Dynamics of organic carbon under different inorganic nitrogen levels and phytoplankton composition

We examined the production and partitioning of organic carbon under different inorganic nitrogen concentrations and phytoplankton composition by incubating unaltered seawater collected from 10, 100, 400 and 700 m depths off Suruga Bay, Japan. Particulate organic carbon (POC) accounted for 60–112% of net accumulated total organic carbon (TOC) during exponential growth and blooms of phytoplankton in all incubations regardless of the concentrations of nutrients and the forms of dissolved inorganic nitrogen (DIN) as well as the composition and size of phytoplankton. After the blooms of phytoplankton, microzooplankton (mainly ciliates) (>20 μm) increased rapidly in majority of incubations. This suggests that a large fraction of new production may also be transferred to higher trophic levels rather than immediately exported. After the depletion of initial DIN, accumulation of both POC and dissolved organic carbon (DOC) increased. DOC accounted for major fraction of TOC in most of the incubations. Continued accumulation of high C/N organic matter implies that recycling through the food web is not necessarily the dominant fate of organic carbon produced under low DIN condition but rather that a major fraction of the organic carbon is exported to deep water through sinking.