PRELIMINARY SURVEY OF BIOLOGICAL INDICATORS IN KANDY LAKE

Kandy Lake is an ornamental fresh water body in Sri Lanka, which was built bythe last king of the Country, the King Sri Wickrema Rajasinghe between 1810 and1812. The quality of the lake water has been investigated since 1979 and theseresults indicated that the lake faces the problem of eutropication. The appearanceof Microcystis bloom during mid April 1999, indicated the severity of the organicpollution. A large number of effluent canals drain in to the lake carrying acontinuous flow of sewage and domestic waste matter. The existing informationsuggests high nitrates and phosphate concentration in the lake.The objective of the present study is a preliminary survey of plankton species,which can be used as biological indicators to determine the water quality an thelevel of eutropication of the lake. A hypothesis has been tested on preliminarybasis, which has to be improved through detailed experimental investigations infuture.Field investigation were carried out in Kandy Lake from Jan.-June 2000.Information collected was analyzed to determine Physico-chemical properties ofthe lake water, composition of plankton species, their relative abundance andrelationship between nutrients and prominent plankton species.Majority of the total plankton density was composed of phytoplankton comparedto zooplankton density of the lake. Therefore it is impossible to use top downcontrol of algal biomass using zooplankton in Kandy lake as in many othertropical water bodiesBacillariophyceae was the most abundant group during most months of the year.Melosira spps also contributed significantly towards the biomass of lake plankton.The Most prominent plankton spps of the lake was Microcystis aeruginosa whichbelongs to the group Cyan bacteria. There was a direct correlation, betweenMicrocystis number and the amount of phosphate present in the lake water, whichcould be used in developing a bio indicator determining trophic status of lakes.At the beginning of the study both. Microcystis density and phosphate levelswere high. With the decrease of phosphate level, Microcystis density decreasedand Melosira species gradually became the domenant species.The Zooplankton composition was made up of Copepods, Cladocerans androtifers. Species composition was high among rotifers and a clear-cut seasonalvariation was observed among them. The majority of the zooplankton was smallin size and their influence on the phytoplankton community could be consideredas negligible due tohigh phytoplankton biomass. All the zooplankton recorded inthe study belongs to the category of typically tropical zooplanktons.There is a significant relationship between Microcystis density andOrthophosphate level which could be used in determining status of eutropicationof the lake