Change of macrobenthic communities in the 1930s, 1970s and 2015 in the mesotrophic Lake Nojiri, Central Japan

We documented the current status of the macrobenthic community of Lake Nojiri (surface area: 4.56 km2; maximum depth: 38.5 m; altitude: 654 m above sea level; mesotrophic lake), and examined changes over time in the densities of chironomids and oligochaetes, by comparison with previous quantitative data of the lake reported in 1931 and 1973. We discussed the succession of benthic macroinvertebrates in relation to changes in the lake bottom environment, as evidence of lake eutrophication. On March 10, 2015, a bathymetrical sampling survey was carried out using a standard Ekman grab at each of the 5 stations (min. ca. 6 m – max. ca. 27 m) in Lake Nojiri. The average densities of the benthic communities for all the stations were ca. 5000m-2, comprised principally of oligochaetes ca. 2800 m-2 (57%) and chironomids ca. 2100 m-2 (43%), whereas their benthic biomass averaged ca. 9.7 g m-2, chironomids ca. 7.6 g m-2 (77%) and oligochaetes ca. 2.3 g m-2 (23%). In the shallower stations, the dominant species was Heterotrissocladius sp., but Chironomus nipponensis Tokunaga was the dominant species in the deeper stations. The densities of C. nipponensis was ca. 14 times higher than those reported by Miyadi in 1931 and 4 times higher than reported by Kitagawa in 1973a. In recent years the density of the oligochaete Tubifex tubifex, has tended to increase and the anoxic- and anaerobic-layer have thickened, especially in deeper regions where they are widely distributed. Moreover, we found differences in the C. nipponensis larval growth rate with water depth, i.e., small IV-instar larvae that dominated at the deepest Station 5. At this station, we suggest that low dissolved oxygen concentration and low water temperature during summer-fall suppresses growth of C. nipponensis. Large environmental changes must have affected chironomid and oligochaete densities and growth rate of C. nipponensis, especially in the deeper regions with low dissolved oxygen concentrations, low water temperature and high organic matter (ignition loss; 15.0% in St. 4 and 13.9% in St. 5) in the sediments. This is a strong evidence that the eutrophication of this lake is continuing.