MECHANISMS ON THE OCCURRENCE OF MICROCYST IS BLOOM AT HARBORS IN THE NORTH BASIN OF LAKE BIWA

Late summer in 1994, Microcystis bloom (dominant species: Microcystis aeruginosa) occurred for the first fime at two harbors in the north ba sin of Lake Biwa. The limit values of TN (total nitrogen) and TP (tota l phosphorus) for the occurrence of Microcystis bloom were ca 30 and 3 mu g at/l, respectively. By comparing with the water quality factors of the harbors where Microcystis bloom occurred and the factors of har bors where Melosira bloom (dominant species: Melosila granulata) did, it was showed that TN, TP, DIP (dissolved inorganic phosphorus) and TN : DIN (dissolved inorganic nitrogen) tended to be higher and TN: TP an d DIN: DIP ratios tended to be lower in the harbors where Microcystis bloom occurred. As the cause of the bloom of Microcystis in the north basin of the Lake Biwa, it was suggested that the suitable environment conditions for the quick growth of Microcystis was brought by the con tinuation of hot and clear weather in 1994. The increase of DIP exudat ion from bottom sediments, the intense consumption of DIN by phytoplan kton, the disappearance of DIN due to nitrification and denitrificatio n near the surface of bottom sediments might cause the increase of DIP and TN: DIN ratio. Further, the relative increase of DON (dissolved o rganic nitrogen) and the inorganization of DON by the symbiotic bacter ia might supplied DIN to Microcystis, as many bacteria with same form were found at the middle layer of mucilage slim of colonies of Microcy stis.