湖水環境の人為的改造と底生有孔虫の群集変化(その3) : 中海東部の過去数10年の環境変化

We report the benthic foraminiferal changes with respect to human activities in brackish environment over several decades. During the past several decades, many reclaimed areas have appeared in the eastern part of Lake Nakaumi, in addition to the dredging of bottom sediments and the construction of a gate for water exchange. The bottom water and foraminiferal fauna might have been influenced directly and indirectly by these human activities .
　The most critical foraminiferal changes were found in the sediments of 60-70 years ago (-1920-30) and in those of 25 years ago (early 1970). We belive the first faunal event was caused by the dredging of sediments in Sakai Strait, which caused salty marine and brackish waters to be exchanged. A 2.8km long embankment constructed in the mouth of Sakai Strait might have facilitated the introduction of marine water into Lake Nakaumi. By this process, Trochammina hadai increased in place of the previously developed Ammonia beccarii fauna in the deeper part of the lake. The second faunal changes might have caused by further introduction of salty water into the main lake area by limiting the opening to Nakaura Gate.
　We attribute the foraminiferal changes to the formation of more stabilized halocline by a consistent input of marine water and the subsequent development of reduced bottom environment.