Community structure of zooplankton and trophic status of some inland reservoirs in the low country intermediate zone of Sri Lanka

Diversity and abundance of zooplankton in relation to nitrate–N and total phosphorus levels of 20 minor and medium scale reservoirs in the low country intermediate zone of Sri Lanka were studied in June-October 2013. Total phosphorus content ranged from 0.029 mg L-1 recorded in Wendaruwewa reservoir to 0.529 mg L-1 recorded in Karangamuwa reservoir and Nitrate-N content varied from 1.186 mg L-1 recorded in Adukkane reservoir to 5.625 mg L-1 recorded in Anukkane reservoir indicating that all reservoirs studied are eutrophic. During the present study, a total of 43 species of zooplankton, comprising 31 species of rotifers, 9 species of cladocerans and 3 species of copepods were recorded. Number of zooplankton species in any one reservoir, which ranged from 4 to 13, was significantly correlated with the nitrate-N content (p<0.05). The most widely distributed zooplankton species was Diacyclops nanus, which was present in 15 reservoirs. Abundance of zooplankton ranged from 4 individuals L-1 recorded for Alona monocantha and Chydorus parvus in Wendaruwewa and Metiyagane reservoirs respectively to 1017 individuals L-1 recorded for Filinia terminalis in Galewela reservoir. Margalef index which ranged from 0.532 in Adukkane reservoir to 2.096 in Dewahuwa reservoir, was significantly correlated with nitrate-N content of water (p<0.05). Pielou index ranging from 0.65 in Galewela reservoir to 0.92 in Polpitiya reservoir and Shannon-Weiner index ranging from 0.91 in Munamaldeniya reservoir to 2.13 in Dewahuwa reservoir were not significantly correlated with nitrate-N and total phosphorus levels of water (p>0.05). Keratella quadrata was present only in Anukkane reservoir where the nitrate-N content was the highest and therefore it may be considered as a bio-indicator of high levels of nitrate-N. Eutrophic conditions of theses reservoirs may possibly be due to runoff of nutrients from agricultural fields as these are located in an intense agricultural area. Therefore, it is necessary to take immediate action to control the overuse of inorganic fertilizer in this area so that contamination of water bodies with nutrients can be minimized.