Processes controlling the distribution and cycling of dissolved manganese in the northern South China Sea

The Northern South China Sea (NSCS) is one of the most important marginal seas in the northwest Pacific Ocean. It receives large amounts of terrestrial inputs, and is affected by intrusion of Kuroshio Water. Dissolved manganese (Mn) can be used as a chemical tracer, facilitating study of fluxes and processes at ocean interfaces. To investigate the behavior and transport of dissolved Mn in the NSCS, we measured the concentrations of dissolved Mn in samples collected in summer 2011 and 2015, during field survey voyages supported by the National Science Foundation of China. The concentration of dissolved Mn ranged from 0.5 to 29.9 nM, with an average of 3.7 nM. Elevated concentrations (>15 nM) of dissolved Mn were found in coastal waters, while low concentrations (<3 nM) were found in areas affected by Kuroshio Water intrusion. There was no significant inter-annual variation in the dissolved Mn concentrations in the study region. Vertical profiles of dissolved Mn in the NSCS were similar to those found in open ocean conditions, with maxima occurring in surface waters and a decrease with depth. Large inputs of dissolved Mn occurred from terrestrial sources to the NSCS coastal areas via the Pearl River. Variations in the distribution of dissolved Mn resulting from intrusion of Kuroshio Water and its interactions with NSCS water were associated with Shelf Water transport. Data on dissolved Mn concentrations across the NSCS slope indicated that the offshore transport of Shelf Water in the upper water layer plays an important role in regulating the distribution of dissolved Mn. A preliminary box model was established to develop a dissolved Mn budget for the NSCS. The flux of dissolved Mn was dominated by exchanges between the NSCS and the northwest Pacific Ocean, with >60% of the material transport flux occurring in the Luzon Strait.