Characterisation of tropical reservoirs in Tamil Nadu, India in terms of plankton assemblage using multivariate analysis

Abstract

The ecological status of reservoirs in Tamil Nadu, India was studied in view of their importance in culture based fisheries. The abundance and distribution pattern of plankton assemblages with respect to important water parameters was selected to characterise the reservoirs. Sampling was carried out seasonally in 22 reservoirs widely ranging in trophic state, during 1993 to 2004. A definite distinction was observed between Western Ghats (WG), rain shadow (RS) and plain land reservoirs (PL) based on plankton communities and environmental factors analysed. Western Ghats reservoirs lying at the head stream of rivers tend to show chemically oligotrophic to mesotrophic status with characteristic dominance of phytoplankters viz., chlorophyta, Selenastrum spp., Ankistrodesmus spp., bacillariophyta, Navicula spp., Nitzshia spp. and Synedra spp. and zooplankton, rotifer, Brachionus spp. and copepod, Diaptomus spp. Eutrophic to hyper-eutrophic status with dominant presence of cyanophyta, Microcystis aeruginosa in all the seasons and lesser presence of Spirulina spp. was observed in many plain land reservoirs while others with low anthropogenic pressure were sub-dominant with bacillariophyta, Nitzshia spp., Synedra spp. and Navicula spp. and chlorophyta Scenedesmus spp. and Selenastrum spp. Blooms of Microcystis aeruginosa occurred in shallow plain reservoirs receiving agricultural, industrial and municipal run-off during monsoon. Rain shadow reservoirs depicted early eutrophy stage as estimated through Carlssons Trophic State Index, represented by species of two groups, chlorophyceae and myxophyceae, distribution being influenced mostly by habitat characteristics. Principal component analysis (PCA) suggested that secchi disc transparency and nutrients were important variables determining the ecological status of the reservoirs. The overall division of the reservoirs is explained by species environmental relationship using BIOENV (Primer 6), which described the positive correlation of temperature, specific conductivity and hardness to plankton composition and numerical abundance. Some plain reservoirs move out of their group to assume a solitary position exhibiting different plankton composition with respect to hydrological characteristics.