Lakes Aha, Baihua and Hongfeng are three artificial Karst reservoirs in the middle of the Yunnan-Guizhou Plateau, China. The variations in microalgal composition, chlorophyll a and net photosynthetic O2 evolution rate from these Karst lakes were investigated. The relationships among total nitrogen, total phosphorus, microalgae compositions, chlorophyll a and net photosynthetic O2evolution rate in the lakes were studied. The microalgal chlorophyll a concentration in Lake Aha correlated with total phosphorus. In contrast, the microalgal biomass and net photosynthetic rate in Lakes Hongfeng and Baihua correlated with total nitrogen. The trophic status and the microalgal composition in the three lakes are influenced by those of water bodies in the catchments, which channel water into the lakes. The variation in the total nitrogen (TN) and total phosphorus (TP) and the ratio of TN/TP in Lakes Aha, Baihua and Hongfeng has not caused large microalgal blooms as are often symptomatic of eutrophication.