Water temperature at the shore and center of Lake Kasumigaura during summer

Abstract

　There is a concern that the water temperature in terrestrial areas may increase owing to global warming. Some lakes, such as Lake Kasumigaura, in Japan have experienced an increase in water temperature of approximately 2°C in both summer and winter. Moreover, lakes, such as Lake Kojima, have witnessed a decrease in water temperature in both summer and winter, and the impact of climate change is different in each lake. However, water temperature changes in lakes are mostly observed offshore, especially at the center of the lake. It has not been confirmed whether the effects of global warming on fish and bivalves, that mainly live in lakeshore areas, are from only the water temperature at the center of the lake.
　In this study, we continuously observed the water temperature at five lakeshore sites in Lake Kasumigaura from August to September 2021. The results of the continuous observations showed that the water temperatures observed 182 times at lakeshore sites during the summer exceeded 30°C on an average, whereas the water temperature observed 36 times at the center of the lake was 30°C. Statistical analysis of the water temperature data that exceeded 30°C in August showed that the water temperatures were significantly higher at the three lakeshore sites than at the center of the lake. The daily variations in water temperature were smaller and more stable at the center of the lake; however, the variations were larger at the lakeshore sites, with variations of 7.4°C at some sites.
　As shown in this study, the water temperature and its daily variations differed greatly between the lakeshore sites and the center of the lake. To estimate the effects of global warming on lake ecosystems, it is important to combine the results obtained from lakeshore sites and the center of the lake.