Trends in the appearance of cyanobacteria and factors influencing the bloom formation of cyanobacteria in two eutrophic reservoirs (Lakes Sagami and Tsukui): 40 years of monitoring

Abstract

Lakes Sagami and Tsukui are reservoirs of nearly equal size, with a short residence time and constructed by connection to the Sagami River. Because of eutrophication of the lakes, cyanobacteria appear every year. However, the occurrence of cyanobacteria in each lake was different. Therefore, in order to investigate these factors, we conducted both biotic and abiotic investigations and compared defined parameters over the past 40 years. Cyanobacteria are found in the surface layer of the lakes. In Lake Sagami, the surface layer was not largely retained and some of the cyanobacteria were found in the outflow water. When the surface layer became disturbed, Dolichospermum dominated, and Microcystis dominated when the layer stabilized due to weather effects. The outflow from Lake Tsukui was taken from a deep layer ( depth 24-32 m ) and the cyanobacteria were not affected by the outflow. Prior to the installation of the aeration systems, Microcystis dominated, and after the installation, the cyanobacteria were dispersed and Nostocales dominated, but Microcystis still dominated when the surface layer stabilized owing to the influence of the weather. As a result, the influence of nutrients and water temperature was limited and the changes in the abundance and dominant species of cyanobacteria may be influenced by the weather, the difference in the outlet structure, and ancillary facilities such as aeration systems. data, Microcystis Nostocales,