Influence of hydrodynamic mixing on the distribution of dissolved organic carbon in the East China Sea and the northwest Pacific

Oceanic dissolved organic carbon (DOC) is one of the largest carbon reservoirs on Earth, and its distribution and behavior play important roles in carbon cycling and biogeochemical processes in the ocean. Here, we report distribution and concentrations of DOC for water samples collected from the shelf-edge and slope regions in East China Sea (ECS) and the Kuroshio Extension (KE) in the northwestern North Pacific (NP) during two cruises in 2014–2015. Concentrations of DOC were 45–88 µM in the ECS and 35–65 µM in the KE. In addition to biological processes, the distribution of DOC is largely controlled by hydrodynamic mixing of different water masses. The intrusion of Kuroshio Current could dilute DOC concentrations at stations in the outer shelf and slope ranges of the ECS. The inverse correlation of DOC with apparent oxygen utilization (AOU) suggests that DOC oxidation only contributes 18 % of the oxygen consumption in the ECS slope waters. In contrast, concentrations of DOC in the KE were significantly lower in surface waters, and a relatively low and stable DOC level (~ 40 µM) was found in deep waters. The observed spatial variations of DOC in the upper 700 m among the stations in the KE were largely influenced by the mixing of the two water masses carried by the two major western boundary currents: Kuroshio and Oyashio that mix and form the KE. The hydrodynamic processes play important roles not only in the distribution of DOC but nutrients as well, thus could have major impact to primary production and ecosystems in the KE region.