Abstract
Lake water collected in mid-August 1991 at a pelagic site in the south basin of Lake Biwa was poured into 2 1 flasks, enriched with various nutrients (control, phosphate, phosphate plus Fe-EDTA, and phosphate plus nitrate), and incubated at 27°C under continuous light (ca. 2, 500 lux) for 26 days in the laboratory.
In all treatments except P+N, Anabaena affinis, which was scarce (50 cells-ml⋅-1) in the original lake water, became predominant, but final cell densities of A. affinis in P and P+Fe-EDTA were several times higher than that in the control. In contrast, several green algae such as Scenedesmus spp., Coelastrum cambricum, C. microporum, Pediastrum duplex and Gloeocystis gigas became predominant in the N+P treatment (at a N: P supply ratio of 20: 1, by weight). These results support the hypothesis that a low nitrogen: phosphorus ratio of lake water is an important factor causing the appearance of Anabaena bloom in the south basin of Lake Biwa.
