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In order to clarify the current status of the benthic community of Lake Shoji, and to examine the difference with the last quantitative data of the lake by Miyadi (1932) and Kitagawa (1973), the distribution of benthic macroinvertebrates in this lake was studied. Also, a comparison of the benthic fauna and density was made between the present and previous studies, with a discussion concerning the succession of benthic macroinvertebrates in relation to changes in the lake bottom environment. On March 1, 2010, and September 7, 2011, bathymetrical sampling surveys were carried out using a standard Ekman-Birge grab at each of the 7 locations in Lake Shoji. The average densities of the benthic communities for all the stations were 4,107 inds. m-2, comprised principally of oligochaetes 50.2%, chironomids 48.6%, and chaoborid 1.2% in March 2010, whereas in September 2011 the average density was 1,185 inds. m-2, and mostly comprised of oligochaetes 86.6% and chironomids 13.4%. On the other hand, the average biomasses (wet weight) of these benthic macroinvertebrates in March 2010 were composed of 33.3% oligochaetes, 62.8% chironomid larvae, and 3.9% chaoborid larvae, as opposed to 48.8% oligochaetes and 51.2% chironomid larvae, respectively, in September 2011. Propsilocerus akamusi (Tokunaga, 1938) and Procladius sp. of chironomid larvae were the dominant species, making up 10.0% and 17.6%, respectively, of the total chironomid fauna in density, and 83.2% and 9.2% in biomass in March 2010. The mean density of P. akamusi was about 2.5 times higher than that reported by Kitagawa in the 1970s. In recent years, the number of P. akamusi larvae has tended to increase; they are widely distributed, and the anoxic-layer and anaerobic-layer are thickening, especially during summer to late fall in Lake Shoji, suggesting that the lake is in the process of eutrophication.
Published online: 17 October 2012
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