Macrozoobenthic community structure in the channel segment, Negombo estuary, Western coast of Sri Lanka

Abstract

Ctrannesl r:gmenot f the Negomboe stuary (7o6'- 7ol2'N, 79o49'- 79o53'E) is very importantf or the continuation of estuarine functrons through exchange of water between sea and the estuary, but are subjected to changes due to vario'rs anthropogenic activities such as planting mangroves, boat and fishing gear operations and discharge of industrial effluents. A survey was carried out in August 2005 to study the community structure of the macrozoobenthic coinmunity in the channel segment in relationt o the environmentalf actorsa nd antlrropogenica ctivities.E leven stations,a ccuratelyr ecorded using a hadd held GPS were sampledu sing a Petersong rab and macrobenthosw ere separatedb y wet sieving ancl identified up to the lowest possible level. At each sampling site, the depth and salinity were measured and the presence of sea grasses and mangroves was recorded. The anthropogenic activities that are carried out at each sampling site such as disposal of sewage and fishing were also noted. I'he diversity of macrobenthos was determined using Shannon-Wiener and Pielou's indices respectiveiy.T he similarities amongt he macrobenthicc ommunitiesa t different sites were determined using Bray-Curtis similaritv coefficient, ordinations of Non-parametric Multidimensional Scaling (MDS) and the BIO-ENV function was used to relate the multivariate community structure to environtnentavl ariablesa nd to determinet he most responsiblev ariablesf or the inter-sitev ariability of the benthic community using PRIMER-5 software package (Version 5.2.2). Thirty-two species of invertebratecso nsistingo f 3 specieso f polychaetesl,4 specieso f gastropods,'7s pecieso f bivalves and 8 specieso f crustaceansw ere recordedd uring the presents tudy.N eritids andH ydrobiirls were the mosi abrndant gastropods whiie Venerids wbre the most abundant bivalves. Salinity of water changed from 17 to 30g/kg and the water depth changedifrom 0.3 to 2.5 m. Highest diversity (2.031) was recorded in association of sea grasses and lowe$t (0) in highly pblluted site near Dutch canal. As indicated by Bray-Curtis similarity index and MDS ordination, two sample sites located at the mouth of the estuarya nd upper left region of Munnakkate clearly separatedfr om the other sites which rnay be explainedb y the presenceo f trvo specieso f biValvesi n theses ites. Resultso f the Spearmanra nk correlationc oefficients for permutationso f envirohmentalv ariableso f the BIO-ENV function indicate that the combination of abundanceo f sea grassesa nd mangr.ovesm ostly affect the abundanc"a :id diverslry, .rlm acrobenthosi n the channels egmento f the Negombo estuary