大正末期から昭和初期に行われた大橋川拡幅以前の宍道湖の塩分

抄録

Lake Shinji from 1635 up to the time of the widening of the Ohashi River, which was carried out from 1924 to 1933 (from the late Taisyo era to the early Syowa era) has been believed as a fresh water lake. This belief is based on previous reports on the damage from salt after the widening, or a diatom analysis of the core sediments of Lake Shinji. To confirm the validity of this belief, the salinity of Lake Shinji before the widening was investigated from various viewpoints from reports on fishery operation, fishery statistics, sediment analysis and a hydraulic study. From this investigation, we propose that Lake Shinji before the widening was an oligohaline lake, with intermittent saline water intrusion from the adjacent Lake Nakaumi depending on meteorological conditions. The intrusion of saline water was considered to increase in summer with a decrease in fresh water inflow, and made brackish water fishes migrate and inhabit Lake Shinji. In the case of a dry year, marine dinophyceae predominated, which was recorded in the core sediments of the lake. Our results strongly suggest that a single approach to reconstructing past environments can be easily misinterpreted, and that an interdisciplinary approach is indispensable for such purpose.