Tropical high mountain lakes in the Andean area of Ecuador are a special type of lake, that had not been studied. They are cold water lakes situated 3,000 to 4,000 m above sea level, directly on the equator. Limnological investigations were undertaken to determine the succession of phyto- und zooplankton and to elucidate basic limnological processes, such as thermal stratification and mixing processes.
Today Lake San Pablo, the largest lake in Ecuador, is a eutrophic lake due to the input of sewage and other nutrients from the catchment area, which originate from intensive agriculture and land erosion. Some data on water chemistry of the lake are presented. Lake San Pablo is a monomictic lake with a short mixing period during July to September. Stratification of the lake and mixing processes caused by nocturnal cooling are of great significance for the eutrophication, which occurred mainly during the last decade.
Initial data on the phytoplankton, zooplankton and macrophytes are provided, which show that the biocoenosis has low diversity. Considering the high nutrient level, phytoplankton biomass is low.