Poster Presentation World Lake Conference 2025

Lake Nishinoko is the greatest inner lake located in the east bank of Lake Biwa (Shiga Prefecture, Japan). In late years, with eutrophication, mass outbreak of the phytoplankton including the outbreak of water-bloom is often seen. This phenomenon occurs harmful effect to freshwater pearls framing and gaining mold smells to tap water. Therefore, improvement of the quality of the water is demanded. In this study, we have monitered investigation into phytoplankton (from April, 2021 to March, 2024) in the west lake heart of Lake Nishinoko. In addition, we examined a relationship of the changing weather around the quality of the water, nutritive salts and with cyanobacteria. Few years after we begin an investigation, the TN/TP ratio tended less than 20 in Lake Nisinoko April through November. For example, a state with density of NO₃-N less than 0.01 mg/L continued from June through September in 2022, and, about a phytoplankton phase. In this term, PO₄-P increased from 0.038 mg/L to 0.133 mg/L. During this term, a tendency to decrease had the TN/TP ratio, and the Dolichospermum (Anabaena) genus was the environment that did predominance at the time. With increase of NO3-N, the Planktothrix (Oscillatoria) genus changed afterwards in October when the TN/TP ratio increased when predominance was to environment to do. In addition, the Planktothrix genus came dominant spices at the time of the investigation in August in 2023, but NO₃-N dried up afterwards, and the Dolichospermum genus changed into the environment that coming dominant spices at the time of the investigation in September. It was suggested by here in a tendency to increase and the quality of the water of the cyanobacteria in Lake Nishinoko that there was linking movement. The inflows of the nitrogen source caused by the seasonal typhoon and heavy rain are considered as an element giving a change in the quality of the water in the summer. In addition, of the stay time by the opening and shutting for downstream water gate of Lake Nishinoko. It is important is not only Lake Nishinoko, and to pay attention to neighboring environment changes. In addition, Shiga prefecture examine various methods for the quality of the water improvement of Lake Nishinoko. We will continue an investigation in future, and we are going to cooperate with the quality of the water improvement activity.

 Fig. Results of water quality and cyanobacteria