28 巻 (2005) 5 号 p. 313-319
The vertical profiles of the physical properties, and nutrient and radio-isotope concentrations of the sediments in Lakes Kasumigaura and Suwa were determined in 2003 and then compared with previous results. Sedimentation rates estimated from the profiles of radio isotopes showed decreasing tendencies in both lakes, which possibly resulted in the difference between the long-term trends of water and sediment concentrations. As regards to phosphorus, it sedimentation rate was in good agreement with those calculated as the difference between the input and output loads divided by its concentration in the surface sediments; on the other hand, a large difference was found in the case of nitrogen, suggesting denitrification. Then, we compare the difference between the nutrient amount accumulated in sediments and the sum of residuals of nutrient budgets in Lake Kasumigaura for 1981 – 2003; the residual nutrient budgets were approximately twofold the former for phosphorus and approximately tenfold for nitrogen. This discrepancy for phosphorus was probably due to the inaccuracy of the calculated parameters for calculation and/or the assumption that the residual nutrient budget is uniformly distributed in the lake.