Large-scale national and prefectural projects have existed to reclaim the marginal area of Nakaumi and Shinjiko Lakes during the past several decades. Original brackish environment may have changed with these reclamation projects associated with many artifictil constructions, which affect directly or in directly life forms living in the lakes. We examined this hypothesis in six columnar sediments (20cm long) from the viewpoint of foraminiferal ecology and paleoecology.
Critical environmental changes were found in two different levels below lake floor, one of which is 4-7cm and the other is 10-17cm. In the frist upper level, a distinct increase of individual number of Ammonia beccarii type 1, which forms the A. beccarii type 1 Assemblage, was recognized. The A. beccarii type 1 Assemblage replaced the Haplophragmoides canariensis Assemblage, in the second deeper level. Ecology of these assemblages indicates the progressed increase of bottom water salinity. These levels are estimated about 15 years and 60 years before the present, respectively. We compared these foraminiferal events with known historical records and found a good correlation between the foraminiferal events and the construction times of artifacts.
We conclude that the sediments and bottom waters of Shinjiko Lake changed their physicochemical characters in association with a consistent salinity input, which was introduced by human activities. Such environmental changes are easily detectable by using foraminiferal analysis, rather than chemical analyses of brackish water.