Silicon budget of eutrophic Lake Kasumigaura, Japan

Abstract

Determining the dynamics of silicon in lakes, one of the essential nutrients for diatoms, is valuable for understanding aquatic environmental problems. The dissolved silicon (DSi) and biogenic silicon (BSi) budgets in Lake Kasumigaura, a shallow eutrophic lake in Japan, during the last three decades were assessed based on the analysis of dated sediment cores and a water quality database. Sediment cores (100 cm long) were taken at the center of Lake Kasumigaura in 2005, 2007, and 2009 and at two other sites in 2007. BSi contents of the dated sediments were determined by wet alkaline digestion. The net sedimentation rates of BSi were defined as the difference between the DSi load from inflowing rivers and the DSi and BSi loads from the outflow of the lake, calculated using DSi concentrations and diatom abundances in the lake from 1980 to 2007 and DSi concentrations of the inflowing rivers during 1994, 2007, and 2009. The gross sedimentation rates of BSi were estimated by multiplying BSi concentrations in lake water by the diatom sinking rate reported by previous studies. Budgetary calculations based on the database showed that 60–70 % of DSi inputs from the inflowing rivers during the 27 years could ultimately be accumulated as diatom frustules in bottom sediments in the lake. The sediment analysis revealed that the amount of BSi accumulated in the lake from 1980 to 2007 was 2.0–2.6 × 1011 g, similar to the amount based on the database of 1.3–2.4 × 1011 g. Although the gross sedimentation rates of BSi likely increased, the net sedimentation rates of BSi decreased significantly from 6–10 × 109 g year−1 in the 1980s to 2–6 × 109 g year−1 in the 2000s, suggesting a fast recycling of BSi in recent years caused by an increase in sediment resuspension and regeneration. The sediment core information and the water quality database can be used for calculating the long-term silicon budgets in Lake Kasumigaura. An increase in the DSi release rates was identified, which is consistent with recent sediment resuspension. Comparing the sediment core information with the database suggests the long-term dissolution of sediment BSi; however, analysis of the BSi content in sediment cores representing a much longer time period is needed to confirm this.