Horizontal distribution and long-term trends in population densities of chironomid larvae in Lake Kitaura, Ibaraki prefecture, Japan

In February 2000, a survey of the horizontal distribution of chironomid larvae was carried out at 60 points in the shallow eutrophic Lake Kitaura situated in the eastern margin of the Kanto region, central Japan. A total of 13 taxa of chironomid larvae were captured with an Ekman-Birge grab in this survey. Larval density (mean +- SD) was 709 +- 458 inds. m(-2) in sandy sediment areas (6 points), in which the abundant species were Polypedilum spp. (57.1%) and Lipiniella sp. (17.1%). On the other hand, the larval density at the stations, other than those in sandy sediments (54 points), was 297 +- 235 inds. m(-2), where three dominant species, Propsilocerus akamusi, Clinotanypus sugiyamai, and Tanypus punctipennis represented 32.3%, 30.3%, and 23.9% of all larvae at these stations, respectively. Chironomus plumosus larvae (an indicator species in eutrophic lakes) showed very low densities at all stations. To examine the collection efficiency of chironomid larvae using the Ekman-Birge grab in the sandy sediment areas, we collected sediment samples from 3 points in each area location using a core sampler in May 2003. It developed that, in the case of Lipiniella sp. larvae, only about 4% of actual larval density could be collected with the Ekman-Birge grab. This finding shows that most Lipiniella sp. larvae inhabit depths in the sandy sediment of this lake than lower the depth at which collection with an Ekman-Birge grab is possible (0-3 cm). Long-term trends in the density and species composition of chironomid larvae in Lake Kitaura were examined using data from previous research reports. The larval density had peaked in 1980, and gradually declined thereafter. Dominant species in the profundal zone of this lake seemed to have changed from C. plumosus and P. akamusi to C. sugiyamai. We discussed the factors involved in the decline in chironomid larval density.