Organic C, N, and biogenic Si (BSi) content, as well as C and N stable isotopes, in sediment cores were measured to elucidate the historical change in primary productivity at Ishikari Bay. The bay is characterized as an oligotrophic coastal water considerably influenced by riverine discharge from the Ishikari River, the second largest river in Japan. Layers with a high C content of 50 mg g−1 and a high C: N ratio of 25 were found in several cores, which coincided with a sedimentation age between the 1960s and 1970s. This was attributed to peat exposure to land surface and the delivery to the bay due to the intensive exploitation of land in the catchment area of the Ishikari River and the frequent floods of that time period. BSi content in the sediment layer may reflect change in the diatomaceous primary productivity in the surface euphotic zone. BSi content increased up to the present at the station nearest to the river mouth, indicating an increase in primary productivity by diatoms. However, the total nitrogen flux from the river has not changed significantly since the 1970s. The transparency in the river water has increased, suggesting a decrease in the loadings of suspended solids from the river due to the construction of revetment walls. Thus, light limitation has been relieved in the area near the river mouth. In semi-enclosed coastal areas, the sedimentary record of organic matter and BSi content may reflect historical changes in primary productivity and riverine influence.
Hidaka et al. (2024) studied this question.