Phytoplankton diversity of a floodplain lake of the Brahmaputra River basin of Assam, north-east India

Study of the phytoplankton communities of Ghorajan Beel, a floodplain lake of lower Assam, from two sampling stations during January-December 2010 revealed a total of 52 species with richness maximum for Chlorophyta. Phytoplankton comprised 44.5 +/- 10.9% and 36.7 +/- 9.6% of net plankton abundance at stations 1 and 2 respectively. Chlorophyta showed quantitative dominance; while Bacillariophyta > Cyanophyta were sub-dominant groups and Dinophyta > Euglenophyta > Chrysophyta exhibited low densities. Both richness and abundance recorded significant monthly variations, with the latter also registering significant temporal variations between the sampling stations. The phytoplankton community showed high species diversity, high evenness index and low dominance index. Various abiotic factors exerted influence on richness and abundance of phytoplankton and their constituent groups. Canonical correspondence analysis (CCA) with eight abiotic factors explained high cumulative variance (73.6%) of phytoplankton assemblages at station 1 while seven abiotic parameters explained 69.9% cumulative variance at station 2.