Ciclo anual del fitoplancton en el lago de Rio Cuarto, Costa Rica (ING)

The annual cycle of phytoplankton and nutrients (nitrates and orthophosphates) of Lago Rio Cuarto, Costa Rica, have been studied from February 1984 through March 1985. Water samples were obtained with a Ruttner and Varn Dorm samplers both in the center of the lake and in litoral zones. Algal number were counted using a Palmer-Maloney counting chamber following iodine sedimentation of algal cells. Fourteen species were identified, with a dominance of Mougeottia genuflexa (Chlorophyta). The peak in density of phytoplankton occurred in April and September/October, 1984 was accompained by an increase in nutrient concentration. Seasonal differences in the amount of rainfall and annual period of deeper circulation were the major factors regulating annual cycle of phytoplankton, via their control of nutrient supply. Because of increase of highly competitive species, the Shannon-Weaver index exhibits the lowest values during the growth maxima of phytoplankton occurrence