Dissolved and particulate organic carbon in the Sea of Okhotsk: Transport from continental shelf to ocean interior

Abstract

Dissolved and particulate organic carbon (DOC and POC) were measured in water columns of the Sea of Okhotsk together with other hydrological and chemical properties, including phytoplankton pigments (Chl‐a) and stable carbon isotopic ratios (δ13C) of POC. Transects from shelf to slope off the northeast coast of Sakhalin showed the existence of a cold‐water mass in the intermediate layer on the slope area having a density of 26.7–27.0 σθ, which is equivalent to the cold and dense water on the shelf (dense shelf water: DSW). The cold intermediate water mass was rich in DOC, POC, and Chl‐a, and the δ13C of POC was high, similar to that in DSW, indicating that the organic matter in the cold intermediate water is exported from the highly productive shelf area by the outflow of DSW. On the other hand, surface waters near the coast of northern Sakhalin had a very low salinity, which must be affected by the fresh water discharge from the Amur River. The low‐salinity water contained a large amount of DOC that was more than 3 times greater than the pelagic surface waters. The linear regression line between the salinity and the DOC in the surface layer of the studied area indicates that the Amur River water contains approximately 690 μM of DOC, and the input of DOC from the Amur to the Sea of Okhotsk was estimated to be approximately 2.5 TgC/yr. The relationships between the temperature and the DOC and POC along a density surface from 26.7 to 27.0 σθ suggest that DOC and POC in the DSW are transported into the intermediate layer of the pelagic region without being removed at the shelf edge. Simple estimates for additions of organic carbon into the intermediate layer of the area off the east coast of Sakhalin suggests that DSW outflow from the shelf brings into this layer approximately 13.6 and 0.9 TgC/yr of DOC and POC, respectively. These fluxes are much larger than the sinking POC flux from the in situ surface water, and so are fundamental in characterizing the biogeochemical cycles in the Sea of Okhotsk.