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Item Prediction of thermal stratification of the Kotmale reservoir using a hydrodynamic model(National Science Foundation:Colombo, 2005) Abeysinghe, K.G.A.M.C.S.; Nandalal, K.D.W.; Piyasiri, S.Item Studies on the population structure of zooplankton in the Kotmale reservoir(National Science Foundation:Colombo, 1998) Piyasiri, S.; Chandrananda, W.P.N.Plankton samples were collected from Kotmale reservoir, at approxiinatelynionthly in tervals from. August 1990 to August 1991. Investigations were focused on species composition, seasonal variation, vertical and horizontal distribution and the size class distribution in the population structure of' zooplankton. Two species of cyclopoids, two species of calanoids,, nine species of' cladocerans and sixteen species of rotifers were recorded. Among these, the most abundant species duringthe study period were Mesocyclops sp., Phyllodiaptoin tts anime. Ceriodal-Vint'a cormita, Keratella troplea, Filina longiseta, Brachi(MIS caudatus and Tricocerce sinulis. A location at the deepest region of the reservol I' closer to the dam was selected as a representative station to investigate ffic vertical distribution of zooplankton. At this location, the vertical distribution patterns of all the zooplankton were almost similar throughout the year. As this location is situated at the down stream region of the river, it was expected to contain most of the zooplankton of the reservoir. The horizontal distribution of' zooplankton did not show much variation. The nauplii stages and cyclopolds showed hio-hest abundance In the population of zooplankton. However, during most of the months, the adult stages of calanoids and cladocerans were highly abundantItem Species composition and population dynamics of the phytoplankton community in Kotmale reservoir(NARESA:Colombo, 1998) Perera, N.; Piyasiri, S.The present work was carried out in the Kotmale reservoir from April 1994 to April 1996 with the objective of studying the species composition and the seasonal,spatial and diurnal distribution of phytoplankton.The phytoplankton of the reservoir was composed of four major families:chlorophyceae,Bacillariophyceae and dinophyceae.Chlorophyceae dominated the phytoplankton,with staurastrum species forming the major component.The results show seasonal variation in the major phytoplankton types of the reservoir that correlates with the water level.During the wetmonths,Staurastrum species dominated the phytoplankton while in the dry months Microsystis aeruginosa became prominent especially in the upstream region of the reservoir.This periodic occurrence of M.aeruginosa bloom was a prominent feature of the impoundment.Although the filamentous diatom,Melosira species and the dinoflagellate,peridinium cinctum formed an important part of the plankton community,no contrasting periodicity was observed as in M.aeruginosa.A clear vertical and diurnal migration pattern was observed among the major phytoplankton types.Microcystis showed a downward movement during noon,while P.cinctum was very abundant in the surface during early morning.The phytoplankton forms a major part of the food consumed by the fish in the Kotmale reservoir and the knowledge gained through this investigation will help to improve the commercial fishery of the impoundment.