Water pollution indices are commonly used for detection and evaluation of water pollution in Mansagar lake (Jalmahal). The indices are broadly characterized into two parts. The physico-chemical indices and the biological indices. The positive significant correlations were however, observed between algal counts and COD (Chemical oxygen Demand), TKN (Total Kjeldhal Nitrogen) and TP (Total Phosphorus). Thus increased availability of N (Nitrogen) and P (Phosphorus) favored algal multiplication resulting in their higher counts. Twenty two algal species were identified in the composite sample collected during the study period (March 2006- August 2008). The occurrence of certain pollution tolerant species such as Microcystis, Oscillatoria, Chlorella, Scenedesmus, Ankistrodesmus, Fragilaria, Euglena and Phacus clearly indicates a high level of organic pollution. Monthly variations in Boyd values were found higher in the year 2006 (0.5-1.27), 2007 (0.6 - 1.27) in comparison to the year 2008 (1.08-1.26) suggesting mild recovery of the lake. Palmer algal genus index found maximum in the year 2006 (22 ± 6.59) decreased in 2007 (17.75 ± 5.94) and 2008 (14.75 ± 1.03). In contrast to Boyd and Palmer indices, Shannon and Weaver’s index found minimum in 2006 (1.97 ± 0.40) increased gradually in 2007 (2.038 ± 0.34) and 2008 (2.296 ± 0.147).

Phytoplankton; zooplankton; physico-chemical characteristics; species diversity