Dissolved Fe in the East China Sea Under the Influences of Land Sources and the Boundary Current With Implications for Global Marginal Seas

The East China Sea (ECS) lies on a wide shelf and embraces one of the world's largest rivers, Changjiang (Yangtze River), on the west and strong boundary current, Kuroshio, on the east, which can act as potentially huge sources of dissolved iron (dFe) to the overlying water. However, their magnitude, distribution, and transformation in the ECS are not yet well characterized. Here, we reported the results from the first large scale investigation of dFe concentrations in the water column from the ECS collected with an X-Vane assemblage in August 2013. Samples from the Changjiang and other coastal rivers were also measured to constrain terrestrial fluxes. In surface waters, dFe concentrations show a strong gradient with patchy nature from the coast adjacent to the river mouth to the mainstream of the Kuroshio Current. The behavior of dFe in the ECS is closely linked to nutrient cycling and development of the sub-surface chlorophyll maximum. Our dFe budget reveals that mass transport over the East China Sea Shelf (ECSS) is dominated by the input from the Taiwan Strait Warm Water, atmospheric deposition, and exchange with the open boundary current further offshore. The incursion of the Kuroshio Current across shelf break is a source of natural fertilization. The export of dFe from the ECSS nourishes other marginal seas (e.g., Japan Sea/East Sea) and the Kuroshio further downstream. Comparison with the literature data provides evidence that marginal seas play a critical role in bridging terrestrial and oceanic spheres in terms of Fe biogeochemistry.