Limnological responses to changes in the thermal mixing regime in Lake Biwa associated with global warming

Abstract

In Lake Biwa, restoration of water quality was achieved through the control of phosphorus discharges from the drainage basin. Recently, additional concerns have arisen regarding accelerating hypolimnetic deoxygenation and changes in the flora and seasonal periodicity of phytoplankton. Based on the analysis of the monitoring survey data, the effects of the changes in lake water mixing due to global warming on phytoplankton dynamics were examined using the seasonal [NO₃] changes in surface water as a marker of the vertical mixing of water column. Autumnal increases in the surface water [NO₃] started one month earlier after 1990. It was concluded that earlier onset of the fall overturn after 1990 was due to warming of the hypolimnion by warm winters after 1990. Because of diverse impacts of climate change and anthropogenic activities on lakes, and less predictability of their cumulative effects, increased knowledge of these effects is an urgent requirement for the progress of limnological science under global warming.