A preliminary survey on macrobenthic communities in Nilwala river and Madu Ganga estuaries in Sri Lanka

Abstract

Macrobenthos are sensitive organisms to water quality and are thus used as indicators of ecosystem health. This study investigated the abundance and diversity of macrobenthos in relation to water quality parameters in Nilwala River (5°57′N, 80°32′E) and Madu Ganga (06°16′25″N, 80°02′05″E) estuaries in Sri Lanka. Triplicate samples were collected from each estuary representing the lower river stretch (L1) and two other locations about 2 km (L2), and 20 km (L3) upstream from L1 from March to October 2022. Dissolved Oxygen, biological Oxygen demand, chemical Oxygen demand (COD), pH, nitrate, orthophosphate, ammonium, turbidity, total dissolved solids, conductivity, and temperature were measured using standard methods. Macrobenthos sampling was done in scoop or surface core samples from the first 10 cm of the bottom sediment and sieved (300 μm-mesh) to separate components for identification. Biodiversity indices, namely, species diversity, species richness, and evenness were calculated using Shannon Weiner’s diversity index (H), Simpson’s dominance index (S), Margalef’s richness index (D), and Pielou's evenness index (J). Forty-seven species of macrobenthos were identified in both locations, while species from families Thiaridae (Mollusca: Gastropoda) and Naididae (Annelida) were the highest. The highest density (526-1824 ind. m-2) and richness (7.06-12.87) were recorded from Nilwala estuary while the highest species diversity (0.1-0.56) and dominance (0.34-0.86) were recorded from Madu Ganga estuary. Pollution Tolerance Index (PTI) values revealed that lower streams in both Nilwala river and Madu Ganga estuaries are highly polluted (6 and 3 respectively; PTI<10 indicates poor quality) compared to the respective upper stretches of the rivers (PTI: 14 and 18). Canonical Correspondence Analysis confirmed that phosphate content, nitrates, COD, and turbidity have a significant correlation with the pollution status of both estuaries.