Physical and biological controls of dissolved manganese on the northern slope of the South China Sea

The South China Sea (SCS) is classified as an oligotrophic ocean because it has low nutrient and chlorophyll concentrations. We examined the biogeochemical cycle of dissolved manganese (DMn) on the northern slope of the SCS during two cruises (October 2014 and June 2015), and the effect of DMn enrichment on the growth of phytoplankton in field incubation experiments. DMn was measured spectrophotometrically by an on-line pre-concentration and flow injection analysis method. The concentration of DMn varied from 0.6 to 32.4 nM in the Pearl River Estuary, and decreased with distance from the coast. Moreover, DMn concentrations declined significantly (50–90%) as salinity increased between the Pearl River and the SCS. Vertical profiles of DMn reflected a typical scavenge-type distribution, and decreased with depth. Mixing between the Shelf Water and the Western Philippine Sea led to a Mn-rich plume (> 4 nM), which extended from the northern shelf into the slope regions of the SCS. The export of Mn-rich shelf waters was an important source of DMn for SCS, and this lateral input from the continental shelf could exceed atmospheric input. The incubation experiments showed that the addition of Mn stimulated phytoplankton growth. These results suggest that Mn may play an important role in regulating phytoplankton growth in the SCS.