The once through condenser cooling water for the Kaiga nuclear power plant, Kaiga of Karnataka state is drawn at the rate of 1,25,205 m3hr-1 from the Kadra reservoir and discharged back to the reservoir through a 1.2 km long open channel. The heated effluents reduce solubility of oxygen and accelerate loss of oxygen locally due to increased biological activity. Applications of biological indices is the effective tool for the assessment of qualities of water and biota. During the study period of two years 49 and 22 genera of phytoplankton & zooplankton respectively were recorded at surface waters. Diversity indices indicate oligotrophic nature of the lake. Dissimilarity was more amongst the plankters in between intake and discharge points. Preference of habitats indicates marginal change of rankings of plankton in zones within 500m and beyond 500m from the point of discharge. Studies revealed that there was negative impact of evaluated temperature on plankton upto 500 m from discharge point.