Seasonal variations of Microcystis populations in sediments of Lake Biwa, Japan

Seasonal variations of colony numbers of Microcystis aeruginosa (Kutz.) Kutz. and M. wesenbergii (Komarek) Komarek in N. V. Kondrat. in sediments of Lake Biwa were investigated over a period of 1 year. At two stations located in the shallow South Basin of Lake Biwa (ca. 4 m water depth), the colony number of Microcystis fluctuated seasonally. The number had a tendency to gradually decrease from winter to early summer, while it increased through mid-summer and autumn. Since the Microcystis population in sediment was rather small, intensive growth and accumulation in the water column should be important for the formation of Microcystis blooms in Lake Biwa. Microcystis colonies in the sediment samples after June were observed to be floating in a counting chamber under a microscope. The observation suggests that the recruitment of Microcystis colonies into the water column mostly occurs in early summer. The number of Microcystis colonies in the deep North Basin of Lake Biwa (70 - 90 m water depth) was larger than in the South Basin. Because the seasonal variation of colony numbers was not observed in the North Basin, and Microcystis cells do not have gas vesicles, these colonies will not return into the water column. The colonies isolated from the sediment of the North Basin were able to grow in cultured conditions, in the same way as those from the sediment of the South Basin. Therefore, Microcystis colonies may survive for a long time under stable conditions of low temperature (ca. 8 degreesC) and darkness, in the sediment of the deep North Basin, accumulating gradually each year.