaleidae
Chironomidae
Plate 5
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