Springer 2006
Hydrobiologia (2006) 553:303–317 DOI 10.1007/s10750-005-0694-3
Primary Research Paper
Benthic, drifting and marginal macroinvertebrate assemblages in a lowland river: temporal and spatial variations and size structure Romina Elizabeth Principe* & Marı´ a del Carmen Corigliano Departamento de Ciencias Naturales, Universidad Nacional de Rı´o Cuarto, A.P. No. 3, X5804 BYA, Rı´o Cuarto, Argentina (*Author for correspondence: Tel.: 54-0358-4676167; Fax: 54-0358-4676230 E-mail:
[email protected]) Received 8 April 2004; in revised form 10 May 2005; accepted 14 June 2005
Key words: benthos, drift, lowland river, macroinvertebrates, marginal fauna, size structure
Abstract Aquatic macroinvertebrates living in anastomosing lowland rivers use different habitats and respond differently to the hydrological regime. In this paper, the structure and composition of benthic, drifting and marginal macroinvertebrate assemblages are analyzed in the lowland river Ctalamochita (Co´rdoba, Argentina). The assemblages were studied in an annual cycle; a comparison among the composition of benthos, drift and marginal fauna was carried out; and size structure of the assemblages was characterized. Samples were obtained from two sites: a rural and an urban site. In total 73 taxa of aquatic macroinvertebrates were collected. Benthos was characterized by Chironomidae and Oligochaeta; marginal fauna was mainly constituted by Coleoptera, Heteroptera, Decapoda, the Trichoptera Nectopsyche sp., Ephemeroptera and Odonata. Drifting assemblage was composed by macroinvertebrates from local and remote upstream benthos, and from the marginal zone. Marginal fauna diversity was higher than benthos and drift. Total biomass of the assemblages pooled together was relatively equitably among size classes. Larger size classes consisted of organisms from the marginal zone whereas the smallest ones were composed by benthic and drifting organisms. In the study area there is habitat partitioning in the lateral dimension of the river. Marginal fauna was more diverse due to the asymmetry of transport and deposit processes, which generate a heterogeneous habitat in the bankside. The relation between fine substrate and high current velocity determines an unstable habitat in the central channel, which makes colonization by benthic macroinvertebrates difficult.
Introduction In anastomosing lowland rivers, the asymmetry of transport and deposit processes generates a great variety of habitats, which are used differently by aquatic macroinvertebrates. Habitats associated with bankside may greatly contribute to faunal diversity (Corigliano, 1989; Ward, 1989; Malmqvist, 2002) by providing refugia during periods of high flow (Coregino et al., 1995; Rempel et al., 1999; Robinson et al 2002). The combined effects of bank morphology, seasonal growth of macro-
phytes and discharge result in a highly diverse and dynamic environment offering a wide range of ecological niches for the faunal community (Armitage et al., 2001). Drift is an important aspect in the study of macroinvertebrate communities (Allan, 1995; Ramı´ rez & Pringle 1998) because it is related to secondary production in water bodies, it is an important source of food for fish, and also an effective way for some aquatic organisms to colonize new areas (Waters, 1972; Williams & Hynes, 1976; Sagar, 1983; Allan, 1995). In anastomosing
304 lowland rivers, drift is composed by organisms from the main channel and from the marginal zone (Cellot, 1989; Corigliano et al., 1998). Organism body size constrains many ecological processes that influence community organization. Body size often influences energy flow and trophic structure, so abundance and biomass of differentially-sized animals should reflect size-specific allocation of total community resources (Peters, 1983), and this allocation may vary among habitats characterized by different abiotic and biotic constraints. In aquatic ecology, size distribution research has focused on size spectrum, which is the concentration of individuals or biomass in logarithmic size classes and the variation in concentrations among classes (Sheldon et al., 1972; Schwinghamer, 1981; Havlicek & Carpenter, 2001; Co´zar et al., 2003). Although considerable research has been carried out on invertebrate size structure in rivers and streams (Poff et al., 1993; Mercier et al., 1999; Schmid et al., 2000; Feldman, 2001); very little is known about comparisons among size distribution of benthos, drift and marginal fauna in a lowland river. In Ctalamochita River, the effects of dams (Corigliano, 1994) and water quality (Gualdoni et al., 1994) were previously studied. However, structure of macroinvertebrate communities are still unknown, and ecological studies about macroinvertebrates occupying bankside habitats and about drifting assemblages are still lacking. In general, lowland rivers of middle-order have received less attention than rivers in mountainous regions and very large rivers. The objectives of this paper are to compare the structure and composition of benthic, drifting and marginal macroinvertebrate assemblages through out an annual cycle in a rural and an urban site; and to characterize the size structure of the assemblages. We hypothesized habitat partitioning between fauna in the central channel and in the marginal zone of the river. In addition, we predicted that diversity would be higher in the marginal zone because of refugia provided by vegetation. We expected that drift would be constituted by organisms from the marginal zone, local and remote upstream benthos; and finally, we expected to observe seasonal variations in the assemblages.
Study area The study was carried out in Ctalamochita River, Co´rdoba, Argentina (Fig. 1). This is one of the most important rivers in the area because it supplies drinking water, irrigation and hydroelectric energy. Ctalamochita River is one of the tributaries of Carcaran˜a´ River and belongs to La Plata river basin. Headwaters are in mountainous regions at about 2000 m a.s.l, where many small streams join to form the main collector at foothills. Then it flows nearly 300 km through the pampean plain in a west-eastern direction into the Carcaran˜a´ River. The study area is situated in Pampean phytogeographical province, very impacted by forestry and agricultural practices (Morrone, 2001). The climate is classified as semi-dry with a tendency to semi-wet. The rainy season starts in October and ends in April with a maximum of 565 mm in this period. The minimum precipitation (143 mm) occurs between April and September (Capitanelli, 1979). Two sampling stations were selected in this study: Villa Marı´ a and Pampayasta (Fig. 1 and Table 1). In these sites the river is anastomosing with vegetated islands and bars, a central channel and secondary channels. Aquatic and semi-aquatic plants like Hydrocotile bonaeriensis develop in the riverbank and the riparian forest is formed by Salix humboltiana and other exotic species of trees and bushes. Pampayasta is a rural site in which there are no major modifications, in neither the channel of the river nor the riparian zone. Villa Marı´ a is an important urban centre of the Co´rdoba province. In this site, the channel of the river has been modified by weirs and the native forest in the riparian zone has been partially replaced by ornamental species. Materials and methods Samples were taken in all seasons in Villa Marı´ a and, in spring and summer in Pampayasta from May 1987 to February 1988. They were obtained from the central channel of the river, from the drift fraction and from the marginal zone. Benthos was sampled in sandy and silty substrate with a hand dipper (0.156 m diameter, 1 l capacity) and a core
305
Figure 1. Study area showing the locations of sampling sites in the Ctalamochita River. Sampling sites are in rectangles. Pampayasta is a rural site and Villa Marı´ a is urban.
sampler (0.03 m diameter, 0.05 m depth). Drift was sampled placing a net (Elliot, 1970)(300 lm mesh, 0.20 · 0.20 m frame, 1.00 m bag depth) in the central channel for 60 min. Drift samples were taken between 10 am and 12 pm in all cases in order to avoid behavioural drift and to allow comparisons among sites and dates. In the present paper, marginal assemblage refers not only to the fauna, which live in the substrate of marginal channels near the shore, but also the swimming macroinvertebrates, which live associated with aquatic plants and roots of riparian vegetation. In order to sample this assemblage, the substrate in the marginal channel was kicked and immediately, samples were taken with a hand dipper (0.156 m diameter, 1 l capacity) filtering 100 l of water through a hand net (300 lm mesh). The hand dipper was swept through the water at the bank/water interface. In all cases, replicate sampling units were taken and pooled for analysis. Samples were fixed with 40% formaldehyde solution. At the laboratory, organisms were sorted, identified to the lowest possible taxonomic level and counted. Conductivity, pH and temperature were measured with
portable sensors on each sampling occasion. Current velocity, depth, channel width and type of substrate were assessed in order to characterize the study sites. Surface current velocity was obtained by timing a bobber (three time average) (Gordon et al., 1994). Average depth was calculated over five measurements from one transversal profile across the channel with a calibrate stick. The relative proportion of substrate was assessed by visual estimation (Gordon et al., 1994). Discharge data were obtained from Villa Marı´ a gauging station (Agua y Energı´ a, 1981) and chemical characterization of water was taken from Nicolli et al. (1985). In this paper the term ‘taxonomic richness’ is used instead of species richness (Malmquist et al., 2000) because not all the identifications were made to the species level. Richness was measured as the total number of taxa recorded. Alpha index of diversity (a) was calculated for each sample allowing comparisons within the study area. This log series index was chosen because of its good discriminant ability and the fact that it is not unduly influenced by sample size (Magurran, 1988). Macroinvertebrate abundance data were standardized by calculating the relative proportion of
306 Table 1. Environmental features of the study sites Pampayasta
Villa Marı´ a
Latitude (S)
32 15¢ 46¢¢
32 35¢ 28¢¢
Longitude (W)
63 39¢ 18¢¢
63 16¢ 23¢¢
River order Altitude (m a.s.l)
7 281
7 198
Basin area (Km2)
4680
5295
Slope (m km)1)
0.13
0.2
Mean velocity (m s)1)
0.5 (0.4–0.6)
0.4 (0.7–0.3)
Mean depth (m)
0.2 (0.2–0.5)
0.4 (0.15–0.7)
Mean width (m)
80 (73–100)
55 (45–63)
Annual mean
30 *
29.9 (5–113)
discharge (m3 s)1) Mean water
27
20 (14–26)
Mean pH
7.9
7.75 (7–8)
Mean conductivity
330
219 (168–271)
temperature (C)
(lS cm)1) Dominant substrate
Sand and silt
Riparian vegetation
Forest of exotic and
Geomorphic pattern
native species Anastomosed
Land use
Rural
Urban
Ranges of values are shown in brackets. In the rural area some ranges are omitted because they do not correspond to an annual cycle. (*) Annual mean discharge in Pampayasta is an approximate value taken from Villa Marı´ a gauging station.
used as an index of ecological stability because it reflects the contribution of marginal and benthic assemblages to the drift and the replacement of the individuals. Macroinvertebrates in all samples were measured to the nearest 0.1 mm using an ocular micrometer in a stereo microscope and their masses were determined from length–mass relationships (Smock, 1980). All individuals were sized along the longest dimension. Cerci, anal gills and antennae were not considered in this length measurement. In each season, differences in size structure of the assemblages were assessed with the Kolmogorov–Smirnov test which is sensitive to differences in the general shapes of the distributions in two populations (Seigel & Castellan, 1988), then size structure of the assemblages was compared in pairs in each season. Size data from all taxa were grouped into log10 size (mg) intervals and biomass in these intervals was determined for benthic, drifting and marginal samples in each season. In order to analyze the size spectrum for the assemblages pooled together, biomass of benthos, drift and marginal fauna were calculated as biomass per 100 m)3. Biomass of benthos per volume unit was calculated considering the hand dipper as a half sphere and the core sampler as a cylinder.
Results each invertebrate taxon collected with respect to total density per sample, in order to allow comparisons between the assemblages since abundance was measured in different units. A Detrended Correspondence Analysis of samples and taxa was carried out using the statistical package CANOCO (Ter Braak & Smilauer, 1999). Log (x+1) transformed abundance data were used. Differences in the DCA scores among the assemblages were tested with one-way ANOVA and the Student– Newman–Keuls’ test (SNK) was used for a posteriori comparisons (p
