AN AUTUMNAL BLOOM OF THE DIATOM MELOSIRA-GRANULATA IN A SHALLOW EUTROPHIC LAKE - PHYSICAL AND CHEMICAL CONSTRAINTS ON ITS POPULATION-DYNAMICS

Temporal changes in population densities of algae and concentrations of chlorophyll-a, particulate organic carbon, and both soluble inorganic and particulate organic fractions of Si, N and P, were surveyed in summer and autumn 1991 in the south basin of Lake Biwa, Japan, at three stations different in the nutritional status. Phytoplankton community during the survey period was characterized by over-whelming dominance by a diatom (Melosira (syn. Aulacoseira) granulata (EHRENB.) RALFS), which was presumably brought about by exceptionally windy weather in this year. Diatom population density and concentrations of biogenic silica and chlorophyll-a were higher in the more eutrophied stations. It seemed unlikely that the diatom population growth was limited by the supply of silicate, except in a short period in October at the station of median nutritional status. Rather, an observation that diatoms tended to decrease in August when dissolved inorganic nitrogen was commonly depleted suggested the diatom population size to have been determined principally by the nitrogen availability. A comparison of relative turnover periods among soluble inorganic N, P and Si supported further that the growth of phytoplankton as a whole was limited by nitrogen supply during August and the first half of September, and by phosphorus supply in the other period. Some potential factors are discussed which would determine the availability of nutrients during the successional passage from summer to autumn in this basin.