Abstract
Modeling the processes by which dissolved inorganic nitrogen and phosphorus contribute to biological productivity would be of assistance in restoring the soundness of the material cycles and food web in semi-enclosed seas. In this study, field observations to investigate the biogeochemical cycle of nutrients and the spatial variations were conducted in Osaka Bay in August and November 2015 to obtain basic data for modeling. The observed results for a stable isotope of dissolved organic carbon showed that dissolved organic matter is generated in the bay in summer in addition to those supplied from rivers. The dissolved organic matter spreads along the conservative mixing line toward the shelf sea. These results suggested that dissolved inorganic nitrogen and phosphorus supplied from rivers were assimilated by phytoplankton and transformed into dissolved organic matter, a large part of which flowed out of the bay before degradation into inorganic matter. The abundance of heterotrophic bacteria and cyanobacteria, and the significant correlations between those and heterotrophic nanoflagellates suggested that a significant amount of nitrogen was taken into the microbial loop by heterotrophic bacteria feeding on dissolved organic matter and cyanobacteria consuming the nutrients.
