Hydrogen sulfide and organic carbon at the sediment–water interface in coastal brackish Lake Nakaumi, SW Japan

The relationship among H2S, total organic carbon (TOC), total sulfur (TS) and total nitrogen contents of surface sediments (0–1 cm) was examined to quantify the relationship between H2S concentrations and TOC content at the sediment water interface in a coastal brackish lake, Nakaumi, southwest Japan. In this lake, bottom water becomes anoxic during summer due to a strong halocline. Lake water has ample dissolved SO4 2− and the surface sediments are rich in planktic organic matter (C/N 7–9), which is highly reactive in terms of sulfate reduction. In this setting the amount of TOC should be a critical factor regulating the activity of sulfate reduction and H2S production. In portions of the lake where sediment TOC content is less than 3.5 %, H2S was very low or absent in both bottom and pore waters. However, in areas with TOC >3.5 %, H2S was correlated with TOC content (pore water H2S (ppm) = 13.9 × TOC (%) − 52.1, correlation coefficient: 0.72). H2S was also present in areas with sediment TS above 1.2 % (present as iron sulfide), which suggests that iron sulfide formation is tied to the amount of TOC. Based on this relationship, H2S production has progressively increased after the initiation of land reclamation projects in Lake Nakaumi, as the area of sapropel sediments has significantly increased. This TOC–H2S relationship at sediment–water interface might be used to infer H2S production in brackish–lagoonal systems similar to Lake Nakaumi, with readily available SO4 2− and reactive organic matter.