The bottom area (ca. 4 m deep) of 25 ha in the south basin of Lake Biwa was intensively dredged.
As a result of dredging, water depth became deep to ca. 15 m. Physico-chemical factors (light attenuation, water temperature, dissolved oxygen, pH, NH₄-N, NO₂-N, NO₃-N, PO₄-P and particulate organic carbon, particulate organic nitrogen and ignition loss of bottom sediment) and the number of Benthic animals were surveyed almost biweekly from March 1980 to April 1981, both in the dredged area (Sta. 1) and an adjacent one (Sta. 2, ca. 4 m deep).
Annual average compensation depth at Sta. 1 was about 5.5 m, and the aphotic layer was found throughout the year. Stratification of water temperature continued from April to the middle of September. Dissolved oxygen was not detected in the bottom layer during June-August, but NH₄-N and PO₄-P were observed in noticeably high concentrations. These nutrients must be produced under anoxic conditions.
There were no differences in the ignition loss and the organic carbon and nitrogen contents between surface sediments of the bottom at Stas. 1 and 2. This finding suggests that the organic matter content of the bottom sediment at Sta. 1 was not reduced by dredging.
Chironomidae were very few at Sta. 1 for the four months from the middle of June to the middle of October. This phenomenon might be related to the anaerobic condition of the bottom sediment during summer.