Long-term changes in phytoplankton community, mainly cyanobacteria in Lake Kasumigaura, Japan

Lake Kasumigaura, the second largest lake in Japan, has faced water quality challenges since the 1970s owing to rapid economic growth, land use changes, and increased wastewater discharge. Moreover, severe cyanobacterial blooms, particularly those of Microcystis, were observed in the 1980s, emphasizing the urgent need for comprehensive ecological monitoring. We present the results of phytoplankton monitoring by the Ibaraki Kasumigaura Environmental Science Center (IKESC) since 2005 and the publication of information on cyanobacterial blooms for citizens since 2012. Our monitoring surveys identified distinct phytoplankton communities in Nishiura and Kitaura, the two main water bodies of Lake Kasumigaura, emphasizing the need for monitoring surveys at various sites in Lake Kasumigaura. Analysis of long-term phytoplankton data from the Lake Kasumigaura Database (National Institute of Environmental Studies, NIES) revealed that Planktothrix is the predominant cyanobacteria in Lake Kasumigaura. At IKESC, we have responded to these potentially socially problematic phytoplankton blooms by releasing the "Bulletin of Cyanobacteria Blooms" in 2012 and the "Bulletin of Cyanobacteria Forecast" in 2020.