Introduction
Biological approaches to evaluating water quality in-volve assessing communities of organisms. “The basis for this approach is that different species have varying tolerances to environmental stressors” [1]. Fish produc-tivity of water bodies is connected to primary production by many intermediate trophic links. The four groups of organisms that appear in The European Water Frame-work Directives WFD (Phytoplankton, Zooplankton, Fish and macrophytes), represent water ecological struc-ture over a range of temporal and spatial scales and func-tional roles. It was recommended that the above biologi-cal indicators and, in addition to a range of supporting hydro-morphological and physico-chemical elements should form the core of any monitoring program on lakes [2]. Seasonal changes in mean temperature, radiations, hydrology and nutrient availability are the most impor-
tant variables which determine plankton abundance [3]. Also the qualitative and quantitative estimates of the plankton provide good indices of quality and productive capacity of water.
The estimation of phytoplankton density, productivity and trophic status of lakes is very important for fisheries management especially in Nigeria because of dominant tilapia fish culture. The trophic status of a water body is usually estimated by values of primary production meas-ured for the growing season. Classification of lakes based on quantitative trophic indicators such as phos-phorus (P) concentration, chlorophyll-a and transparency also allow the trophic status of lakes on a large gradient to be defined [4]. However, these parameters are not al-ways relevant for short-term evolution within any given lake, especially when variations in phosphorus concen-tration are low. The values of chlorophyll-a (μg•l−1) are mostly used as the basic criteria, because it is relatively