Primary Research Paper

Benthic, drifting and marginal macroinvertebrate assemblages in a

lowland river: temporal and spatial variations and size structure

Romina Elizabeth Principe* & Marıa del Carmen CoriglianoDepartamento de Ciencias Naturales, Universidad Nacional de Rıo Cuarto, A.P. No. 3, X5804 BYA, Rıo Cuarto,Argentina

(*Author for correspondence: Tel.: 54-0358-4676167; Fax: 54-0358-4676230 E-mail: principe@exa.unrc.edu.ar)

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 dif-ferently to the hydrological regime. In this paper, the structure and composition of benthic, drifting andmarginal macroinvertebrate assemblages are analyzed in the lowland river Ctalamochita (Cordoba,Argentina). The assemblages were studied in an annual cycle; a comparison among the composition ofbenthos, 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 macroinver-tebrates were collected. Benthos was characterized by Chironomidae and Oligochaeta; marginal fauna wasmainly constituted by Coleoptera, Heteroptera, Decapoda, the Trichoptera Nectopsyche sp., Epheme-roptera and Odonata. Drifting assemblage was composed by macroinvertebrates from local and remoteupstream benthos, and from the marginal zone. Marginal fauna diversity was higher than benthos anddrift. Total biomass of the assemblages pooled together was relatively equitably among size classes. Largersize classes consisted of organisms from the marginal zone whereas the smallest ones were composed bybenthic and drifting organisms. In the study area there is habitat partitioning in the lateral dimension of theriver. Marginal fauna was more diverse due to the asymmetry of transport and deposit processes, whichgenerate a heterogeneous habitat in the bankside. The relation between fine substrate and high currentvelocity determines an unstable habitat in the central channel, which makes colonization by benthicmacroinvertebrates difficult.

Introduction

In anastomosing lowland rivers, the asymmetry oftransport and deposit processes generates a greatvariety of habitats, which are used differently byaquatic macroinvertebrates. Habitats associatedwith bankside may greatly contribute to faunaldiversity (Corigliano, 1989; Ward, 1989; Malmq-vist, 2002) by providing refugia during periods ofhigh flow (Coregino et al., 1995; Rempel et al.,1999; Robinson et al 2002). The combined effectsof bank morphology, seasonal growth of macro-

phytes and discharge result in a highly diverse anddynamic environment offering a wide rangeof ecological niches for the faunal community(Armitage et al., 2001).

Drift is an important aspect in the study ofmacroinvertebrate communities (Allan, 1995;Ramırez & Pringle 1998) because it is related tosecondary production in water bodies, it is animportant source of food for fish, and also aneffective way for some aquatic organisms to colo-nize new areas (Waters, 1972; Williams & Hynes,1976; Sagar, 1983; Allan, 1995). In anastomosing

Hydrobiologia (2006) 553:303–317 � Springer 2006DOI 10.1007/s10750-005-0694-3

lowland rivers, drift is composed by organismsfrom the main channel and from the marginal zone(Cellot, 1989; Corigliano et al., 1998).

Organism body size constrains many ecologicalprocesses that influence community organization.Body size often influences energy flow and trophicstructure, so abundance and biomass of differen-tially-sized animals should reflect size-specificallocation of total community resources (Peters,1983), and this allocation may vary among habi-tats characterized by different abiotic and bioticconstraints. In aquatic ecology, size distributionresearch has focused on size spectrum, which is theconcentration of individuals or biomass in loga-rithmic size classes and the variation in concen-trations among classes (Sheldon et al., 1972;Schwinghamer, 1981; Havlicek & Carpenter, 2001;Cozar et al., 2003). Although considerable re-search has been carried out on invertebrate sizestructure in rivers and streams (Poff et al., 1993;Mercier et al., 1999; Schmid et al., 2000; Feldman,2001); very little is known about comparisonsamong size distribution of benthos, drift andmarginal fauna in a lowland river.

In Ctalamochita River, the effects of dams(Corigliano, 1994) and water quality (Gualdoniet al., 1994) were previously studied. However,structure of macroinvertebrate communities arestill unknown, and ecological studies about macr-oinvertebrates occupying bankside habitats andabout drifting assemblages are still lacking. Ingeneral, lowland rivers of middle-order havereceived less attention than rivers in mountainousregions and very large rivers.

The objectives of this paper are to compare thestructure and composition of benthic, drifting andmarginal macroinvertebrate assemblages throughout an annual cycle in a rural and an urban site;and to characterize the size structure of theassemblages. We hypothesized habitat partitioningbetween fauna in the central channel and in themarginal zone of the river. In addition, we pre-dicted that diversity would be higher in the mar-ginal zone because of refugia provided byvegetation. We expected that drift would be con-stituted by organisms from the marginal zone, lo-cal and remote upstream benthos; and finally, weexpected to observe seasonal variations in theassemblages.

Study area

The study was carried out in Ctalamochita River,Cordoba, Argentina (Fig. 1). This is one of themost important rivers in the area because it sup-plies drinking water, irrigation and hydroelectricenergy. Ctalamochita River is one of the tribu-taries of Carcarana River and belongs to La Platariver basin. Headwaters are in mountainousregions at about 2000 m a.s.l, where many smallstreams join to form the main collector at foothills.Then it flows nearly 300 km through the pampeanplain in a west-eastern direction into the Car-carana River. The study area is situated in Pam-pean phytogeographical province, very impactedby forestry and agricultural practices (Morrone,2001). The climate is classified as semi-dry with atendency to semi-wet. The rainy season starts inOctober and ends in April with a maximum of565 mm in this period. The minimum precipitation(143 mm) occurs between April and September(Capitanelli, 1979).

Two sampling stations were selected in thisstudy: Villa Marıa and Pampayasta (Fig. 1 andTable 1). In these sites the river is anastomosingwith vegetated islands and bars, a central channeland secondary channels. Aquatic and semi-aquaticplants like Hydrocotile bonaeriensis develop in theriverbank and the riparian forest is formed bySalix humboltiana and other exotic species of treesand bushes.

Pampayasta is a rural site in which there are nomajor modifications, in neither the channel of theriver nor the riparian zone. Villa Marıa is animportant urban centre of the Cordoba province.In this site, the channel of the river has beenmodified by weirs and the native forest in theriparian zone has been partially replaced by or-namental species.

Materials and methods

Samples were taken in all seasons in Villa Marıaand, in spring and summer in Pampayasta fromMay 1987 to February 1988. They were obtainedfrom the central channel of the river, from the driftfraction and from the marginal zone. Benthos wassampled in sandy and silty substrate with a handdipper (0.156 m diameter, 1 l capacity) and a core

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sampler (0.03 m diameter, 0.05 m depth). Driftwas sampled placing a net (Elliot, 1970)(300 lmmesh, 0.20 · 0.20 m frame, 1.00 m bag depth) inthe central channel for 60 min. Drift samples weretaken between 10 am and 12 pm in all cases inorder to avoid behavioural drift and to allowcomparisons among sites and dates.

In the present paper, marginal assemblagerefers not only to the fauna, which live in thesubstrate of marginal channels near the shore, butalso the swimming macroinvertebrates, which liveassociated with aquatic plants and roots of ripar-ian vegetation. In order to sample this assemblage,the substrate in the marginal channel was kickedand immediately, samples were taken with a handdipper (0.156 m diameter, 1 l capacity) filtering100 l of water through a hand net (300 lm mesh).The hand dipper was swept through the water atthe bank/water interface.

In all cases, replicate sampling units were takenand pooled for analysis. Samples were fixed with40% formaldehyde solution. At the laboratory,organisms were sorted, identified to the lowestpossible taxonomic level and counted. Conductiv-ity, pH and temperature were measured with

portable sensors on each sampling occasion. Cur-rent velocity, depth, channel width and type ofsubstrate were assessed in order to characterize thestudy sites. Surface current velocity was obtained bytiming a bobber (three time average) (Gordon et al.,1994). Average depth was calculated over fivemeasurements from one transversal profile acrossthe channel with a calibrate stick. The relative pro-portion of substrate was assessed by visual estima-tion (Gordon et al., 1994). Discharge data wereobtained from Villa Marıa gauging station (Agua yEnergıa, 1981) and chemical characterization ofwater was taken from Nicolli et al. (1985).

In this paper the term ‘taxonomic richness’ isused instead of species richness (Malmquist et al.,2000) because not all the identifications were madeto the species level. Richness was measured as thetotal number of taxa recorded. Alpha index ofdiversity (a) was calculated for each sampleallowing comparisons within the study area. Thislog series index was chosen because of its gooddiscriminant ability and the fact that it is not un-duly influenced by sample size (Magurran, 1988).

Macroinvertebrate abundance data were stan-dardized by calculating the relative proportion of

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 Marıa is urban.

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each invertebrate taxon collected with respect tototal density per sample, in order to allow com-parisons between the assemblages since abundancewas measured in different units. A DetrendedCorrespondence Analysis of samples and taxa wascarried out using the statistical package CANOCO(Ter Braak & Smilauer, 1999). Log (x+1) trans-formed abundance data were used. Differences inthe DCA scores among the assemblages weretested with one-way ANOVA and the Student–Newman–Keuls’ test (SNK) was used for aposteriori comparisons (p<0.05).

The drift/benthos ratio and drift/marginalfauna ratio were calculated for the most frequentand abundant taxa in order to analyze their per-sistence. Drift ratios were calculated as the per-centage of benthic and marginal taxa in drift(percentage of the taxon in drift/percentage of thetaxon in benthos). The drift/benthos ratio can be

used as an index of ecological stability because itreflects the contribution of marginal and benthicassemblages to the drift and the replacement of theindividuals.

Macroinvertebrates in all samples were mea-sured to the nearest 0.1 mm using an ocularmicrometer in a stereo microscope and their masseswere determined from length–mass relationships(Smock, 1980). All individuals were sized along thelongest dimension. Cerci, anal gills and antennaewere not considered in this length measurement.

In each season, differences in size structure ofthe assemblages were assessed with the Kolmogo-rov–Smirnov test which is sensitive to differencesin the general shapes of the distributions in twopopulations (Seigel & Castellan, 1988), then sizestructure of the assemblages was compared inpairs in each season.

Size data from all taxa were grouped into log10size (mg) intervals and biomass in these intervalswas determined for benthic, drifting and marginalsamples in each season. In order to analyze the sizespectrum for the assemblages pooled together,biomass of benthos, drift and marginal fauna werecalculated as biomass per 100 m)3. Biomass ofbenthos per volume unit was calculated consideringthe hand dipper as a half sphere and the coresampler as a cylinder.

Results

The Ctalamochita River is a typical anastomosinglowland river (Table 1). Sand predominated in thethalweg channel; fine sand, silt and clay in sec-ondary lateral channels. Current velocity peakedin summer (0.70 m s)1) and was the lowest in au-tumn (0.35 m s)1). Water is classified as semi-hardsince values of CaCO3 were 90 mg l)1. Values ofsulphate were 120 mg l)1 and chlorides were69.9 mg l)1.

A total of 73 taxa of macroinvertebrates wereidentified (Table 2). The most common orders ofinsects were Heteroptera, Coleoptera, Diptera andEphemeroptera. Benthos consisted mostly ofChironomidae and Oligochaeta; drift was com-posed by Ephemeroptera, Trichoptera and Chi-ronomidae and the marginal fauna includedDecapoda, Coleoptera, Odonata, Trichoptera,Heteroptera and Ephemeroptera. Odonata and the

Table 1. Environmental features of the study sites

Pampayasta Villa Marıa

Latitude (S) 32� 15¢ 46¢¢ 32� 35¢ 28¢¢Longitude (W) 63� 39¢ 18¢¢ 63� 16¢ 23¢¢River order 7 7

Altitude (m a.s.l) 281 198

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

discharge (m3 s)1)

30 * 29.9 (5–113)

Mean water

temperature (�C)27 20 (14–26)

Mean pH 7.9 7.75 (7–8)

Mean conductivity

(lS cm)1)

330 219 (168–271)

Dominant substrate Sand and silt

Riparian vegetation Forest of exotic and

native species

Geomorphic pattern 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 Marıa gauging station.

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Table 2. List of aquatic macroinvertebrates of different assemblages collected in the study sites of the Ctalamochita river: urban site

(U) and rural site (R)

Taxa Study site % Frequency of occurrence

Benthos Drift Marginal fauna Total

Nematoda U R 0.0 14.3 14.3 8.3

Oligochaeta

Nais sp. U R 20.0 57.1 85.7 50.0

Chaetogaster sp. U R 10.0 0.0 14.3 8.3

Pristina sp. U R 10.0 14.3 57.1 25.0

Tubificidae U R 30.0 28.6 85.7 45.8

Lumbriculidae U R 30.0 0.0 28.6 20.8

Acari U R 0.0 42.9 28.6 20.8

Crustacea

Copepoda U R 0.0 85.7 42.9 37.5

Hyallela curvispina Shoemaker R 0.0 14.3 14.3 8.3

Palaemonetes argentinus Nobili U 0.0 0.0 57.1 16.7

Macrobrachium borelii Nobili U R 0.0 0.0 100.0 29.2

Aegla uruguayana Schmitt R 0.0 0.0 14.3 4.2

Collembola

Isotomidae (Iso) U R 0.0 14.3 100 33.3

Poduridae U R 0.0 14.3 42.9 16.7

Ephemeroptera

Paracloeodes sp. U R 0.0 57.1 100.0 45.8

Camelobaetidius penai Traver & Edmunds U 0.0 0.0 14.3 4.2

Apobaetis sp. U R 0.0 14.3 14.3 8.3

Caenis sp. U R 0.0 14.3 57.1 20.8

Leptohyphes sp. (Lep) U R 0.0 42.9 71.4 33.3

Traverella sp. U 0.0 0.0 14.3 4.2

Odonata

Agrionidae (Agr) U R 0.0 14.3 100.0 33.3

Lestidae (Les) U R 0.0 0.0 57.1 16.7

Aeshna sp. U 0.0 0.0 14.3 4.2

Phyllocycla sp. U R 0.0 28.6 71.4 29.2

Progomphus sp. U R 20.0 28.6 28.6 25.0

Heteroptera

Ranatra sp. U 0.0 0.0 14.3 4.2

Belostoma elegans (Mayr) U R 0.0 0.0 71.4 20.8

Ambryssus ochraceus Montandon U 0.0 0.0 14.3 4.2

Neoplea maculosa (Berg) (N. mac) R 0.0 0.0 28.6 8.3

Buenoa fuscipennis (Berg) U 0.0 14.3 0.0 4.2

Sigara denseconscripta (Berdın) U R 0.0 0.0 42.9 12.5

Trichocorixa mendozana Jaczewski (T. men) R 0.0 0.0 14.3 4.2

Mesovelia mulsanti (White) U 0.0 0.0 14.3 4.2

Hydrometra argentina Berg (H.arg) U 0.0 0.0 42.9 12.5

Lipogomphus lacuniferus Berg (L .lac) U R 0.0 0.0 14.3 4.2

Trichoptera

Smicridea sp. U R 0.0 57.1 42.9 29.2

Continued on p. 308

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Table 2. (Continued)

Taxa Study site % Frequency of occurrence

Benthos Drift Marginal fauna Total

Metrichia sp. R 0.0 0.0 14.3 4.2

Nectopsyche sp. U R 0.0 57.1 100.0 45.8

Coleoptera

Suphis sp. U R 0.0 0.0 28.6 8.3

Hydrocantus sp. (Hyd) U R 0.0 0.0 57.1 16.7

Suphisellus sp. U R 0.0 0.0 42.9 12.5

Desmopachria sp. (Des) R 0.0 0.0 14.3 4.2

Liodessus sp. U R 0.0 42.9 100.0 37.5

Lancetes sp. U 0.0 0.0 14.3 4.2

Ranthus sp. U 0.0 0.0 14.3 4.2

Neogyrinus sp. U 0.0 0.0 14.3 4.2

Gymnochthebius sp. (Gym) U R 0.0 0.0 42.9 12.5

Berosus pedregalensis Jensen-Haarup (B. ped) R 0.0 0.0 14.3 4.2

Berosus patruelis Berg (B. pat) U 0.0 0.0 14.3 4.2

Berosus pallipes Brulle (B. pall) U R 0.0 0.0 42.9 12.5

Berosus sp. U 0.0 0.0 14.3 4.2

Derallus paranensis Oliva U 0.0 0.0 14.3 4.2

Tropisternus lateralis limbatus Brulle (T. lat) U 0.0 0.0 14.3 4.2

Tropisternus flavescens Orchymont U 0.0 0.0 14.3 4.2

Enochrus sp. U R 0.0 0.0 57.1 16.7

Heterocerus sp. U 0.0 0.0 14.3 4.2

Elmidae U 0.0 0.0 14.3 4.2

Diptera

Tipulidae U R 0.0 28.6 14.3 12.5

Simulium sp. R 0.0 0.0 14.3 4.2

Ablabesmyia sp. (Abl) U R 0.0 0.0 14.3 4.2

Polypedilum sp. U R 30.0 100.0 71.4 62.5

Parachironomus sp. R 0.0 0.0 14.3 4.2

Goeldichironomus sp. (Goe) R 0.0 0.0 14.3 4.2

Chironomus sp. U R 10.0 0.0 28.6 12.5

Cryptochironomus sp. (Cry) R 0.0 14.3 14.3 8.3

Saetheria sp. U R 50.0 57.1 42.9 50.0

Pseudochironomus sp. R 0.0 0.0 14.3 4.2

Paratanytarsus sp. (Par) U 0.0 0.0 28.6 8.3

Rheotanytarsus sp. R 0.0 0.0 14.3 4.2

Onconeura sp. U R 0.0 57.1 42.9 29.2

Thienemanniella sp. U R 10.0 14.3 57.1 25.0

Nanocladius sp. U R 0.0 14.3 28.6 12.5

Cricotopus sp. U R 0.0 71.4 57.1 37.5

Frequency of occurrence is expressed as percentage. In brackets: taxa codes used in DCA analysis, some species’ names were used in

full form.

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Decapoda Macrobrachium borelli and Palaemone-tes argentinus were present in all samples from themarginal zone. Heteroptera were found in allmarginal samples, but the predominant taxa dif-fered seasonally. In summer Neoplea maculosapredominated; in autumn, Hydrometra argentina;in winter, Belostoma elegans and in spring, Sigaradenseconscripta. Chironomidae were recorded inall samples. Benthos included Saetheria sp. in allseasons; Polypedilium sp. and Thienemanniella sp.were more frequent in the marginal zone andCricotopus sp. was frequent in drift.

Biomass and abundance of macroinvertebrateswere higher in the rural site (Fig. 2). Richness persample ranged from 1 to 38. The highest values ofAlpha index and taxonomic richness were found in

the marginal zone (Fig. 3) while the lowest werefound in benthos. Of a total of 73 taxa, 72 werefound in the marginal zone, 30 in drift and 11 inbenthos.

The results of the DCA ordination showed that32.9% of species abundance was accounted by thefirst four ordination axes (Eigenvalues: Axis 1: 0.66,Axis 2: 0.31, Axis 3: 0.19, Axis 4: 0.10; Total inertia:3.82). DCA resulted in three main groups of sam-ples (Fig. 4) based on the assemblages. DCA axis 1showed a separation between benthos andmarginalfauna. Benthic samples from silty substrates (BA1and BSp2) were associatedmainly with Oligochaetawhereas benthic samples from sandy substrateswere associated with the Chironomidae Saetheriasp. DCA axis 2 separated drift frommarginal fauna

Figure 2. Total density and biomass of macroinvertebrates from different assemblages in autumn (A), winter (W), spring (SP) and

summer (SU). Notice that a logarithmic scale is used in the graphs instead of a normal scale because densities in the rural site were

much higher than in the urban site and abundances could not be observed in a normal scale.

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and benthos. Drift was more related to marginalassemblage since the ordination scores of benthoswere significantly different from those of the otherassemblages, as measured by one-way ANOVA(F: 11.10; p <0.001). Benthic samples from siltysubstrate were not taken into account for ANOVAin order to allow the detection of differences amongthe other ordination groups.

The drift ratios (Table 3) indicated that sometaxa have more predisposition to drift and conse-quently a low stability in benthos or in the mar-ginal assemblage. As DCA showed, some of thesetaxa were more associated with the marginalassemblage (e.g.: Onconeura sp.) and others weremore related to benthos (e.g.: Saetheria sp.).

Sizes of marginal fauna were more variablethan sizes of invertebrates from other assemblagesin all seasons (Fig. 5). Size structure of benthos,drift and marginal fauna showed differences infrequency distribution of organism body length(Fig. 6). Size distribution of benthos was signifi-cantly different from drift in all seasons exceptSpring (Table 4) and it was different from mar-ginal fauna in all seasons except in Autumn. Sizedistribution of drift was significantly differentfrom the marginal fauna in all seasons.

Biomass of individual macroinvertebrates ran-ged from 0.0001 to 313 mg (Fig.7). Total biomassof the assemblages pooled together was relativelyequitably among size classes (Fig. 8). The smallestsize classes were mainly composed by benthic and

drifting organisms and the largest were formed bymarginal macroinvertebrates. However, biomassof benthic organisms was higher than biomass ofmarginal organisms. Biomass of drifting organ-isms was the lowest in all size classes, so drifthardly contributed to the size spectrum.

Discussion

Spatial and temporal analysis

Our results showed a low diversity in benthos ofthe lowland Ctalamochita River since this assem-blage was mainly constituted by two taxa of Chi-ronomidae and three of Oligochaeta. In ananastomosing reach, the interaction among currentvelocity, slope and type of substrate causes a greatmobility in the substrate and generates an unstablehabitat for benthic organisms (Allan, 1995). On theother hand, the highest diversity were obtained inmarginal assemblage. In lowland rivers, marginalzones become very important because they allowthe establishment of more species due to the het-erogeneous environment and refugia provided byvegetation (Ward, 1989; Coregino et al., 1995;Robinson et al., 2002; Malmqvist, 2002).

In the present study, DCA showed differentgroups of benthic, drift and marginal samples.This result may suggest a spatial rather thantemporal segregation of the assemblages. The use

Figure 3. Taxonomic richness and diversity of benthos (B), drift (D) and marginal fauna (M) in all seasons in both study sites in the

Ctalamochita River. Richness and diversity of all the assemblages are in a single graph in order to remark differences among them.

Alpha index (Log series) of diversity allows to compare different assemblages because is unduly influence by sample size.

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of refugia and colonization patterns of macroin-vertebrates may explain the relative persistence ofthe assemblages in the temporal dimension. Dur-ing floods, refugia may be important as reservoirsof species and after these periods, habitats may becolonized by the same taxa from refugia. Differ-ences between benthic and marginal fauna revealthe importance of considering the lateral dimen-sion of the four-dimensional river for the analysis

of macroinvertebrate community structure in alowland river (Ward, 1989).

As it has already been reported (Cellot, 1989;Corigliano et al., 1998), some macroinvertebrateswere found only in drift. This finding suggests thatat least some elements of the drift may be derivedfrom rather remote upstream habitats (Elliot,1967; Obi & Corner, 1986). Drift samples formed agroup in DCA indicating that drift composition

Figure 4. DCA ordination of samples (circles) from all assemblages and macroinvertebrate taxa (crosses) in the study sites of the

Ctalamochita River. A biplot of samples and taxa was performed in order to relate taxa to assemblages. Taxa codes are defined in

Table 2. Assemblages: benthos (B), drift (D) and marginal fauna (M). Seasons: autumn (A), winter (W), spring (Sp), summer (Su).

Study sites: urban site (1), rural site (2).

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was different from benthic and marginal assem-blages. This difference may be explained takinginto account that not all organisms have the same

predisposition to enter into the water column(Waters, 1972; Allan, 1995), consequently a dif-ferent assemblage is conformed.

Table 3. Drift ratios for all major drifting macroinvertebrates in the Ctalamochita river

Autumn Winter Spring Summer Mean

D/B D/M D/B D/M Urban site Rural site Urban site Rural site

D/B D/M D/B D/M D/B D/M D/B D/M

Nais sp. 0.00 – 0.94 – 1.19 1.70 0.15 0.00 0.28 0.32 0.57

Pristina sp. 0.00 0.00 0.21 0.71 0.23

Tubificidae 0.00 0.00 0.00 0.00 – 0.15 0.02 0.03 0.02

H. Curvispina – 1.13 1.13

Paracloedes sp. – 0.02 0.00 0.00 – 0.31 – 0.73 – 2.53 0.59

Caenis sp. 0.00 – 0.11 0.00 0.00 0.03

Leptohyphes sp. – 0.27 0.00 0.00 – 1.80 – 0.35 0.48

Agrionidae 0.00 – 0.16 0.00 0.00 0.00 0.00 0.03

Phyllocycla sp. 0.00 0.00 0.00 – 0.74 – 0.50 0.25

Progomphus sp. 0.00 – 0.58 – 18.71 6.43

Smicridea sp. – – – – – 13.39 – 5.39 0.00 9.39

Nectopsyche sp. 0.00 0.00 – 0.60 – 0.08 – 0.94 0.00 0.27

Liodessus sp. – 0.22 0.00 0.00 0.00 0.00 0.00 0.03

Tipulidae – – – 1.08 1.08

Polypedilum sp. – 0.10 – 1.88 – – – 0.89 – – 0.06 0.50 0.68

Saetheria sp. 0.00 0.00 0.00 0.00 0.28 33.15 0.06 – 0.13 – 0.00 3.36

Onconeura sp. – – 0.00 – – – 8.02 – 4.13 4.05

Thienemanniella sp. 0.00 – 3.97 0.00 0.00 0.00 0.79

Cricotopus sp. 0.00 – – – 3.78 – 0.08 0.00 – – 0.96

Drift ratios were calculated in relation to benthos (% taxa in drift/% taxa in benthos) and marginal assemblage (% taxa in drift/% taxa

in marginal fauna) in order to evaluate ecological stability in both assemblages. The gap indicates that taxa were not found in that

period. (–) Taxa registered only in drift in that period. Notice that drift ratio cannot be calculated when taxa were only registered in

drift because % of benthos (or % marginal fauna) is zero.

Figure 5. Size variations of macroinvertebrate body length in benthos (B), drift (D) and marginal fauna (M) in all seasons of the study

sites of the Ctalamochita River.

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Contrary to what we expected, taxonomicrichness in the urban site was higher than in therural site. Although, there were also differences inmacroinvertebrate abundance between sites, itmust be considered that the rural site was

sampled only in two seasons and that there wereno replicates of the sites. Consequently, signifi-cant differences could not be assessed. Furtheranalyses with replicates of each site must beperformed.

Figure 6. Size distributions of macroinvertebrate assemblages from the urban site in the Ctalamochita River. Different letters indicate

significant differences among size distribution of the assemblages tested by Kolmogorov Smirnov test. p values of K–S test are in

Table 4.

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Size structure

The most general theory of community sizestructure (Thiel, 1975) hypothesizes that reduced

levels of resource supply lead to communities withsmaller individual size. In our study, the largestorganisms were found in the marginal zone, andbenthos and drift were constituted by the smallestorganisms. Some researchers have reported a pre-dominance of young organisms in drift which ap-pears to be a frequent result in studies of driftingassemblages and suggests that one role of drift isdispersal (Waters, 1972; Williams & Hynes, 1976;Sagar, 1983; Allan, 1995). Large marginal organ-isms were not recorded in drift because they aremostly swimming species, which have low predis-position to drift. Moreover, marginal macroin-vertebrates may use aquatic plants and roots ofriparian vegetation as refugia.

Benthos may be constituted by small organismsdue to the unstable substrate, which does not allowthe deposit of great amounts of organic matter. Inthe bankside of the river, low current velocity andthe supply of riparian organic matter, may provideenough resources to allow the development oflarger organisms.

In Ctalamochita River, total biomass of theassemblages combined was relatively equitably

Table 4. Results of Kolmogorov-Smirnov test examining dif-

ferences among size distributions of the assemblages

Seasons Assemblages compared D statistic p value

Autumn Benthos–Drift 1.42 0.01

Benthos–Marginal fauna 5.62 0.10

Drift–Marginal fauna 5.62 0.01

Winter Benthos–Drift 0.76 0.01

Benthos–Marginal fauna 10.15 0.01

Drift–Marginal fauna 10.15 0.01

Spring Benthos–Drift 0.65 0.20

Benthos–Marginal fauna 7.89 0.01

Drift–Marginal fauna 7.89 0.05

Summer Benthos–Drift 0.86 0.05

Benthos–Marginal fauna 4.48 0.05

Drift–Marginal fauna 4.48 0.01

Assemblages of the urban site of the Ctalamochita River are

compared in pairs in each season. Significant p value are in

bold.

Figure 7. Range of body sizes (mg dry mass [DM]) of dominant macroinvertebrates collected in benthos, drift and marginal fauna in

the study sites of Ctalamochita River.

314

among size classes. Poff et al. (1993) described sizestructure of a metazoan community, finding abimodal size spectrum for benthos. These authorsdemonstrated that meiofaunal-sized animals can-not be discounted to assess community size struc-ture, because meiofauna biomass provokesirregularities in size spectrum. In our study, mei-ofauna was not assessed so, probably this is areason of finding a spectrum near to flatness.

Although larger organisms were found in themarginal zone, the total biomass of benthos washigher than biomass of the marginal assemblage.This result is not expectable since it may suggest ahigher benthic production in an unstable habitatlike the central channel of the river, wherethe substrate is easily removed by the current.

Probably, the explanation to this finding lies in thelife history of benthic predominant taxa, and intheir patterns of colonization from different habi-tats of the river. Further research must be carriedout to explore this finding.

Conclusion

Our results have shown that in the study area thereis habitat partitioning in the lateral dimension ofthe river. Marginal fauna was more diverse, indi-cating that communities distribute mainly laterallydue to the asymmetry of transport and depositprocesses, which generate a heterogeneous habitatin the bankside. The relationship between the

Figure 8. Biomass size spectra for all macroinvertebrates collected in benthos, drift and marginal fauna in the study sites of the

Ctalamochita River. Crosses represent data from all assemblages pooled together.

315

heterogeneity of the marginal zone and dischargeis a functional characteristic of any river-flood-plain system that is likely to exert a major influ-ence on biodiversity patterns. Two differentprocesses, erosive flooding and flow pulses, actingat different time-scales control connectivity in ariverine landscape (Tockner et al., 2000).

Flood pulse and the variations of water level inthe Ctalamochita River, may act on differenttemporal and spatial scales as environmental fac-tors that cause an intermediate disturbance in themarginal zone of the river. These processes maygenerate a high diversity of species, as it is estab-lished in the intermediate disturbance hypothesis(Connell, 1978), because bankside provides refugesduring periods of disturbance (Coregino et al.,1995; Rempel et al., 1999; Armitage et al., 2001;Robinson et al., 2002). In the central channel ofthe Ctalamochita River, floods may cause a highdisturbance since the substrate is easily removedby the current. Consequently, only species that areable to tolerate this unstable habitat can live there.As a result, an assemblage with low diversity ofspecies is conformed.

Acknowledgements

This study has been supported by Secretarıa deCiencia y Tecnica, Universidad Nacional de RıoCuarto. We wish to thank to Mabel Gualdoni,Ana Oberto and Graciela Raffaini for their helpwith identifications of macroinvertebrates, andMarcelo Ardon Sayao and Minor Hidalgo forEnglish assistance and constructive suggestions.We also thank the anonymous referees forthoughtful reviews of the manuscript and forconstructive suggestions for its improvement.

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