Bottom environments causing the extinction of macrobenthic fauna in the stagnant period in Lake Hamana on the Pacific coast of central Japan.

Abstract

The surveys were carried out at 22 stations in Lake Hamana including the satellite lakes from June through October, 1981 and in April, 1982. The three regions of the lake that were divided by the degrees of median diameter (Mdφ) of the sediments coincided with the regions characterized by the other properties of the sediments and also by the macrobenthic community. Region A; The southern half of the main lake and the southern parts of Lake Shonai with the phi values of sediments of less than 4φ, Region B; The central parts of Lake Shonai with those of 4 to 8φ, and Region C; The northern half of the main lake with the two adjoining lakes and the inner parts of Lake Shonai where the values were more than 8φ. The fauna never became afaunal in regions A or B. However, in region C, the concentration of organic substances in the sediments was extremely high. And the mean values of bottom dissolved oxygen contents of less than 1.4mg/l and redox potentials in the sediments of less than -150mV were observed in June in the region where the macrobenthic fauna was poor, and became completely extinct from July through Setpember. The afaunal state continued till October. Therefore, those levels of dissolved oxygen contents and redox potentials of the bottom in June could be numerical indicators of the onset of the extinction of macrobenthic fauna. The causes which would result in the extinction of the fauna were also discussed in connection with the formation of a thermo-halocline in region C.