Biogeochemical alteration and fluxes of dissolved organic matter and nutrients in coastal bays

Abstract

We measured dissolved organic carbon (DOC), δ13C-DOC, fluorescent dissolved organic matter (FDOM), dissolved inorganic and organic nitrogen (DIN; DON), dissolved inorganic phosphate (DIP), and radium isotopes (223Ra, 224Ra, and 226Ra) in three different bays: Gwangyang Bay (GB), Suyoung Bay (SB), and Ulsan Bay (UB), Korea. The water residence times based on Ra isotopes were approximately 15 (±7) for GB and 1–3 days for SB and UB. UB and SB showed clear two (terrestrial and marine) endmember mixing trends for DOC, DIN, and DIP. In addition, significant “excess” DOC (10–50% of outer bay) was observed for SB under depleted nutrient conditions. GB, which had the longest water-residence time, showed depleted DIN and DIP, with large “excess” DOC (40–60% of the outer bay) and DON (10–40% of the outer bay) concentrations. The excess DOC observed in SB and GB was found to be marine in origin, although there was a slight influence of terrestrial DOC in the low-salinity waters of SB based on δ13C-DOC values. Terrestrial humic-like FDOM was conservatively mixed in the three bays. The net fluxes of these components, estimated using the water residence times, suggest that GB is a significant source of DOM (DOC, DON, and humic FDOM) but a sink of DIN and DIP. SB and UB are the sources of inorganic nutrients as well as DOM to the open ocean. Therefore, our study reveals that the residence time of coastal embayment play an important role in the biogeochemical production and alteration of nutrients and DOM.