Studies on the Sedimentation in Lake Biwa from the Viewpoint of Lake Metabolism

Abstract

SummaryThe rate of deposition at every ten meters depth was measured by using so-called sediment-trap in Lake Biwa, the largest lake in Japan. The rate of deposition near bottom ranged from 3–6 g/m2/day as dry weight in April and May to 0.1 g/m2/day in winter, in rather good agreement with the amount of water outflow of the lake. The yearly rate of deposition was calculated as 208 g/m2/year. From spring to summer, the maximum values were obtained in upper layers, then the rate was decreased with depth. In autumn, the maximum values were found in deeper layers, while no marked change was observed in winter. There was almost no relationship in vertical distribution between the rate of deposition and the amount of seston.A considerable amount of organic nitrogen still remains in deposit and receives the decomposing process during its sinking in water, whereas, the most of organic phosphorus is decomposed rapidly in trophogenic layer. Consequently, the larger part of phosphorus in deposit is composed of inorganic phosphorus, such as that of in the silicate.The age of bottom sediment was estimated from the yearly depositing rate of silicate in deposit and the contents of silicate in the core sample of sediment. The yearly rates of decomposition in bottom sediment were calculated as 680 mg/m2/year for the nitrogen and 130 mg/m2/year for the phosphorus.The annual total amounts of organic nitrogen in deposit which had been taken at 20 m and 50 m depth were only ⅛ and 1/20 of its supply out of the primary production and the inflowing seston. furthermore, only one fifth of the organic nitrogen in deposit which had reached the bottom surface remained as permanent sediments. In the case of phosphorus, the changes were not so clear. It can be said that the most of the decomposing process takes place in the trophogenic layer, a result confirming that one of Ohle (1962).