Assessing the roles of artificial vs. natural impacts on brackish lake environments: foraminiferal evidence from Lake Nakaumi, southwest Japan

Abstract

During the 20th century, many Japanese brackish lakes have been significantly changed by human activity such as construction of water barrier gates, dikes, and coastal embankments. In addition to these modifications, sea level changes have influenced brackish biotas. However, few reports have discussed artificial and natural interactions in these brackish lakes. To evaluate these artificial and natural influences, the environment of Lake Nakaumi has been reconstructed from the 1920s to the present using foraminiferal analysis. Four periods of environmental changes can be recognized by factor analysis: 1920s-'30s, 1940s-'50s, 1960s-'70s, and the 1980s to the present. These four periods show that both man-made and natural impacts are significant for brackish biotas. When both impacts combine, the changes became more significant. Originally less mobile lower waters of Lake Nakaumi became activated during the middle 1940s-early '50s, coinciding with the development of the Ammonia and Milliolinella assemblages, which are the most diversified biotas of the 20th century. This period of environmental change is associated with sea level rise in the Sea of Japan. A large-scale land reclamation project produced significant changes in Nakaumi waters from the middle to late 1970s on. Topographically, Lake Nakaumi is almost closed. It was further enclosed by artificial construction. This created stable lower waters below a well-developed halocline, permitting occupation of eutrophic Nakaumi Proper Water (NPW) as indicated by the development of the Trochammina hadai assemblage.