Seasonal diurnal and vertical distribution patterns of cyclopid copepods in Kotmale reservoir

Vertical distribution of cyclopid copepcds was studied in Kotmale reservoir,in order to determine the variation in zooplankton at different depths. Theirseasonal distribution and migratory patterns were also studied. Vertical and horizontal sampling were done at monthly intervals fromAugust 1990 to December 1991. Two diurnal samplings were done; onewhen water level was low (40m) and the other when it was high (62m). The two cycIopid species identified were Mesocyclops leukarti andThermocyclops crassus. CycIopid adults and their immature stages were abundant at all depths.However their population density decreased with increasing depth. Highestdensity was found within the range of surface to ten meters. They showed striking diurnal migration which was independent of theintensity of sunlight. The migratory pattern of adults and young was notmuch different. The seasonal variation observed did not show the same pattern duringboth sampling years. During the latter part of the study (October to December1991)a large decrease in cyclopid population was observed which was correlatedwith the collapse of green algae population and the appearance of algal bloomcaused by blue green algae (Microcystis aeruginosat.