Population dynamics of Propsilocerus akamusi and Chironomus plumosus (Diptera: Chironomidae) in Lake Suwa in relation to changes in the lake's environment

Abstract

Lake Suwa is a shallow eutrophic lake in central Japan. We have investigated the long-term population dynamics of chironomids in this lake. The objective of this study was threefold: (1) clarifying temporal variations in the emergence pattern of P. akamusi midges, (2) assessing dynamics in density, biomass and distribution pattern of Propsilocerus akamusi and Chironomus plumosus larvae in this lake, and (3) determining key factors that may affect these variations, as well as comparing the results of this study to those of previous reports. Emergence periods of P. akamusi midges from the lake have tended to become shorter in recent years. Furthermore, the mean daily catch of midges exhibits a clear decrease over time. In contrast, average densities of P. akamusi and C. plumosus larvae in the lake were 73.3±48.0 (SD) and 3.2±9.4 individuals per m2, respectively. Compared to 1986, this amounts to a decrease of about 1/70 and 1/130, respectively. Finally, the organic matter in the upper layer of the sediment has also been decreasing. This suggests that the basic conditions necessary for chironomid larvae habitats, especially quality and quantity of food, seem to be changing in Lake Suwa as water quality improves.