Studies on the modern sedimentary environment in Lake Hamana on the Pacific coast of Central Japan.

Abstract

The study dealt primarily with some physicochemical properties of the bottom sediments of Lake Hamana and the behavior of phytoplankton in the lake water and the distribution of pollen grains and spores in the sediments. Lake Hamana is divided into three regions; 1) the northern half of the main lake with two adjacent small lakes, 2) the southern half of the main lake and 3) Lake Shonai. The former two regions of the main lake are divided by the bathymetrical lines of 4 to 8m across the middle of the lake. The lines were revealed to correspond roughly to the isovalue lines of Md φ of 4 to 8φ of the sediments, of ignition loss of 3 to 10%, of total carbons of 1.0 to 3.0%, and of total nitrogens of 0.1 to 0.3%. They also correspond to the isovalue lines of pollen grains and spores of 104 to 5×104 grains per g lyophilized sediment. Almost the same values were observed in the inner part of Lake Shonai as those in the northern half. All of the maximum values of these parameters were obtained in the northwestern part of the main lake and in the inner part of Lake Shonai. These values ranked with the higest levels found in the aquatic areas of Japan. In the present study, the sources of sediments supply to the stations in the main lake, where the accumulation is most evident, was examined using indicators, such as phytoplankton and pollen grains and spores. It was concluded that the main supply source is the river basin of the Miyakoda River. The organic substances and pollen grains and spores might mainly be transported through Lake Inasahosoe by the large amounts of flux of Miyakoda River and accumulated in the northern part of the main lake because of the lack of a vertical circulation due to water stratification, the tidal pressure at the bottom layer of the outer sea water and the existence of the topographical boundary around the middle of the main lake.