Increase in silicon concentrations and release from suspended solids and bottom sediments in Lake Kasumigaura, Japan

Increasing trends of dissolved Si measured by a colorimetric method and ICP (DSicol and DSiICP, respectively) and total Si concentrations were detected at the center of Lake Kasumigaura during 1980-2006 (mean DSicol concentration in the 1980s and 2000s was 1.3 and 4.0 mg l(-1), respectively). The observation of such trends is rare; therefore, the elucidation of the causes could be useful to understanding silicon dynamics in inland waters. Based on statistical analysis, we found that the increases in DSicol and lithogenic Si accounted for most of the total Si increase (44 and 45%, respectively) and that biogenic Si, consisting of diatom frustules, also increased with them. Increases in DSiICP concentration were not detected near the mouth of the inflowing rivers, suggesting that the increase was caused by in-lake processes. Because the increase in suspended solids (SS) caused by sediment resuspension had been observed in the lake for the same period, we assumed that the Si release from SS containing diatom frustules contributed to the increase. The results of the laboratory experiments in which surface sediments were stirred in lake waters showed that the change in DSicol concentration depended mainly on SS concentration, water temperature and the elapsed time of diatom frustules dissolution. An estimation of the released amount of Si from SS using the sediment resuspension model was (1.0-2.7) x 10(9) g year(-1) in the 2000s, which was about 30-90% of the increase in the DSicol outflow of 3.0 x 10(9) g year(-1) from the 1980s to the 2000s. We also determined the Si release rates from bottom sediments through laboratory experiments. The Si amount released from bottom sediments in the lake in the 2000s was estimated to be 4.3 x 10(9) g year(-1), which was about 2-4 times higher than the estimated Si amount released from SS. These findings suggest that the sediment resuspension might be the cause of the latest DSi increase.