A long-term eutrophication process observed from the changes in the horizontal distribution of profundal oligochaete fauna in mesotrophic-eutrophic Lake Kawaguchi, Japan

The purpose of this study was to record the oligochaete fauna in Lake Kawaguchi and clarify their horizontal distribution. In addition, annual changes in the lake were recorded. Samples were collected between 5 March 1993 and 7 March 2006 by using a standard Ekman-Birge sampler at 22 stations, and multipoint sampling surveys were carried out. As a result of the study, a total of 8 species belonging to 6 genera and 3 subfamilies were identified. The average density of oligochaetes for the entire lake was 5247 ± 3873 ind M^(-2) and the average wet weight of oligochaetes was 25.5 ± 23.4 g M^(-2) in 1993, compared to 1075 ± 676 ind M^(-2) and 2.7 ± 1.7 g M^(-2), respectively, in 2006. Tubifex tubifex had inhabited the entire lake bottom in 1993, but Limnodrilus spp. inhabited only the center of the lake in 2006. In recent years, the total number of oligochaetes has shown a tendency to increase, compared with the past record from the 1970s, suggesting that eutrophication is an ongoing process.