Studies on the population structure of zooplankton in the Kotmale reservoir

Abstract

Plankton samples were collected from Kotmale reservoir, at approximately monthly intervals 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 during the study period were Mesocyclops sp.,Phyllodiaptomus annae, Ceriodaphnia cornuta, Keratella tropica, Filina Longiseta, Brachionus caudatus and Tricocerca similis. A location at the deepest region of the reservoir closer to the dam was selected as a representative station to investigate the 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 cyclopoids showed highest abundance in the population of zooplankton. However, during most of the months, the adult stages of calanoids and cladocerans were highly abundant.