Factors controlling the C:N:P stoichiometry of dissolved organic matter in the N-limited, cyanobacteria-dominated East/Japan Sea

Abstract

The vertical and horizontal distributions of dissolved organic carbon (DOC), nitrogen (DON), and phosphorus (DOP), together with dissolved inorganic nitrogen (DIN), phosphorus (DIP), silicate (DSi), and photosynthetic pigments, were measured in the euphotic layer (0–100m) along the south–north transect of the East/Japan Sea (EJS) in May 2007. In the mixed layer, the DIN concentrations were lower than 3μM, and the DIN:DIP ratios were lower than 10. Under this DIN-limited condition, cyanobacteria (20–65%) and diatoms (20–50%) dominated the phytoplankton community. The concentrations of DOC (60–83μM) and DOP (0.1–0.4μM) in the EJS fell into a range similar to those found in the major oceans, whereas the concentration of DON (2–7μM) was lower than that in the Pacific Ocean (7–13μM). The correlation analyses showed that the distributions of DOC, DON, and DOP in the study region were affected mainly by the physical characteristics of water masses. The lower DON concentrations, higher DOC:DON ratios, and lower DON:DOP ratios in the EJS, relative to the major oceans, together with a strong negative correlation between the concentrations DON and zeaxanthin, suggest that DON is preferentially taken up by cyanobacteria in this environment. Thus, our results show that the C:N:P stoichiometry of dissolved organic matter (DOM) in the EJS is controlled mainly by the physical and biological characteristics of water masses.