Abstract
Bathymetric distribution of aquatic oligochaetes was studied at 8 stations with different depths (mean depth 16.7 ± 9.4 m, min. 5.3 m, max. 29.4 m) in July 2012 in mesotrophic Lake Kizaki, Nagano Prefecture, Central Japan. The average density was 10424 ± 5346 individuals m-2. Oligochaetes were numerically dominant at all sampling stations. High density, sometimes more than 10000 individuals m-2,was recorded at 3 stations (St. 4, 5, and 6) deeper than 18 m. Maximum density was 20561 ± 13864 individuals m-2 at Station 4 (depth 29.4 m, ignition loss of sediment 13.5%, bottom water temperature 8.1 °C and DO of bottom water 0.12 mg l-1). We identified a total of 5 genera and 7 species belonging to three subfamilies: i.e., 1 species of Naidinae, 2 species of Rhyacodrilinae and 4 species of Tubificinae. Dominant species (in order of abundance) were Tubifex tubifex (Müller, 1774) (8550 ± 6317 individuals m-2; 82.0%) and Limnodrilus spp. (L. hoffmeisteri Claparède, 1862 and L. claparedeianus Ratzel, 1868) (1710 ± 1383 individuals m-2; 16.4%). Tubifex and Limnodrilus were obtained from every station, and the density of T. tubifex tended to be higher as the depth became greater, while Limnodrilus spp. was also widely distributed, but with a peak value at a depth of 5.3 m in the shallowest station. From the submerged plant zone, Ophidonais serpentina (Müller, 1773) was collected. An earlier study by Hirabayashi & Hayashi (1994) showed that in 1985 the average density of aquatic oligochaetes was 435 ± 428 individuals m-2, oligochaetes dominated in only four locations and were distributed over the whole lake approximately equally. Comparisons of our data with the earlier studies have shown that oligochaete density has increased greatly according to Hirabayashi & Hayashi (1994). We suggest that a decrease in dissolved oxygen concentration of bottom water was due to an increase in organic matter content of the sediment. As a result, T. tubifex could expand their habitats in the profundal zone.
