Modeling phytoplankton production in Ise Bay, Japan: Use of nitrogen isotopes to identify dissolved inorganic nitrogen sources

An important aspect of the nitrogen cycle in coastal environments concerns the source of the nitrogen used in primary production. Phytoplankton production in Ise Bay, one of the most eutrophic embayments in Japan, is supported by external nitrogen derived from rivers and the ocean, and regenerated nitrogen formed in hypoxic water within the bay. We evaluated the contribution of each source of dissolved inorganic nitrogen (DIN) to phytoplankton production in Ise Bay. A unique three-dimensional ecosystem model including nitrogen isotopes (δ15N) was developed based on precise observations. Model results revealed that DIN (=ammonium + nitrate) consumption by phytoplankton exceeds the DIN supply from the rivers and ocean, indicating that a large amount of phytoplankton production in Ise Bay depends on regenerated DIN within the bay rather than on newly supplied DIN. However, the ratio of consumption to external supply differs seasonally. Distributions of simulated δ15N clearly showed the source of nitrogen incorporated by phytoplankton in each source. The intrusion depth of oceanic water changes from the bottom to the middle layer in spring. Oceanic nitrate is transported into the euphotic layer by the middle layer intrusion and stimulates phytoplankton production at the bay mouth. The subsurface chlorophyll maximum layer then develops. In autumn, however, the intrusion depth of oceanic water changes from the middle layer to the bottom layer. Regenerated NO3−, which is accumulated in the hypoxic water mass, is uplifted and continuously supplied to the euphotic layer. These results imply that phytoplankton production in Ise Bay is mainly dominated by the internal cycle rather than the external supply.