Size structure of benthic invertebrate assemblages in a Mediterranean river

Size structure of benthic invertebrate assemblages in a Mediterranean riverAuthor(s): Angelo G. Solimini, Alessio Benvenuti, Raffaella D’Olimpio, Marco De Cicco, andGianmaria CarchiniSource: Journal of the North American Benthological Society, Vol. 20, No. 3 (September2001), pp. 421-431Published by: Society for Freshwater ScienceStable URL: http://www.jstor.org/stable/1468039 .

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421

J. N. Am. Benthol. Soc., 2001, 20(3):421–431q 2001 by The North American Benthological Society

Size structure of benthic invertebrate assemblagesin a Mediterranean river

ANGELO G. SOLIMINI1, ALESSIO BENVENUTI, RAFFAELLA D’OLIMPIO,MARCO DE CICCO, AND GIANMARIA CARCHINI

Laboratory of Experimental Ecology and Aquaculture, Department of Biology, University of RomeTor Vergata, Via della Ricerca Scientifica, 00133 Rome, Italy

Abstract. We examined the temporal, longitudinal, and among-substrate variation of the size struc-ture of the invertebrate assemblage in a Mediterranenan river (Aniene River, central Italy). Samplingwas conducted with quantitative methods on different substrates every 2 mo at 2 sites for 1 y and at9 additional sites in spring 1997. Water-quality variables, periphyton standing crop, and physicalcharacteristics of sites were also recorded. The Aniene River showed large longitudinal changes inoverall physical features, periphyton biomass, and water quality that were reflected in dramaticchanges of the taxonomic composition of the invertebrate assemblage. Insects were numerically dom-inant in the upper reach and codominant with gastropods in the middle reach, whereas oligochaetes,crustaceans, and chironomids were predominant in the lower and polluted reach. Despite thesechanges, the shape of the size structure of the invertebrate assemblage was similar among sites, dates,and substrates, with limited departures from the average size spectrum. Body mass alone accountedfor the largest part of the variance of the abundance per size class (48%) whereas date, site, andsubstrate accounted for an additional 13%. These data support early observations of size spectruminvariance from North American streams and rivers, reinforcing the view that size-dependent pro-cesses may structure lotic benthic assemblages.

Key words: size spectra, animal abundance, body mass, Aniene River, linear models, substrate.

The structure of natural assemblages is gen-erally described using species composition andabundance. Because benthic invertebrate assem-blages are taxonomically heterogeneous, thisapproach demands great taxonomic expertiseand is time consuming (Warwick 1988). Benthicecologists, following early studies on marine(Sheldon et al. 1972) and lake plankton com-munities (Peters 1983, Sprules and Munawar1986, Ahrens and Peters 1991), have used sizestructure of invertebrate assemblages as an al-ternative to taxonomic description (Strayer1991). In the size spectrum approach, the den-sity of organisms is grouped into successivelogarithmic size classes, making it convenient tocompare assemblages of different species, bio-geographic regions, or ecosystems (Sprules andMunawar 1986, Hanson et al. 1989, Strayer 1991,Poff et al. 1993).

Abiotic factors in lotic systems are highly var-iable in time and space (Hynes 1970, Minshall1988, Palmer et al. 1997, Palmer and Poff 1997)and often account for a significant proportion ofthe variation of community patterns in terms ofspecies diversity, abundance, biomass, and pro-

1 E-mail address: [email protected]

duction (Hynes 1970, Furse et al. 1984, Camargo1992, Downes et al. 1993). In contrast, the fewstudies done on stream invertebrate size spectrahave shown a remarkable regularity in spaceand time (Morin 1997). Size spectra did not varyamong streams (Morin and Nadon 1991), sub-strates (Bourassa and Morin 1995), seasons (Cat-taneo 1993, Morin et al. 1995), and phosphorousand periphyton gradients (Bourassa and Morin1995). However, the longitudinal pattern of theinvertebrate size spectrum on different sub-strates along an entire river course (i.e., fromheadwater to downstream) remains to be de-scribed.

The small variation of stream invertebrate sizespectra may arise from the similarity of sitesfrom which size spectra have been described todate. Almost all published size spectra are fromNorth America, primarily from Canada, andthere are few data from other biogeographicalregions (but see Ramsay et al. 1997) or fromstreams with different hydrological regimes ornutrient status (Morin 1997). In the Mediterra-nean region, the hydrological regime makesstreams and rivers ecologically different fromthose of more mesic or xeric areas, causing ma-jor differences in ecosystem patterns and pro-



422 [Volume 20A. G. SOLIMINI ET AL.

FIG. 1. Monthly discharge regime at 2 gauging sta-tions of the Aniene River (historical data). Upper-reach gauging station (closed circles) is 1 km up-stream of S5; lower-reach gauging station (open cir-cles) is at S10. Data are monthly averages collectedfrom 1935 to 1990 by the local water authority (IstitutoIdrografico e Mareografico di Roma, unpublisheddata).

TABLE 1. General physical features of sites at the time of sampling. Values of channel characteristics areusually averages of several transects.

Site Reach

Elevation(m abovesea level)

Channelwidth

(m)Water depth

(cm)

Currentvelocity(m/s)

Prevalent substrates(% area covered)

S1S2S3S4S5S6S7S8S9S10S11

UpperUpperUpperUpperMiddleMiddleMiddleMiddleMiddleLowerLower

1000660450400340325300250220100

50

25

137

19251319182031

21.220.929.534.847.549.141.651.971.2

160.0170.0

1.671.932.311.941.141.791.470.90.22

,0.2,0.2

Coarse gravel and cobbles (58%)Coarse gravel and cobbles (66%)Coarse gravel and cobbles (70%)Boulders and cobbles (53%)Cobbles and coarse gravel (61%)Coarse and fine gravel (76%)Cobbles and coarse gravel (78%)Coarse gravel (68%)Coarse gravel (67%)Sand (.90%)Sand (.90%)

cesses (recently reviewed by Gasith and Resh1999) and making these systems a good refer-ence for comparisons of size spectra patterns.

The aim of this paper is to describe the sizestructure of benthic invertebrate assemblagesand to analyze 1) longitudinal variation, 2) var-iation among substrates, and 3) temporal vari-ation in a Mediterranean river (Aniene River,central Italy). We also will compare the inver-tebrate size spectra of this Mediterranen riverwith these found in temperate North Americanstreams.

Study Site

We sampled 11 sites along the Aniene, a 5th-order river that joins the Tiber River just inside

the city of Rome. The Aniene is 116 km long,and flows from an elevation of 1200 m abovesea level (asl) to 23 m asl, with an average slopeof 1.3% and a mean annual discharge of 27.3m3/s. Mean monthly discharge varies season-ally with maxima of .35 m3/s in winter andminima of ,18 m3/s in summer (Fig. 1). Casoriaand Bajetti (1994) classified the climate of thebasin as Meso-Mediterranean. The basin covers1466 km2 and can be divided into 3 subbasinswith different geomorphological attributes andland uses (Casoria and Bajetti 1994). The uppersubbasin is formed by Mesozoic calcareousrocks, the terrain is mountainous (the highestpeak is 2176 m asl), and land use is mainlywoodland and pastureland. We sampled 4 sitesin this reach (S1–S4). The middle subbasin isformed by Tertiary calcareous sandstone andclays, and land use is nonintensive agriculture.We sampled 5 sites in this reach (S5–S9). Thelower subbasin is formed by continental Qua-ternary sediments, and is heavily urbanized:93% of inhabitants of the entire basin are locat-ed in this area. In this last reach, the river re-ceives the sewage outflows from several smalltowns and villages and several industries. TheAniene enters the Tiber River inside the city ofRome, which causes a significant decrease ofwater quality (Solimini et al. 1997, Solimini etal. 2000). We sampled 2 sites in this reach (S10–S11), the last one just inside the urban area ofRome. Physical characteristics of sampling sitesand location within the basin are reported inTable 1. The Aniene River has higher nutrientconcentrations and a different hydrologic re-



2001] 423INVERTEBRATE SIZE SPECTRA IN A MEDITERRANEAN RIVER

TABLE 2. Substrates sampled for benthic invertebrates at each month 3 site combination. Substrate codesare as follows: 1 5 boulder, 2 5 cobble, 3 5 coarse gravel, 4 5 fine gravel, 5 5 detritus, 6 5 sand, 7 5 aquaticvegetation.

Site Nov 1996 Jan 1997 Mar 1997 Apr–May 1997 Jul 1997 Sep 1997

S1S2S3S4

1, 2, 3, 4, 5, 6 1, 2, 3, 4, 5, 6 1, 2, 3, 4, 5, 6 1, 2, 3, 4, 5, 61, 2, 3, 4, 5, 61, 2, 3, 4, 5, 61, 2, 3, 4, 5, 6

1, 2, 3, 4, 5, 6 1, 2, 3, 4, 5, 6

S5S6S7S8S9S10S11

1, 2, 3, 4, 5, 6 1, 2, 3, 4, 5, 6 1, 2, 3, 4, 5, 6 1, 2, 3, 4, 5, 61, 3, 4, 5, 61, 2, 3, 5, 6, 71, 2, 3, 5, 61, 3, 4, 5, 6, 71, 75, 6

1, 2, 3, 4, 5, 6 1, 2, 3, 4, 5, 6

gime than other streams and rivers from whichinvertebrate size spectra have been described.

Methods

Periphyton and physical and chemical variables

Study sites, each consisting of a 100-m stretchincluding riffle and pool habitats, were sampledfrom November 1996 to December 1997 forphysical and chemical variables and periphytonstanding crop. Water (single sample of 500 mL)was collected monthly at each sampling site ina dark polyethylene bottle and processed in thelaboratory for NO2-N, NO3-N, NH4-N, and sol-uble reactive phosphorous (SRP) concentrationsfollowing standard methods (APHA 1989,Wetzel and Likens 1991). Conductivity, dis-solved oxygen (DO), and pH were measuredwith portable instruments in the field.

Chlorophyll a was used as a measure of pe-riphyton standing crop. Seven microscope slides(25 3 75 mm) were stacked over the substratesurface at random locations within each site.Slides were left in the field for ;60 d to provideenough time for algal colonization and replacedwith new ones at the time of sample collection.Upon retrieval, slides were wrapped in alumi-num foil and frozen for later analysis. Within 30d of collection, slides were soaked in 90% ace-tone at 48C for 24 h, and chlorophyll a was ex-tracted and determined by spectrophotometryfollowing the procedure outlined in Wetzel andLikens (1991).

Invertebrates

Sampling. Benthic invertebrates were sam-pled every 2 mo, beginning in November 1996,at selected sites representative of the upper (S1)and middle (S5) reaches of the river (Table 2).In addition, we sampled 9 more sites betweenApril and May 1997 (Table 2). Three randomsamples were collected from each substrate typeat each site 3 date combination (total n 5 348).Substrate categories were determined as fol-lows: boulder .256 mm, cobble 64–256 mm,coarse gravel 16–64 mm, fine gravel 2–16 mm,sand ,2 mm. We also included 2 qualitative cat-egories: detritus and aquatic macrophytes. Notall substrates were available for sampling at allsites (Table 2) because substrate composition ofsites varied along the river.

Fine gravel and sand were sampled with anEkman grab (225 cm2) to a depth of 5 to 10 cm,and the material collected was passed througha 0.25-mm-mesh net. Coarse gravel and cobbleswere sampled with a Surber sampler (900 cm2)with a 0.25-mm mesh, disturbing substrates toa depth of 5 to 10 cm. Boulders were sampledby scraping and brushing an area of 314 cm2

into a 0.25-mm-mesh net, the lower edge ofwhich was held against the boulder surface. De-tritus and vegetation were sampled by collect-ing materials from a 450 cm2 area into a 0.25-mm-mesh net. All samples were preserved in90% ethanol. In the laboratory, samples werewashed through nested sieves (mesh sizes: 4.0mm, 1.0 mm, and 0.3 mm) and invertebrateswere removed under 23 magnifier lenses and




preserved in a solution of 75% ethanol and glyc-erine. Sorted material was routinely checked formissed animals (,10% of total), which wereadded to the processed sample. We subsampledthe material in the 0.3-mm-mesh sieve using aFolsom plankton splitter when .500 individualswere present.

Dry mass measurements. Dry mass (DM) ofeach collected individual was estimated frommeasures of the body area using taxon specificarea–DM relationships (Bernardini et al. 2000).Relationships were calculated from a randomsubsample of individuals for which both areaand DM were measured (Solimini 1999). Bodyareas were measured with an image analysisprogram (Image Tool/2.0, University of TexasHealth Science Center, San Antonio), after im-ages of animals were digitized using a framegrabber (Miro Video DC20) connected to a vid-eo camera (Sony Handycam). Dry mass was de-termined with a Sartorius balance after ovendrying at 608C for 7 d. Invertebrates weregrouped into 19 log10 size classes ranging from2 mg to 5.2 3 105 mg DM (class upper limit).

Data analysis

We used Generalized Linear Modeling (GLM)(Neter et al. 1996) to examine the effects of sub-strate, sampling date, and site on invertebrateabundance per size class. We ran 2 analyses.The 1st analysis accounted for mass, date, site,substrate, and interactions as sources of varia-tion and included only sites S1 and S5, whichwere sampled every 2 mo. The 2nd analysis in-cluded all sites sampled in spring only. In bothGLM analyses, linear, quadratic, and cubicterms of size were entered into the model toaccount for curvilinearity after orthogonaliza-tion with the Gram–Schmidt procedure to min-imize multicollinearity (Legendre and Legendre1998). Abundance and mass, (mass)2, and(mass)3 were log10 transformed to stabilize thevariance and linearize the relationship; sam-pling dates and sites were entered into the mod-el as dummy variables. A small constant (7.5)equal to ½ the minimum density we could de-tect in the grab samples was added to densityraw data before the log10 transformation.

Results

Water quality declined downstream; lower-reach sites (S10 and S11) had the highest values

of NO2-N, NO3-N, NH4-N, and SRP and the low-est concentrations of DO (Table 3). Differencesin water-quality variables among other siteswere less evident and were obscured by intra-site annual variability. Periphyton chlorophyll agradually increased between upper- and mid-dle-reach sites, and decreased again at S9 andthe lower-reach sites (S10 and S11).

We measured ;80,000 invertebrates out of the;215,000 collected, and obtained size spectrafor 348 samples. Almost all the spectra collectedin spring were unimodal, with the mode at 64to 128 mg, a minimum size of 2 mg, and a max-imum size of 0.5 g (Fig. 2). Taxa compositionand total abundance differed among upper,middle, and lower reaches (Fig. 3). In the upperreach, the assemblage was composed mainly ofinsects (.80% of all organisms in a given sizeclass). In the middle reach, gastropods weredominant in the smaller size classes, whereasoligochaetes, crustaceans, and insects weredominant in the medium and larger size classes.In the lower reach, oligochaetes, crustaceans,and insects were dominant in all size classes.

Sampling date, site, and substrate accountedfor a small portion of the variability in abun-dance per size class. In the 1st GLM analysis(only sites S1 and S5, over 6 sampling dates,Table 4), the model with mass alone explained48% of the variance of invertebrate density. Ex-plained variance increased by 13% when weadded date, site, and substrate to the model (R2

improved from 0.48 to 0.61, Table 4), and by 8%more when we included all 2-way and 3-wayinteractions (R2 improved from 0.61 to 0.69, Ta-ble 4). Invertebrate abundance at both sitespeaked in July (Fig. 4). In the 2nd GLM analysis(all sites, but in spring only, Table 5), the modelwith mass alone also explained a large portionof the variability of invertebrate density (53%).Explained variance increased only 4% when weincluded site and substrate in the model (R2 im-proved from 0.53 to 0.57, Table 5), and by 9%more when we included all 2-way and 3-wayinteractions (R2 improved from 0.57 to 0.66, Ta-ble 5).

The abundance of invertebrates per size classwas highest on boulders, detritus, and (less of-ten) on cobble at upper-reach sites (Fig. 5). Thisgeneralization was not possible at middle-reachsites because abundance per size class peakedon different substrates depending on the site. Inparticular, abundance peaked on detritus and




TABLE 3. Annual mean (61 SD) of water-quality variables (n 5 13) and periphyton chlorophyll a (Chl a, n5 6) recorded from 1996 to 1997 along the Aniene River. DO 5 dissolved oxygen, SRP 5 soluble reactive P.

SiteChl a

(mg/m2) pH

Conduc-tivity

(mS/cm)DO

(mg/L)SRP

(mg/L)NO2-N(mg/L)

NO3-N(mg/L)

NH4-N(mg/L)

S1 7.1(1.6)

8.2(0.2)

409(3)

9.7(0.3)

0.29(0.09)

0.002(0.0004)

0.63(0.07)

0.02(0.01)

S2 10.3(1.4)

8.2(0.2)

309(4)

10.0(0.2)

0.19(0.04)

0.004(0.0008)

0.82(0.09)

0.03(0.01)

S3 12.7(3.8)

8.2(0.2)

338(4)

10.0(0.4)

0.42(0.18)

0.003(0.0004)

0.90(0.07)

0.02(0.01)

S4 17.5(5.0)

8.4(0.1)

340(1)

10.0(0.2)

0.25(0.14)

0.005(0.0006)

1.07(0.10)

0.03(0.01)

S5 14.7(2.7)

8.2(0.1)

374(4)

9.2(0.4)

0.28(0.06)

0.009(0.0018)

1.05(0.09)

0.09(0.03)

S6 25.9(7.4)

8.0(0.1)

351(3)

9.5(0.3)

0.21(0.07)

0.007(0.0018)

0.95(0.09)

0.06(0.02)

S7 33.0(11.2)

7.0(0.1)

802(12)

8.2(0.2)

0.33(0.12)

0.004(0.0006)

0.77(0.08)

0.05(0.02)

S8 24.7(6.4)

7.6(0.1)

518(13)

9.0(0.2)

0.33(0.15)

0.010(0.0017)

1.04(0.07)

0.05(0.01)

S9 5.5(1.7)

7.8(0.1)

516(11)

9.0(0.3)

0.37(0.19)

0.011(0.0013)

1.12(0.08)

0.10(0.02)

S10 7.2(4.2)

7.2(0.1)

1071(75)

6.9(0.4)

0.68(0.21)

0.027(0.0043)

1.14(0.08)

0.31(0.04)

S11 3.2(3.2)

7.3(0.1)

1173(68)

6.5(0.3)

0.49(0.09)

0.055(0.0041)

1.44(0.06)

0.75(0.15)

sand at S5 (Fig. 5); on boulder, fine gravel, andsand at S6; on aquatic vegetation and detritus atS7; on coarse gravel and detritus at S8; and onfine gravel and sand at S9. Substrates in the low-er-reach sites showed similar patterns, but thetotal abundance on sand at S11 was by far thelargest of the samples collected in this study(Fig. 5).

Discussion

Longitudinal variation of size structure

Size structure in the Aniene River was stableamong sites, with small departures from the av-erage spectrum at lower-reach sites (Fig. 2).Other authors have shown that the shape of thesize spectrum is similar among streams (Morinand Nadon 1991, Cattaneo 1993, Bourassa andMorin 1995, Morin et al. 1995), but our datashowed that this regularity also holds along thelength of a river, despite dramatic changes ingeology, land use, substrate composition, waterquality, and periphyton standing crop. Thesechanges were clearly reflected in the taxonomic

composition of the size spectra (Fig. 3), with anincrease of noninsect taxa between upper-reachsites and middle-reach sites, followed by anabrupt loss of taxa in the lower-reach sites. Us-ing a more detailed taxonomic analysis, Fochettiet al. (1998) showed that the similarity of AnieneRiver invertebrate assemblages among upper-and middle-reach sites was ,30%, and ,15%among middle- and lower-reach sites. Neverthe-less, the size structure was very similar amongreaches, reinforcing the hypothesis that size-de-pendent processes may structure benthic com-munities (Strayer 1991, Rodriguez and Magnan1993).

Total invertebrate abundance per size classwas more variable among sites than the shapeof the size structure. In particular, abundance ofall size classes was higher in the middle reachthan in the upper reach, probably because of thelarger quantity of periphyton available in theformer. A positive correlation between periph-yton and total abundance per size class has al-ready been reported (Morin et al. 1995, Bour-assa and Morin 1995). Even higher abundance




FIG. 2. Average size spectra in spring 1997 at each sampling site and average spectrum by reach. Points aremean (61 SD) density of invertebrates in each size class when all samples are averaged by site (n 5 18 forupper-reach sites, S5, S7, and S9; n 5 15 for other middle-reach sites; n 5 6 for lower-reach sites).

was observed in the lower reach in size classesbetween 100 and 1000 mg DM, corresponding tothe size classes where large Chironominae andTubificidae were very abundant. However, theresponse of the size spectrum to water pollutionwas not consistent in the lower reach. Site S10showed less abundance per size class but highernumbers of large individuals (mostly taxa tol-erant to organic pollution such as Gammaridae,Asellidae, and Erpobdellidae), resulting in a flatsize spectrum. In contrast, S11 showed a nar-rower spectrum because of the absence of large-sized taxa. The resulting average size spectrumfor the reach was similar to spectra of otherreaches.

Variation among substrates

Invertebrate size structure in lakes differsamong macrophyte and sediment substrates(Hanson et al. 1989, Hanson 1990, Rasmussen1993; but see Rodriguez and Magnan 1993 for

a different result). By comparison, invertebratesize structure in streams is similar on all sub-strates, with some variability in the meiofaunalrange of the size spectrum (Morin et al. 1995,Bourassa and Morin 1995). In our GLM analysis,substrate type was significant (Tables 4, 5), butadded little to the total explained variance, sup-porting the view that substrate has low influ-ence on the shape of size spectra.

Total abundance per size class was more var-iable among substrates than the size spectrumshape (Fig. 5). The pattern of variation reportedhere illustrates the combined effect of site andsubstrate type. In the upper-reach sites, theabundance of invertebrates peaked on coarsesubstrates such as boulders and cobbles, where-as in the middle and lower reaches invertebrateabundance peaked, with minor exceptions, onfine substrates such as fine gravel and sand.Generally, the abundance of invertebrates is ex-pected to be highest on medium to large sub-strates such as cobbles or coarse gravel (see the




FIG. 3. Relative abundance of dominant taxa ineach size class along the Aniene River in spring 1997.Points represent the density of each taxon relative tothe total density per size class in that reach.

TABLE 4. Results of sequential General LinearModeling (GLM) analysis on invertebrate abundanceper size class at site S1 (upper reach) and S5 (middlereach). All variables were statistically significant (p ,0.001, n 5 4104). Mean square error (MSE) and coef-ficient of determination (R2) are shown for each step.

Variable MSE R2

Log10(mass), log10(mass2), log10(mass3)DateSubstrateSiteDate 3 siteSite 3 substrateDate 3 substrateMass 3 dateMass 3 substrateMass 3 siteMass 3 date 3 substrateMass 3 date 3 siteMass 3 site 3 substrate

0.4510.3970.3660.3430.3320.3290.3180.3040.2990.2910.2900.2850.279

0.4820.5440.5800.6070.6190.6240.6380.6540.6610.6700.6730.6790.687

FIG. 4. Average size spectra at each sampling date(n 5 18) for sites S1 (upper reach) and S5 (middlereach). Bars are 61 SD.

review by Minshall 1984). The heterogeneity ofavailable surface of these substrates offers pro-tection from predators (Williams and Feltmate1992) and a large accumulation of organic mat-ter (Rabeni and Minshall 1977). The peaks ofinvertebrate density recorded on boulders at theupper-reach sites probably reflected the fact thatboulder surfaces were heavily covered by mossand associated organic matter, forming highlyattractive patches for invertebrates. However, inmiddle and lower reaches, boulders were nolonger covered by moss, and sand was richer in




TABLE 5. Results of sequential General LinearModeling (GLM) analysis on invertebrate abundanceper size class at the 11 sampling sites in spring 1997.All variables were statistically significant (p , 0.001,n 5 3021). Mean square error (MSE) and coefficient ofdetermination (R2) are shown for each step.

Variable MSE R2

Log10(mass), log10(mass2), log10(mass3)SubstrateSiteSite 3 substrateMass 3 substrateMass 3 siteMass 3 site 3 substrate

0.4160.4050.3790.3430.3310.3160.306

0.5290.5410.5720.6180.6310.6490.664

FIG. 5. Average size spectra by substrate at one up-per- (S1), one mid- (S5), and one lower-reach site (n5 18 for S1 and S5; n 5 3 for S11). Bars are 61 SD.

organic matter (A. G. Solimini, unpublisheddata). Bourassa and Morin (1995) reported thatfine gravel supported as many invertebrates ascoarser substrates, mostly because of the pref-erence for this substrate by the numericallydominant chironomids. In the Aniene River, chi-ronomids, as well as oligochaetes, gastropods,and Gammaridae were very abundant on sand.Finally, invertebrate abundance also peaked ondetritus at many sites in both the upper and themiddle reaches. This result was expected be-cause invertebrates use detritus accumulationsas a food source, as a shelter from excessive cur-rent velocity, and as a refuge from predators(Dobson 1994, Murphy et al. 1998).

Temporal variation

Temporal variability of size distribution hasbeen examined in 2 other studies on lake (Han-son et al. 1989) and stream communities (Morinet al. 1995). The latter revealed that, even if sizedistribution is affected by recruitment of smallorganisms in the warmer months, the averagedistribution varies little among dates. Our dataconfirm the observation by Morin et al. (1995)because, in the 2 sites where samples were col-lected through the year, average spectra bydates were similar in shape, with some peaks inwarmer months (Fig. 4). These peaks probablyreflected life cycles of dominant invertebratespecies and were not consistent between the 2sites because of the different assemblage com-position. In fact, at S1 the assemblage was al-most entirely composed of insects whose life cy-cles are strictly constrained by the adult phase,

whereas at S5 noninsect taxa were also abun-dant.

Comparisons with previous studies

The shape of the biomass size distribution ofthe Aniene River invertebrate assemblage wassimilar to that observed in previous studies, butit was shifted to the right (Fig. 6). However, thisresult may arise from the different methodsused in processing samples. In particular, themesh size used in collecting and sieving benthic




FIG. 6. Biomass size spectra of the invertebrate as-semblage in the Aniene River (Italy, this study), theYealm River (Britain, Ramsay et al. 1997), GooseCreek (United States, Poff et al. 1993), and Outaouaisstreams (Canada, Morin 1997).

samples can influence the lower end of the sizespectrum (Rodriguez and Magnan 1993). Oursampler mesh size was 0.25 mm and the finestsieve mesh size was 0.3 mm, compared to the0.063-mm-mesh size for both the sampler andthe finest sieve used by other investigators, sowe probably underestimated the invertebrateabundance in the small size classes. We can es-timate that the first 4 size classes of our spectraare biased, assuming that the 0.3-mm-meshsieve retains all animals .2 mm length (L) (58 mg DM using a width to length ratio of 1:7and the coarse estimate of DM 5 L3; Morin andNadon 1991). This estimate is conservative be-cause an unknown proportion of individuals ofa size smaller than the screen mesh is usuallyretained during the sieving process (see Ramsayet al. 1997). Therefore, although we could notreadily assess the invertebrate density in themeiofaunal size range, similarities of the AnieneRiver size spectrum with other streams and riv-ers in the macrofaunal size range appear real,and what causes this convergence deserves fur-ther study.

Acknowledgements

This work was partially supported by a grantfrom Simbruini Regional Park (Convenzione trail Parco Regionale dei Simbruini ed il Diparti-mento di Biologia, Universita di Roma Tor Ver-gata) and by a MURST grant to G. Carchini; A.G. Solimini was supported by a PhD scholarship

of the University of Rome La Sapienza. Wethank S. Cataudella and E. Ciccotti for allowingus the use of their lab facilities and A. Ruggierofor periphyton analyses. P. Ramsay, A. Morin,and C. Hakenkamp kindly provided their bio-mass data for the Yealm River, Outaouaisstreams, and Goose Creek. G. Carr, D. Rosen-berg, J. Stephenson, D. Strayer, and 2 anony-mous referees made useful comments and im-proved the manuscript. We are especially grate-ful to A. Morin for invaluable advice, reviews ofthe manuscript, and stimulating thinking dur-ing a short visit of one of us (AGS) to the Uni-versity of Ottawa.

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Received: 24 July 2000Accepted: 17 April 2001



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Size structure of benthic invertebrate assemblages in a Mediterranean river