A Hypothesis on Formation of the Subsurface Chlorophyll Maximum Observed in Lake Biwa in September of 1994

Abstract

The vertical distribution of chlorophyll differed in. its pattern between September 1993 when monthly precipitation was 200-350 mm in July and August (similar to an average year) and September 1994 when it was well below the average (8-50mm). Chlorophyll was more abundant in the epilimnion in 1993, while in 1994 it was less in the mixed layer but instead showed a peak in the thermocline. Algal biomass decreased in the mixed layer due to the absence of nutrients from river water under drought conditions in September 1994. As a result, light intensity sufficient for photosynthesis could penetrate into the thermocline. Nutrients available for algae were supplied by physical and chemical processes from the hypolimnion and/or from turbid water transported from the shore-bottom into the thermocline.The subsurface chlorophyll maximum seems to have been formed by in situ growth of algae in the thermocline relatively rich in nutrients under sufficient light conditions.