2023 Volume 46 Issue 2 Pages 47-60
In the aerobic layer in the upper part of meromictic Lake Abashiri, blue-green algae grow and Anabaena has easily been the dominant species in summer since the 1950s. However, in recent years, other blue-green algae such as Microcystis are coming prevalent. We carried out an investigation of water quality and phytoplankton community up to three times a month in order to clarify the reason for this change. Dissolved inorganic nitrogen (DIN) loading to the aerobic layer was greatly affected by the influx of river water, and dissolved inorganic phosphorus (DIP) loading to the aerobic layer was greatly affected by the supply from the anaerobic layer. It was found that DIN is easily depleted in the aerobic layer in summer, and the amount of Anabaena, which is capable of atmospheric nitrogen fixation, easily increases. However, it is considered that when the effect of river water becomes large, the amount of DIN loading increases and the amount of blue-green algae other than Anabaena easily increases. Recent changes in agricultural and other activities in the basin and an increase in river flow rate, which results in greater DIN load in the influent river water, are considered to be the main factors behind the changed in the dominant species of blue-green algae in Lake Abashiri.
