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.
