composition and distribution of benthic macroinvertebrates in

121 Acta Limnol. Bras., 18(2):121-132, 2006

Composition and distribution of benthic macroinvertebrates inAmericana Reservoir (SP, Brazil).

PAMPLIN1, P.A.Z . , ALMEIDA2, T.C.M. & ROCHA3, O .

1 Depar tamento de Química , Cent ro de C iênc ias da Natureza , Un ivers idade Federa l do P iau í ,

Campus Ministro Petrônio Portel la, SG-02, CEP: 64.049-550, Teresina, PI , Brasi l .

e -mai l : pazpampl in@cl ick21 .com.br2 Laboratório de Ecologia Aquática, Centro de Ciências da Terra e do Mar, Universidade do Vale

do I tajaí , Rua Uruguai 458, CEP 88302-202, I tajaí , SC, Brasi l . e-mai l : t i to@unival i .br3 Departamento de Ecologia e Biologia Evolut iva, Universidade Federal de São Car los, Rodovia

Washington Luís km 235, CEP 13565-905, São Carlos, SP. e-mail : [email protected]

ABSTRACT: Composition and distribution of benthic macroinvertebrates at Americana Reservoir (SP, Brazil).The structure of benthic community at the Americana Reservoir was studied in two distinctperiods of one year comprised between 1997-1998. Field sampling was conducted in dryand rainy periods in 15 and 13 sites, respectively. Nutrient (nitrate, nitrite, ammonium, totalphosphorus and orthophosphate) analyses revealed that the reservoir is in advancedeutrophication process. Bottom fauna of the reservoir was composed by 19 invertebratetaxa, belonging to the fol lowing faunist ic groups: Gastropoda, Ol igochaeta, Hirudinea,Dip tera (Chaobor idae, Ch i ronomidae, Cera topogonidae and St ra t iomyidae ) andEphemeroptera. Chironomidae was the richest group, being represented by 6 taxa. Themajority of species showed low densit ies, being that the oligochaetes formed the mostimportant components of benthos densities, and Limnodrilus hoffmeisteri was the dominanttaxa, representing almost 50% of total benthos. Comparing both periods, major densitieswere registered in dry season for most taxa. Cluster analysis revealed a longi tudinalgradient with total densities decreasing from riverine portion towards the dam. The actualstructure of benthic community at the Americana Reservoir is probably a consequence ofthe advanced state of environmental degradat ion. In addit ion, the existence of exot icspecies, represented by the gastropod Melanoides tuberculata and the leech Barbroniaweberi, can also be contributing to the biodiversity losses in this community.Key-words: benthic macroinvertebrates, eutrophication, biodiversity, exotic species.

RESUMO: Composição e distribuição de macroinvertebrados bentônicos na represa de Americana (SP,Brasil). A estrutura da comunidade bentônica da represa de Americana foi estudada emdois períodos distintos. As coletas foram realizadas nos períodos seco e chuvoso, respec-tivamente, em 15 e 13 estações de amostragem. A análise da concentração de nutrientes(nitrato, nitrito, amônia, fósforo total e ortofosfato) revelou que a represa encontra-se emavançado processo de eutrofização. A fauna de macroinvertebrados bentônicos da repre-sa foi composta por 19 taxons, pertencentes aos seguintes grupos: Gastropoda, Oligochaeta,Hi rud inea, D ip tera (Chaobor idae, Chi ronomidae, Ceratopogonidae e St ra t iomyidae) eEphemeroptera. Chironomidae foi o grupo com maior riqueza, estando representado porseis táxons. A maioria dos táxons ocorreu em baixas densidades. Os oligoquetos forma-ram o maior componente da fauna bentônica e Limnodrilus hoffmeisteri foi o táxon domi-nante, representando cerca de 50% do bentos total . Comparando ambos os per íodosamostrados, as maiores densidades foram registradas no período seco para a maioria dostáxons. A análise de cluster revelou um gradiente longitudinal, com a densidade total dafauna bentônica diminuindo da região lótica em direção à barragem. A estrutura atual dacomunidade bentônica na represa de Americana é provavelmente uma conseqüência doadiantado estado de degradação ambiental. A ocorrência de espécies exóticas represen-tadas nesse reservatório pelo gastrópode Melanoides tuberculata e pelo hirudíneo Barbroniaweberi, pode estar também contribuindo para a perda de diversidade nesta comunidade.Palavras-chave: macroinvertebrados bentônicos, eutrofização, biodiversidade, espécies exó-t icas .

PAMPLIN, P.A.Z. et al. Composition and distribution of benthic macroinvertebrates...122

Introduction

Freshwater ecosystems are inhabitedby great variety of organisms. Zoobenthosis characterized as a group of invertebrates,which spend at least part of their life cycleat the bottom substrate in the water bodies.In many reservoirs, benthic community isrepresented by three main groups:ch i ronomid la rvae, o l igochaetes andmol lusks. Ka jak ( 1988) and Covich et a l .(1999) have pointed out the importance ofthese organisms in the cycling of nutrientsand in the energy transfer trough aquaticfood-chains. Addit ional ly, they have beendescr ibed as good ind icators o f waterquality (Saether 1979; Lang & Reymond 1996).

Accord ing to Schindler & Scheuere l l(2002) , lent ic ecosystems ( inc lud ingreservoirs) are complex and composed bydis t inc t habi ta ts . As a consequence thecomposition, abundance and distribution ofbenthic organisms can be determined by aset o f b io t ic and ab io t ic fac tors , e . g .compet i t ion, predat ion, food avai lab i l i ty ,substrate type, oxygen concentrat ion andothers (Prat et al., 1992; Jónasson, 1996).

In Brazi l , a country reservoir -oriented(Tundis i e t a l . , 1993) , much l imnologica linformation (both abiotic and biotic aspects)

has been published, especially in the lasttwo decades. On the other hand, studiesconcern ing benth ic communi ty are s t i l lscarce in reservoirs (Valent i & Froehl ich,1988; Moret to e t a l . , 2003) , probably bydi f f icu l t ies in sampl ing procedures andtaxonomic identification, to be solved in thefuture, wi th equipment development andpubl ica t ion o f appropr ia te taxonomica lmanuals.

In sp i te o f th is s i tuat ion , someknowledge regarding the bottom fauna ofAmer icana Reservoi r has been prov ided(Strixino, 1971; Rocha, 1972; Shimizu, 1978;1981 ; Valent i & Froehl ich, 1986; 1988) ,a l though much of these resul ts wereunpubl ished.

The present s tudy is par t o f anextensive environmental and l imnologicalresearch in Americana Reservoir carried outin the last decade. We analyzed thecomposition, abundance and distribution ofmacroinvertebrate benthic assemblages inAmericana Reservoir in two distinct periods.

Study Area

Amer icana Reservoi r (22 o44´S and47o19’W; 530 m a.s.l.) is located in the cen-tral-east region of São Paulo State (Fig. 1).

Figure 1: Map of Americana Reservoir (SP, Brazil), showing of the sampling stations and some morphometricfeatures

123 Acta Limnol. Bras., 18(2):121-132, 2006

Covering an area of 13.8 km2 and with amaximum volume of 106 x 10 6 m 3, thereservoir is placed in the Atibaia River. I twas constructed at the end of the forties inthe las t century for mul t ip le purposes,inc lud ing hydroelect r ic i ty generat ion ,recreat ion, f ishery , i r r igat ion and watersupply.

Placed in a region with high populationdensity and heavy industrial development,both accelera t ing natura l resourcesdegradation, particularly that of the aquaticecosystems. The surroundings of America-na Reservoir are occupied by sugarcaneplantations on the right margin and by urbansettlements in the opposite side, and natu-ra l vegeta t ion is res t r ic ted to somefragments. Regarding the climate, there aretwo d is t inc t per iods : dry win ter (Apr i l –September) and rainy summer (October –March) with an average monthly precipitation51 .5 ± 13 .1 mm and 163.3 ± 43.4 mm,respectively (Coelho, 1993). More detai ledinformat ion regarding the region and thereservoir can be found in Espíndola et al.(2004) .

Material and methods

Three sediment samples were collectedusing an Ekman-Birge grab (15 x 15 cm) in15 sampling stations in two periods: July/1997 and January/1998. In the last period,the la rge and dense macrophyte s tandsprevented sampling at stations E14 and E15.The material was washed through a sievewith 210 µm-mesh aperture and preservedon 10% formalin.

In the laboratory, the organisms weresorted, identified and counted. Oligochaetesand chironomids larvae were slide-mountedwi th Hoyer so lu t ion to ev idence somemorphological characteristics of taxonomicinterest . Organisms were ident i f ied downto genus or species level , was based inguides and keys of Brinkhurst & Marchese(1991) for Ol igochaeta, Tr iv inho-Str ix ino &Strixino (1995) and Epler (1995) for Diptera-Chironomidae, and Merritt & Cummins (1996)for other insect groups. Density and relativeabundance were calculated.

Addi t iona l sediment samples werecol lec ted to determine organic mat tercontent (loss ignition after 6 hours at 550oC)and granulometry (adapted f rom Suguio,1973). Water temperature, pH, conductivityand dissolved oxygen concentrat ion werealso measured at each s i te us ing amultisensor apparatus HORIBA U-10 model.Nut r ient concent ra t ions (n i t ra te , n i t r i te ,

ammonium, to ta l phosphorus andorthophosphate) f rom surface water werequantif ied as described in Koroleff (1976),Golterman et al. (1978) and Mackereth et al.(1978). Water transparency was determinedby Secchi disk disappearance.

The existence of significant differencesamong water or sediment var iab lesbetween the sampling periods was testedby Mann-Whitney test (Zar, 1998). CanonicalCorrespondence Analys is (CCA) wasper formed in order to revea l probablere la t ionships of macro inver tebra tesassemblages and environmental variables(depth, sand, clay+silt , and organic mattercontent in the sediment ) . S igni f icance ofcanonicals axis and variables were verifiedby Mantel test (Ter Braak, 1986; Legendre &Legendre , 1998) . Only taxa wi th re la t iveabundance greater than 1%, in at least onesampl ing per iod, were used in the CCAanalysis (10 of 19 taxa). Biological data werepreviously t ransformed by squared root ,whereas data from abiotic variables werenot transformed.

Results

L imnolog ica l features of Amer icanaReservoir are summarized in Tab. I . OnlySecchi disk readings and the concentrationsof nitrate, ammonium and total phosphatewere signif icantly dif ferent (Mann-Whitneytest) among the periods. The values of pHindicated a s l ight ly acid condi t ion in thedry period; in the rainy period, both acidand alkal ine condit ions were found, withpH values ranged from 5.99 to 9.30. Themin imum conduct iv i ty reg is tered in thereservoir was 82 µS.cm - 1 and the maximumvalue was 240 µS.cm - 1, revea l ing anenvironment r ich in nutr ients. In general ,the h ighest nutr ient concentrat ions wereregistered in the dry season (Tab. I). Nitrateconcentrat ions var ied f rom 1217 µg.L - 1 to1945 µg .L - 1 i n t he d ry pe r iod , and f rom564 µg.L - 1 to 1409 µg.L-1, in the rainy season.Nitrite concentrations ranged from 44 µg.L - 1

to 275 µg.L - 1 and 32 µg.L - 1 to 179 µg.L - 1 inthe dry and rainy periods, respectively. Forammonium, the concentrat ion varied from164 µg.L - 1 to 1520 µg.L - 1 in dry period andfrom 35 µg.L-1 to 744 µg.L-1 in the rainy. Thispat te rn was a lso fo l lowed by the to ta lphosphorus and orthophosphate, althoughmean concentrations were stil l high in therainy period.


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125 Acta Limnol. Bras., 18(2):121-132, 2006

A complex thermal s t ructure wi thseveral micro-stratif ications was observedin Americana Reservoir , especial ly in thera iny per iod (F ig . 2 ) . Water temperatureranged f rom 17.80 oC to 20.70 oC in dryperiod; while in the rainy period, it variedfrom 25.60oC to 32.10oC. In both sampling

per iods, anoxia was observed i n t h erese rvo i r , e spec i a l l y i n t he d ry pe r i od( F i g . 2 ) . The h ighest concent ra t ions ofd issolved oxygen were 5 .37 mg.L - 1 and14.94 mg.L - 1 in dry and ra iny per iods ,respectively.

Figure 2: Profiles of temperature (full circle) and dissolved oxygen concentration (empty circle) in AmericanaReservoir (SP, Brazil). The profiles correspond to sampling stations E6 (upper region), E10 (middleregion) and E13 (dam region) in both periods.


Considering all samples taken, a totalof 5119 benthic invertebrates were countedfor the sediment of Americana Reservoir .The benth ic fauna cons is ted of 19invertebrate taxa, as shown in Tab. II. Dipterawas the group wi th the h ighest speciesr ichness being represented by 9 taxa, 6belonging to the Chironomidae family. BothHi rud inea and Ol igochaeta wererepresented by 4 taxa each, while only one

taxon was co l lec ted for Ephemeroptera(Insecta) and Gastropoda (Mollusca), in thisreservoir. The mean densities in the dry andrainy seasons were 4256 ± 6300 ind.m -2 and2475 ± 2714 ind.m -2, respectively (Tab. I I ) .Tubificid Limnodrilus hoffmeisteri, was themain species, corresponding to 49.9 % oftotal benthos, whi le Branchiura sowerbyiand Dero (Aulophorus) sp. representedabout of 10.5 % each. Other taxa found in

mean density ± s.d. (ind.m-2)

Dry Season Rainy Season

Relative Abundance (%)

in this study

GASTROPODA

Melanoides tuberculata 11 ± 50 138 ± 232 1.7

HIRUDINEA

Helobdella stagnalis 386 ± 1038 51 ± 135 7.5

Helobdella brasiliensis 16 ± 50 2 ± 6 0.3

Gloiobdella obscura 314 ± 623 3 ± 12 5.7

Barbronia weberi 164 ± 623 - 3.2

OLIGOCHAETA

Branchiura sowerbyi 616 ± 1038 209 ± 56 10.5

Limnodrilus hoffmeisteri 1899 ± 5791 1212 ± 2787 49.9

Opystocysta funiculus 86 ± 278 65 ± 120 1.9

Dero (Allophorus) sp. 333 ± 278 35 ± 674 10.5

INSECTA - EPHEMEROPTERA

Campsurus sp 246 ± 517 26 ± 74 3.5

INSECTA - DIPTERA

Stratiomyidae

Labostigmina cf. fenestrata - 2 ± 6 < 0.1

Chaoboridae

Chaoborus sp 34 ± 80 8 ± 14 0.5

Ceratopogonidae

Culicoides sp 9 ± 39 21 ± 74 0.4

Chironomidae

Ablabesmya annulata group 28 ± 69 10 ± 73 0.5

Aedokritus sp 68 ± 264 - 0.9

Chironomus decorus group 28 ± 39 178 ± 499 2.4

Goeldichironomus pictus 13 ± 22 9 ± 25 0.3

Polypedilum sp 3 ± 11 - < 0.1

Tanypus stellatus 1 ± 6 7 ± 25 0.1

Mean density 4256 ± 6300 2475 ± 2714

Richness 18 16

Table II: Composit ion, mean density and abundance of benthos community in Americana Reservoir (SP,Brazil ) , sampled in July 1997 (dry season) and January 1998 (rainy season).

127 Acta Limnol. Bras., 18(2):121-132, 2006

Americana reservoir had relative abundanceslower than 3.5 %, except leeches Helobdellastagnalis and Gloiobdella obscura, with 7.5% and 5.7 % of the total, respectively.

Ol igochaeta was the main group,represent ing 69.12 % and 81.63 % in thedry and rainy periods, respectively (Fig. 3).

Hirudinea occurred mainly in the dry period,being the second group with 22.72 % ofto ta l benthos. Gast ropod Melanoidestuberculata was practically absent in the dryperiod (0.22 %), but increasing to 5.59 % inthe ra iny per iod. In th is per iod, a h ighpercentage of Chi ronomidae was a lso

Figure 3: Relative abundance of the main benthic macroinvertebrate groups in Americana reservoir (SP,Brazil ) , sampled in July 1997 (dry period) and January 1998 (rainy period).

registered, representing 8.22 % of the to-tal, against 2.64 % in the other period.

In both sampl ing per iods , h igh to ta ldensi t ies were recorded in the r iver ineregion, decreasing towards the dam (Fig. 4).This pattern was mainly influenced by the

densities of Oligochaeta and Hirudinea. Inthe dry period, mean density declined from17733 ind.m-2 in the riverine zone (E1 and E2)to 1481 ind.m -2 near the dam; while in therainy period, mean density at the riverineregion was 7844 ind.m -2 and 15 ind.m -2 near

Figure 4: Longi tudinal distr ibut ion of total benthic macroinvertebrates densi ty in Americana Reservoir(SP, Brazil ) , ; sampled in July 1997 (dry period) and January 1998 (rainy period).


the dam. In the dry per iod, Gast ropodadensities were almost similar in all samplingstations (mean density of 11 ind.m -2) ; whilein the rainy period, major densities (up to133 ind.m - 2 ) were reg is tered in sampl ingstations located close to the riverine zone.Regard ing the Chi ronomidae, densit iesaccounted were below 250 ind.m -2, exceptin s i t es E12 (d ry pe r iod ) and E3 ( r a inyper iod ) where the dens i t i es were 2266ind.m -2 and 1422 ind.m -2, respectively.

Canonical Correspondence Analysis(CCA) the two first axes accounted for 24.5%of the variance on the species composition,

of which 86% was expla ined by theenvi ronmenta l var iables (Table I I I ) . Sand(Fcal=4.11; p=0.005), depth (Fcal= 1.90; p=0.07)and o rgan i c ma t t e r ( F ca l= 1 . 84 ; p=0 .075 )were s ign i f i can t va r iab les . In canon ica la n a l y s i s , t h r e e m a j o r g r o u p s w e r epointed out as shown in Fig. 5) . The firstassocia t ion (Group I ) was determinednegat ively by axis 1 , whi le associat ion 2(Group I I ) and 3 (Group I I I ) were def inedposi t ive ly and negat ive ly by ax is 2 ,respect ive ly . Group I gathered r iver inesta t ions (E1 and E2) and had a majorcor re la t ion wi th f ine sediment f rac t ion

Figure 5: Ordination diagram of benthic macroinvertebrates in Americana Reservoir (SP, Brazil ) based ona canonical correspondence analysis (a) sampling stat ion and t ime score representation; and(b) species score representation. Arrows represents the environmental variables.

Axes 1 Axes 2 Accumulated percentage of species 16.5 24.5

Accumulated percentage of variables 57.8 86.1 F calculated – Mantel’s test 4.152 2.092

Mantel’s test p 0.020 0.025

Table I I I : Resul ts f rom Canonical Correspondence Analysis of species composit ion and environmentalvar iables (sand, clay+si l t , organic matter , and depth) . Accumulated percentages of explainedvariations of axes 1 and 2 and environmental variables.

129 Acta Limnol. Bras., 18(2):121-132, 2006

(clay+silt) and organic matter. This group wasre la ted to the abundance of L imnodr i lushof fmeis ter i , Chi ronomus decorus ,Barbronia weberi and Gloiobdella obscura.At the Group II, depth was the major variableand the Branchiura sowerbyi was the mainspecies . The mayf ly Campsuruscharacterized the Group III, that was formedby E12 (both periods) and E13 (only in therainy period), and was associated with thesand sediment fraction.

Discussion

Accord ing to Ravera ( 1996) , thestructure of aquatic communities dependson a setting of environmental variables andon the spec ies adapta t ion to spec i f icenvironments. Water analysis of AmericanaReservoir reveals that the reservoir is aneutrophic ecosystem in accelerated processof degradation. This fact can probably bepointed out as the major cause of significantmodifications in the structure and dynamicsof the benthic fauna of that reservoir , asfound in Kuibyshev Reservoi r, Russ ia(Zinchenko, 1992) and in Pawnee Reservoir,United States (Popp & Hoagland, 1995).

Reservoirs have generally a reduceddiversity of benthic fauna as compared withnatural lakes (Lindergaard, 1995). In Ameri-cana Reservoir, the community structure ofbenthic macroinvertebrates was very simple,had few faunis t ic groups, low speciesnumber in each group and the majority ofthe spec ies occur r ing wi th very lowabundance. The dominant species ,L imnodr i lus hof fmeis ter i , i s an organic -to lerant spec ies , wor ldwide d is t r ibuted.High organic matter content (up to 10% inthe majority of sites) and a poor-oxygenatedenvironment near the sediment contributedto i ts h igh densi t ies as ev idenced bycanonica l ana lys is . Gong & X ie (2001 )observed that the abundance of L .hoffmeisteri in a Chinese shallow lake waspositively correlated with the eutrophicationdegree. According to Mi lbr ink ( 1994) andLang ( 1997) , some aquat ic o l igochaetes( inc luding of L. hof fmeis ter i ) a re ab le tosurvive for prolonged periods of anoxia andhigh organic loadings.

In prev ious s tudies in Amer icanaReservoi r , Sh imizu ( 1978) and Valent i &Froehl ich ( 1988) reg is tered the re la t iveabundance of oligochaetes as 56.52 % and67.18 % of to ta l benthos (exc lud ing

microcrustaceans, rotifers and bryozoans),respectively. The tendency of increase inoligochaetes abundance was verified in ourstudy, where they represented 72.77 % ofbottom fauna. In contrast, chironomids andmollusks populat ions drast ical ly decl inedalong the time. Shimizu (1978) reported thatthe relative abundance of chironomids andmollusks were of 19.85 % and 10,31 %; whilein our study the percentage decreased to4.57 % and 1.74 %, respectively.

Considering the insects, al though atlow densi t ies , they formed the h ighestrichnest group, especially well representedby Chi ronomidae. Th is d ip teran fami lytogether with the oligochaetes, are in factthe most conspicuous and re levantcomponents of benth ic communi t ies inalmost al l f reshwaters. In addi t ion to theenvironmental degradat ion, i t is possiblethat the low abundance of insects found insediment of Americana Reservoir is due tothe preference of these organisms for thefree floating vegetation banks, especiallyof Eichhornia crassipes, Pist ia strat iotes,Sa lv in ia aur icu la ta and Polygonum spp(Tavares et a l . , 2004) , which providediversified habitats for them, with wide foodresources, breeding p laces and refugiaagainst predators (Bechara , 1996) . InAnhumas Reservoir, Corbi & Trivinho-Strixino(2002) reg is tered h igher r ichness anddensities of macrobenthic fauna in placesr ich in aquat ic vegeta t ion. Thepredominance of mayf ly Campsurus instation E12 (especially, in dry period) wasprobably re la ted to low depth ,predominance of sand, low organic mattercontent and h igh d issolved oxygenconcentrat ion.

Rocha ( 1972) and Sh imizu ( 1978)recorded the presence of gast ropodsAncyl idae and Planorbi idae, and bivalvesSphaer idae, in Amer icana Reservoi r .However, they practical ly disappeared atthe end of 70 ’s decade (Sh imizu, 1981 ) .Valenti & Froehl ich (1988) only registeredthe presence of gast ropods, but d id notrefer the families. In our study, the uniquespecies of gastropod found was Melanoidestuberculata, although in low densities. Thisexot ic spec ies might have cont r ibuted,together environmental degradation, for thedisappearance of those nat ive species ofmol lusks. This af ro -as iat ic Thiar idae hasbeen in t roduced in many count r ies ,especia l ly in the Car ibbean area, as acompet i to r fo r na t ive mol lusks tha t a re


vectors of disease (Pointier et al., 1993). InBrazi l , i t was f i rs t ly registered in middlesixties in the city of Santos, São Paulo State(Vaz et a l , 1986) , f rom which rapid lydispersed to other localities, being currentlyfound in many Brazi l ian regions (Simone,1999) .

Longi tud ina l ly , the major i ty o f theinver tebrates of the Amer icana reservoi rpre ferent ia l ly inhabi ted wel l oxygenatedareas, r iver ine zone and upper reservoi rsection. In the middle and near the dam,where anoxic condition in the bottom werereg is tered in both sampl ing per iods,densities were lower and many taxa wereabsent. This result corroborated with otherstudies on benthic fauna that pointed outdepth, dissolved oxygen and substrate, asthe main factors contro l l ing thei r spat ia ld is t r ibut ion (Pet r id is & S in is , 1993; D iGiovanni et al., 1996; Moretto et al., 2003).In Amer icana Reservoi r, spat ia lheterogeneity was also demonstrated forphytoplankton (Falco & Cali juri , 2002) andzooplankton (Zanata & Espíndola, 2002). Forthe former, Ch lorophyceae (main lyMonoraphidium griffithii ) dominated riverinezone; whi le in the dam region,Cyanophyceae (pr inc ipa l ly Microcyst isaeruginosa) had major abundance (Falco &Calijuri, 2002). Regarding the zooplankton,Zanata & Espíndola (2002) have found thatthe dens i ty o f ro t i fe rs decreased f romupst ream towards the dam, whi le thedensi t ies of microcrustaceans (copepodsand cladocerans) increased. As previouslymentioned, the existence of dense standsof macrophytes in the middle and near damareas of Americana reservoir created newhabitats for the bottom fauna.

In summary, we concluded that thestructure (composit ion and abundance) ofbenthic community in Americana Reservoirhas been h igh ly impacted as aconsequence of human activities, especiallyby eut rophicat ion and organic pol lu t ion.According to Zinchenko (1992) and Popp &Hoagland (1995) anthropogenic impacts inf reshwaters ecosystems, especia l ly theincreasing load of materials, is a major factorcorrelated with species loss, particularly ofinsects. The reduction and even extinctionof many bot tom inver tebra tes cancompromise the whole ecolog ica lfunct ion ing of an aquat ic ecosystem,af fect ing the energy t ransfer through thetrophic food chain, and the nutrient cyclingboth by nut r ient excre t ion and mater ia lsuspending to the water column.

Acknowledgements

We acknowledge Dra Alaíde A. Fonse-ca-Gessner, Dr Donald Klemm, Dr RobertoG. Alves, Dra Maria L. Felipe-Bauer and DrLuis R. L . S imone for taxonomicalassistance. First author had financial supportfrom FAPESP (Proc No 97/0377-1).

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Received: 30 March 2006Accepted: 27 June 2006

composition and distribution of benthic macroinvertebrates in

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