Long-Term Eutrophication in Mesotrophic-Eutrophic Lake Kawaguchi, Japan, Based on Observations of the Horizontal Distribution of Profundal Chironomid Larvae and Oligochaetes

The distribution of benthic macroinvertebrates in Lake Kawaguchi (maximum depth 16.1 m; mean depth 9.3 m) was studied on 7 March 2025. The benthic animals identified were aquatic oligochaetes, chironomid larvae, shellfish, and others. The mean density of oligochaetes, which is the dominant group, was 2,457±1,247 individuals/m2, followed by chironomid larvae at 816±391 individuals/m2. The larvae of Propsilocerus akamusi were the most abundant species at 669±358 individuals/m2, followed by Chironomus plumosus at 109±114 individuals/m2. Other chironomids (38±75 individuals/m2) were also captured. Benthic communities were collected at all stations, but each taxa had its own characteristics. Oligochaetes and C. plumosus were widely distributed throughout the lake, whereas the distribution of P. akamusi was skewed toward the western part of the lake. In comparison with previous studies, P. akamusi larvae were now found to be the most abundant chironomid species in this lake, accounting for an increased percentage of the chironomid community, while C. plumosus larvae has decreased in recent years. In addition, the increased levels of organic matter in the upper layer of sediment of this lake suggest that eutrophication is ongoing. Although Lake Kawaguchi was classified as a mesotrophic-eutrophic lake in previous studies, this classification should be reconsidered in light of the circumstances mentioned above. We suggest that this lake be ranked as a eutrophic lake based on a long-term investigation of the changes in chironomid fauna.