Abstract
In early summer of 1987 the micro alga occured in large quantities as plankton in the eutrophicated Sagami Reservoir of Kanagawa, Japan. The composition of planktonic comunity at that time differed distinctly from that of late years. It consisted almost of the mono-alga that, by its biomass, brough about the yellowish green water bloom during the periodJune to July. In the case of its mass occurence, thenumber of cells per 1 ml was about 2 million. Thealga was studied in the living stage from the various viewpoints of taxonomy and defined by the following features.
Colonies 4-celled to about 32 celled, with cellsplaced at the ends of remnants or strands thatwere formed by the rupture and gelatinization ofthe mother cell wall. Mucilage almost absent, butin the case of presence badly visible. Adult cells spherical 4.0-6.5μm in diameter, chloroplastcup-shaped with pyrenoid. Asexual reproductionthrough autospores. Autospores spherical-oval to spherical, formed 2 or 4.
According to each kind of the observed results, it is satisfactory to consider this micro alga as
Dictyosphaerium pulchellum Wood var. minutum DEFL.1926
It appears to be the first report, in Japan, that thegenus Dictyosphaerium perticipated in the yellowishgreen water bloom in the eutrophic waters.
