Distribution of dissolved organic carbon in the East China Sea

A high-temperature combustion (HTC) method with high precision (±0.3–1.2 μM) was applied to measure the distribution of dissolved organic carbon (DOC) in the East China Sea. The seawater samples were collected near the shelf edge in the autumn (October–November 1995) and along a transect on the shelf (the PN line) in the spring (April 1996). In the surface mixed layer, DOC concentrations varied from approximately 65–75 μM, within the range generally reported for other oceanic regions. In general, the distribution of DOC seemed to be controlled by hydrography (σt) rather than biology (chlorophyll a). However, our method also detected small variations (usually <3 μM) within the surface mixed layer, implying photochemical degradation of DOC in the top surface and a temporal accumulation of DOC of intermediate reactivity. Depth-integrated DOC increased by 13 g C m−2 from the autumn to the spring in the 100–200 m layer around the shelf edge, comparable to the annual particle flux from the euphotic zone, suggesting substantial downward export of DOC in this area. Terrestrial input of DOC was estimated to be 4.8×1012 g C yr−1 in the shelf area relatively close to the river mouth of the Changjiang.