Population pressure, urbanization, industrialisation and increased agricultural practices have significantly contributed to the pollution and toxicity of aquatic ecosystems. Pollutants bring about a change not only in physical and chemical quality of water but also modify the biotic components resulting in the elimination of some, probably valuable species. Eutrophication of our natural ecosystems due to sewage disposal and other human activities has become a common feature these days. It is, therefore, necessary to monitor the trophic level of the aquatic habitats either by chemical or by biological methods. Attempts have been made by many workers to decide the trophic status of water bodies based on phytoplankton groups or species. Microscopic suspended algae or phytoplankton occur in different forms such as unicellular, colonial or filamentous, which are mainly photosynthetic in nature. Zooplankton were found to graze on these phytoplanktons.

Phytoplanktons are one of the most rapid detectors of environmental changes due to their quick response to toxins and other chemicals. Pollution stress reduces the number of algal species but increases the number of individuals. A marked change in algal community severely affects the species diversity (Biligrami, 1988). Eutrophication or organic pollution of aquatic ecosystem results in replacement of algal groups. It has been observed that many species are sensitive to the nutritional loading but equally good numbered are pollution tolerant. A number of reports are available on pollution indicating or pollution tolerant algal species. Similarly, a good number of indices have also been evolved to determine the trophic level of fresh water ecosystems like Nygaard`s algal indices, Shannon and Weiner`s species diversity indices and Palmer`s algal pollution index.

Certain species of phytoplankton grow luxuriantly in eutrophic waters while some species cannot tolerate waters that are contaminated with organic or chemical wastes. Some of the species that indicate clean waters are Melosira islandica, Cyclotella ocellata and Dinobryon. The pollution indicating plankton includes Nitzschia palea, Microcystis aeruginosa and Aphanizomenon flosaquae. The latter two species have been found to produce toxic blooms and anoxic conditions. Some algae were found to cause noxious blooms in polluted water that tastes bad with intolerable odour. Plankton adapt quickly to the environmental changes because of their short life cycles. Their standing crop and species composition indicate water quality. Plankton influence on factors such as pH, colour, taste and odour. This is mainly because of the small size and great numbers. Often their scant distribution along with their transient nature cannot be totally relied upon for assessing the water quality (APHA, 1985).

Algae are said to be simple plants inhabiting various kinds of habitats. Generally they are present in almost all natural water bodies. The nutrient deficient natural water, harbouring low populations of algae, on addition of nutrients increases the growth of algae. The water appear dark green on excessive algal growth or the algal blooms. These water blooms occur in highly enriched waters specially that receiving sewage waste (Trivedy and Goel, 1984).

In the present work an attempt has been made to assess the distribution pattern of phytoplankton in Sharavathi River Basin. Comparative study of various stations of the reservoir (lacustrine ecosystem) and streams (lotic ecosystem) is unique. This type of study is new to the Sharavathi River basin. As the area of the Linganamakki reservoir is about 2,000 sq. km, it is quite possible that there could be some difference in phytoplankton composition among the different reservoir stations. Similarly, as the streams selected for this study are feeding this reservoir and are flowing from different areas of the catchment, there is a possibility of change in the phytoplankton composition in these streams.

Further, as these streams are flowing through the different regions of Western Ghats and feeding the Linganamakki reservoir there could be difference in species composition, diversity, richness and dominance of phytoplankton between reservoir and streams. Thus, the study was intended to know diversity status of phytoplankton among these aquatic ecosystems.

The specific objectives are -