Trends of Cyanobacterial Blooms in the Brackish Lake Shinji －Transition of about 40 Years since 1976－

Abstract

Cyanobacterial blooms caused by the genus Microcystis have been confirmed to occur in Lake Shinji. In this study, we used preserved specimens of the genus Microcystis to measure cell diameter and intercellular distance. Differences in cell diameter were observed, with M. ichthyoblabe being 3.3 to 4.0 μm and M. aeruginosa being 4.5 to 4.9 μm. The distance between M. aeruginosa cells was relatively small with an average of 1.1 to 2.8 μm, whereas that between M. ichthyoblabe cells was from 0.9 to 5.7 μm. It was found that there are cases where they are densely packed and where they are relatively dispersed. Regarding the water quality in the upper layer of Lake Shinji when cyanobacterial blooms occurs, the average Cl⁻ concentration for two months before the occurrence is often less than 2,000 mg L^-1. It became clear that the PO₄-P concentration tended to be higher when cyanobacterial blooms occurred.