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 o f chlorophyll-a, particulate organic carbon, and both soluble inorgani c and particulate organic fractions of Si, N and P, were surveyed in s ummer 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 t his year. Diatom population density and concentrations of biogenic sil ica and chlorophyll-a were higher in the more eutrophied stations. It seemed unlikely that the diatom population growth was limited by the s upply of silicate, except in a short period in October at the station of median nutritional status. Rather, an observation that diatoms tend ed to decrease in August when dissolved inorganic nitrogen was commonl y depleted suggested the diatom population size to have been determine d principally by the nitrogen availability. A comparison of relative t urnover periods among soluble inorganic N, P and Si supported further that the growth of phytoplankton as a whole was limited by nitrogen su pply 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.