Benthic invertebrates of a tropical estuary in the western coast of Sri Lanka.

Spatial distribution, diversity of macrobenthos and their relationships between physico-chemical parameters of the water and sediments were studied in the Negombo estuary (7°6' -7°12'N, 79°49' -79°53'E), western coast of Sri Lanka from January to December 2005. Benthic samples were taken from 37 sampling sites scattered over the estuary using van Veen grab sampler. The depth, pH, temperature, salinity, dissolved oxygen content, nitrate nitrite phosphate contents in the water, sea grass biomass, abundance of mangroves and sediment texture were determined at each sampling site. The similarities among the macrobenthic communities at different sampling sites were determined using Bray-Curtis similarity coefficient and ordinations of Non-metric Multidimensional Scaling (MDS). A total of 9062 benthic invertebrates consisting of 36 species of polychaetes, 24 species of gastropods, 16 species of bivalves and 13 species of crustaceans were recorded. The ranges of Species richness, Shannon Weiner (diversity) index (H'), Simpson index (J') and Pielou's index of the sampling sites were 0-27, 0-2.85, 0-0.95 and 00.87respectively. Low or zero diversity was recorded from deeper areas and high abundance and diversity were noted at the marginal areas of the estuary. Significance differences between groups of sampling sites which were clustered together with the 30% similarity level of the Bray Curtis similarity analysis were noted. The ordinations of Non-parametric Multidimensional Scaling (MDS) indicated that the clustering of sites which were located in the northern region of the estuary was based on the abundance of polychaetes. Spearman rank correlation coefficients for permutations of environmental variables indicated that the combination of salinity, depth, nutrient contents of water and sea grass biomass mostly affect the abundance and diversity of macrobenthos in the Negombo estuary.