Phytoplankton community structure in the dry zone reservoirs of Sri Lanka at the declining phase of the tropical monsoon

Abstract

We studied the phytoplankton community structure and their relationship to the physicochemical properties of water by selecting five major freshwater reservoirs; Bandagiriya, Kattakaduwa, Lunugamvehera, Ridiyagama, and Tissa in the Southern dry zone of Sri Lanka during the declining phase of tropical monsoon rainfall (December 2019 to January 2020). Fifty-seven species, dominated by Cyanophyta (63%), followed by Chlorophyta, Bacillariophyta, and Euglenophyta were identified morphologically and through DNA sequencing. The majority of Cyanophyta were filamentous forms (62%) with approximately 88% being cyanotoxin producing species. The species composition of phytoplankton communities is reservoir specific. Microcystis was dominant in Lunugamvehera while the diatom Melosira was dominant in Ridiyagama, Kattakaduwa and Tissa. The lowest phytoplankton density and diversity were observed in Bandagiriya mainly due to high turbidity and total suspended solids which interfere with light penetration through the water column. Physicochemical properties of water were significantly different among reservoirs, leading to reservoir specific correlations between phytoplankton density and physicochemical properties of water which might have been driven by the inter-correlative effects of biotic and abiotic factors at the time of sampling. Therefore, the interactive effects might be responsible for the observed variations in phytoplankton community composition. Thus, the present study provides important information on the phytoplankton community structure at the onset of successional episodes in five tropical freshwater reservoirs in relation to their spatial variations in hydrological regimes and physicochemical properties. Such data would provide essential information for planning and implementation of reliable and efficient strategies for monitoring, sampling, forecasting, and managing algal blooms.