Seasonal Diurnal and Vertical Distribution Patterns of Cyclopid copepods in Kotmale Reservoir

Abstract

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