Dissolved and particulate organic carbon in the southern East China Sea

This work reports distributions of dissolved organic carbon (DOC) and particulate organic carbon (POC) observed along a cross-shelf transect in the southern East China Sea from 1994 to 1996. DOC concentrations were relatively high (>85 μM) in inner shelf and slope waters but low (ca. 65 μM) around the shelf break, where Kuroshio upwelling occurred. Such distribution pattern showed little temporal variations. Colloidal organic carbon (COC), defined as dissolved organic carbon with molecular weight above 1000 Da, was further separated for analysis. Although the coastal water in the inner shelf and the Kuroshio surface water over the slope had similar DOC concentrations, the partitions between COC and the low molecular weight DOC were different. The coastal water contained less COC than the oligotrophic slope water, suggesting a lower production rate and/or a higher breakdown rate in the coastal water. The POC distribution showed a decreasing trend from the inner shelf to the slope with a local maximum at the shelf break, where POC was enriched due to enhanced primary productivity induced by upwelling. The average POC inventory was about 1/10 that of the DOC. A mid-depth maximum was repeatedly observed above the slope, indicating lateral transport of POC offshore from the shelf. The net exports of DOC and POC from the shelf are estimated to be 414 and 106 Gmol C/yr, respectively, but the POC export along the bottom boundary layer is unaccounted for in our assessment.