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The distribution of benthic macroinvertebrate on 8 March 2009 was studied in Lake Saiko (73.2 m maximum depth). The average density of the benthic community for the entire lake was 19 583 individuals m−2, being comprised of oligochaetes and chironomid larvae, with densities of 18 163 (92.8%) and 1274 (6.5%), respectively. The average wet weights of oligochaetes and chironomid larvae were 15.96 and 1.67 g m−2, respectively. The oligochaetes inhabited the entire lake bottom, with their densities being higher in the transitional region (20–40 m) and deeper region (>40 m) than in the shallower region. However, the densities of chironomid larvae were low in the deeper region (>40 m), with only a few chironomid larvae being found in the centre of the lake (>60 m). The most abundant species (Micropsectra chuzeprima) exhibited the widest distribution (from 10.1 to 65.5 m depth), followed by Polypedilum nubeculosum. The results of this study also were compared to previous data on oligochaetes and chironomid larvae reported by Kitagawa (1973). The density of these animals increased throughout the whole lake in 2009. However, the chironomid species compositions did not changed. The larval distribution pattern of chironomid changed since Kitagawa’s study, being distributed from 11 to 68 m in 1973. In contrast, almost all chironomid larvae were found in the transitional (e.g. M. chuzeprima) and shallower (e.g. Procladius choreus and P. nubeculosum) regions in 2009, with only a few individuals being observed in the deeper regions (>60 m). In contrast, the oligochaete density increased in the deeper regions. Large environmental changes must have affected the oligochaetes and chironomids densities, especially in the deeper regions with low dissolved oxygen concentrations. Consequently, the distribution pattern of oligochaetes and chironomids in the lake has changed. These study findings suggest the lake is experiencing increasing eutrophication.

The chironomid fauna in the deep lake, Lake Sai, Japan, was investigated in March, 2009. Thirteen (8 males and 5 females) chironomid midges were collected with a sweeping net around the boat house and 81 (44 males and 37 females) chironomid midges emerged in the laboratory. They were identified as 11 species belonging to four subfamilies, i.e., 7 species of Chironominae, 1 species of Orthocladiinae, 1 species of Prodiamesinae and 2 species of Tanypodinae. At the deepest Station 4 (65.5 m in depth), we could not collect chironomid midges. However, only Micropsectra chuzeprima Sasa, 1984 was collected at Station 5 (56.6 m). At Stations 6 (29.6 m) and 3 (18.8 m), two species, M. chuzeprima and Procladius choreus (Meigen, 1804), were collected. On the other hand, at the shallowest Station 1 (4.9 m), we collected nine chironomid species, and the most numerous species in this collection was Polypedilum nubeculosum (Meigen, 1804) with a total of 41 individuals collected. In previous reports, a total of 16 species were collected from around the lake. We additionally describe 10 species from Lake Sai in this study. Only one species, P. nubeculosum, was common to the earlier report and the present study. As a result, there are 26 species recorded from Lake Sai., Article, Medical Entomology and Zoology. 62(4):243-247 (2011)