Preliminary Report on Diel Changes in Dissolved Oxygen Concentration in Filamentous Green Algal Mats in Littoral Zone of Lake Biwa

During the summer time (June - August) of 1994 and 1995, filamentous green alga Spirogyra sp. largely dominated the littoral zone in the north basin of Lake Biwa. Algal mats which were mainly composed of Spirogyra had a thickness of over 20 cm. Diel changes in dissolved oxygen concentra-tion were measured in the Spirogyra community to predict the effect of Spirogyra growth on the lake's littoral ecosystem. A decrease in dissolved oxygen concentration was clearly observed at night. It was suggested that the reduction in dissolved oxygen concentration by the Spirogyra community had a harmful influence on other organisms at this study site.