Analysis of Historical Trend of Eutrophication in Lake Nakaumi, Japan, Using Concentrations of Several Index Elements in Sediment Cores

Abstract

To elucidate the historical trend of eutrophication in Lake Nakaumi over the last 100 years, we measured the concentrations of total organic carbon (TOC) , total nitrogen (TN) , total sulfur (TS) , and molybdenum (Mo) in sediment cores collected at two sites in the lake. TOC and TN in the sediments were derived mainly from planktonic organisms, and their concentrations increased abruptly after the 1950s. Most of the TS in the surface layer of 0–2 cm was present as sulfides, indicating that large amounts of sulfides are deposited around the sediment surface. Assuming that sulfides were formed mostly in the upper layer of 0–10 cm, the vertical profile of TS concentration in layers deeper than 10 cm suggests that the amount of sulfides formed in the sediment cores increased after the 1950s. This increase may be attributable to the expansion of the area of anoxic water around the lake bottom owing to the increase in primary production after the 1950s. The vertical profile of Mo concentration in the cores was similar to that of TS concentration. It is likely that the vertical profile of Mo concentration is also related to the historical trend of primary production in Lake Nakaumi.