Temporal change in the organic carbon concentration of surface sediment at Lake Shinji during the past 30 years

The organic carbon concentration of surface sediment influences bottom water hypoxia. To investigate whether the organic carbon concentration of surface sediment can be used to reconstruct temporal changes in organic carbon concentration, surface sediments from Lake Shinji, Shimane Prefecture, Japan were studied. Samples were analyzed using ignition loss. For comparison with earlier studies, the techniques used by these studies were also used herein, and a conversion formula developed to calculate equivalent organic carbon concentrations. The converted organic carbon concentrations at the same locations in 1982, 1997 and 2016 were compared, and found to be significantly lower in 1997. Since the organic carbon concentration decrease in the 1990s was not detected by an earlier study, it was thought that the decrease for around ten years may have been obscured during early diagenesis. In addition, the organic carbon concentration was significantly higher in sediments collected from the western part of Lake Shinji where a halocline exists. When monitoring hypoxia in brackish lakes, attention should be paid to areas with a halocline as well as the deeper areas.