AbstractIn Japan, there was an unusually cool and rainy summer in 1993, and an unusually hot and dry summer in 1994. We investigated chemical species in the waters of Lake Biwa during these two years. Samples were collected vertically from stations off Omimaiko in the mesotrophic northern basin and at a dredged area in the eutrophic southern basin, and surface water samples were also collected from three other stations at about one month intervals. We also studied chemical species in precipitation collected at Otsu, on the southeast shore of the southern basin, and sediment cores collected from the southern basin. The features of the lake water were very dependent on the summer climate. In 1993, abundant rain and weak thermal stratification supported high nutrient concentrations in the epilimnion. In 1994, nutrients were depleted and a high transparency and low concentration of chlorophyll a were observed in the epilimnion of the northern basin. This is because the supply of nutrients was limited owing to little rain and a well‐developed thermocline. However, a great deal of dissolved reactive phosphorus (DRP) occurred until late August in the southern basin. Local anoxic conditions were extensively developed in 1994, since a large amount of macroalgae grew and regressed in the southern basin owing to a high water temperature (30°C) and decline in the water‐level. Presumably DRP was desorbed from ferromanganese oxides that were reduced in the anoxic sediments and the DRP found its way into the lake water. Although it has been reported that the concentration of phosphorus in the lake water had not significantly increased in the last two decades, phosphorus which had steadily accumulated in the sediments had a crucial effect on the quality of the lake w