Progress of eutrophication and change of chironomid fauna in Lake Yamanakako, Japan

Benthic macroinvertebrate distribution was studied on April 9, 1994, and March 2, 2003, in Lake Yamanakako. The average density of the benthic community for the entire lake was 3168 ind. m−2, comprising principally oligochaetes (41.0%) and chironomids (59.0%) in 1994. In 2003, the benthic community for the entire lake was 1847 ind. m−2, principally consisting of oligochaetes (69.9%) and chironomids (30.1%). In 1994, the larval density of Propsilocerus akamusi was 3.5 times that of Chironomus nipponensis and in 2003 the figure was 5.7 times. However, the larval biomass of P. akamusi was 2.1 times greater than that of C. nipponensis in 1994 and 2.8 times greater in 2003. The larval density of Tanypodinae decreased drastically, by about 12-fold, from 1994 to 2003. P. akamusi larvae were particularly abundant at the lake center in 1994, but they inhabited the entire lake bottom in 2003. P. akamusi density was closely related to water depth and ignition loss. C. nipponensis larvae also showed the widest distribution pattern in 2003, whereas their larvae had inhabited the northeastern parts and the lake center in 1994. Recently, the number of C. nipponensis larvae in Lake Yamanakako is tending to decrease, whereas that of P. akamusi larvae is increasing, suggesting ongoing eutrophication.