Seasonal variations of dominant phytoplankton, chlorophyll a and nutrient levels in the pelagic regions of Lake Biwa.

Abstract

In order to clarify the present trophic status of Lake Biwa, the largest, monomictic lake in Japan, seasonal variations of dominant phytoplankton, chlorophyll a and nutrient levels were surveyed for offshore waters of its north (main) and south (subsidiary) basins from February to December 1981. Annual means of Secchi disc transparency in the north and south basins were 4.8 and 1.7 m, respectively. In both basins, diatoms were most predominant in winter and early spring, and green algae in other seasons. Besides these algae, Uroglena americana and Anabaena macrospora formed dense blooms for short periods. Two peaks of chlorophyll a concentration in the surface water were observed in both basins, one in May and the other in October. Annual mean concentrations of chlorophyll a in the surface waters of the north and south basins were 9.1 and 10.1 g El-1, respectively, while those of total phosphorus were 9.2 and 16.7 g El-1, respectively. High concentrations of nitrate plus nitrite nitrogen were detected in both basins except for three months from August to October. It was concluded that phosphorus was the most limiting nutrient for phytoplankton growth for the most part of the year, but nitrogen seemed to be also limiting during August to October. Trophic degrees of both basins were intermediate between mesotrophic and eutrophic in terms of chlorophyll a and total phosphorus concentrations.