Abstract
Fauna and seasonal change of larval populations of Chironomidae were investigated in the eutrophic Lake Yunoko from May 1988 to June 1989. A total of 1, 213 adults belonging to 16 species were obtained by rearing of larvae in sediment collected from 10 stations as well as those in epiphytic matter on submerged plants, mainly Elodea nuttallii (PLANCH)ST. JOHN collected from littoral zones. Diclotendipes lobiger (KIEFFER) and Psectrocladius yunoquartus SASA emerged from sediment samples at≤7 m depth, Tanytarsus nippogregarius SASA et KAMIMURA emerged at≤10 m. Chironomus nipponensis TOKUNAGA, collected from all the stations, was the only species emerging at>10 m depth. C. nipponensis and T. nippogregarius constiuted 93 % of the total adults emerging. From the epiphytic matter on submerged plants, nine species were obtained, of which D. lobiger, P. yunoquartus and Cricotopus trifasciatus (MEICEN), constituted 96 % of the total.
Annual mean density and annual mean biomass of chironomid larvae at 7 m depth were, 2, 210 m-2 and 3.8 g d.w.⋅m-2, respectively, in which C. nipponensis larvae comprised 1800 m-2 (81%) and 3.5 g d.w.⋅m-2 (93%), respectively. Annual mean biomass of Oligochaeta was as high as 13.7 g d.w.⋅m-2.Population of C. nipponensis consisted of two cohorts which overlapped each other. One emerged in April with earlier emergence in the preceding October. The other emerged during late May to July. A large amount of inflowing hot ground water elevated the bottom water temperature, enabling C. nipponensis to reach maturity within a year.
Five out of 15 species recorded in the 1979-1981 survery were not collected in the present study. Changes in submerged plants in both species composition and biomass might have affected chironomid communities.
