Distribution of dissolved iron in the Pearl River (Zhujiang) Estuary and the northern continental slope of the South China Sea

Iron (Fe) has been recognized as a key trace element that limits primary production in large oceanic areas. In the oligotrophic South China Sea (SCS), few Fe data have been reported due to the difficulties in collecting trace metal clean seawater samples. Here, we report dissolved Fe (dFe) datasets from the Pearl River estuary and the northern continental slope of the SCS. Samples were collected in October 2014 using our homemade towed-fish and X-Vane sampling system. dFe experienced considerable removal at a salinity < 12, and then increased in the higher salinity area within the estuary. The estimated dFe flux out of the Pearl River to the SCS was less important than the contribution from atmospheric dust deposition to the SCS. Surface dFe concentrations ranged from 0.17 nM to 1.01 nM, averaging 0.50 ± 0.17 nM over the northern continental slope of the SCS. At an ~ 1000 m full water column station, dFe minimum values of 0.15 nM were observed, coincident with the subsurface Chl-a maximum at a depth of 65–70 m. dFe gradually increased with depth to 0.6–0.8 nM at over the slope, due to the remineralization of sinking particles. A regenerated Fe:C ratio of 1.6 μM/M was derived from the relationship between dFe and apparent oxygen utilization. Our results suggested that Fe may be high enough to support phytoplankton growth in the surface water of the north continental slope area of the SCS.