Abstract
The dissolved (D) trace metals zinc (Zn), cadmium (Cd), cobalt (Co), copper (Cu), iron (Fe), manganese (Mn), nickel (Ni), titanium (Ti), lanthanum (La), Yttrium (Y) and lead (Pb) were analysed via ICPMS in samples from the Ross Sea obtained during a cruise between 20 December 2013 and 5 January 2014. The concentrations of DZn, DCd, DCo, DCu, DFe, DMn, DNi and DTi were significantly lower in the Antarctic surface Water (AASW) compared to the other deeper water masses, indicating biological uptake and possibly scavenging. In the AASW, DLa and DY were higher than in Winter Water (WW). This can be explained by a spring source from ice melt followed by loss during summer and autumn, probably due to passive adsorption. Dissolved Pb was low (16 pM) and no distinction between water masses was possible. Akin to the macro-nutrients nitrate and silicate, the modified Circumpolar Deep Water (mCDW) shows elevated DCd compared to the shelf water masses. Sea ice melt and ice sheet melt released DZn, DFe, DMn, DNi, DY, DLa, and probably DPb into the Ross Sea. However, only DFe, DMn, DY and DLa are transported into the Antarctic Circumpolar Current (ACC) with the outflowing High Salinity Shelf Water (HSSW). The bottom nepheloid layer (BNL) released DFe, as well as DMn and DCu, into the HSSW whereas lateral transport from land formed a source of DMn and DFe. One station in the Ross Sea polynya was resampled after two weeks, during which time the thickness of the BNL increased, with accompanying increases in DFe and DMn near the seafloor. In the surface layer nutrients (including micro-nutrients) were depleted further. The uptake slopes/stoichiometric ratios of DZn, DCd and DCo versus phosphate indicated that the distribution of these metals is related to uptake as well as the composition of the phytoplankton community. Estimated stoichiometric ratios of Zn and Cd relative to P were higher at a station dominated by Phaeocystis antarctica than at diatom-dominated stations, implying a higher utilisation of these metals by P. antarctica.
Highlights
The growth of Southern Ocean phytoplankton is primarily limited by low dissolved Fe (DFe) concentrations and availability of light, making the Southern Ocean the largest of the world’s high nutrient low chlorophyll areas (de Baar et al, 1990; Martin et al, 1990; Boyd et al, 2012)
The difference between LSSW and High Salinity Shelf Water (HSSW) depends on the extent of salt rejection in the sea ice, where LSSW was only present at the stations in the eastern Ross Sea (St. 16, 17 and 18)
A bottom nepheloid layer (BNL) was observed above the bottom in the Ross Sea Polynya in HSSW and at two stations in MSW (Gerringa et al, 2015, 2019)
Summary
The growth of Southern Ocean phytoplankton is primarily limited by low dissolved Fe (DFe) concentrations and availability of light, making the Southern Ocean the largest of the world’s high nutrient low chlorophyll areas (de Baar et al, 1990; Martin et al, 1990; Boyd et al, 2012). In the Ross Sea Polynya, DFe is often the primary nutrient that is limiting to phytoplankton growth (Sedwick et al, 2000, 2015; Marsay et al, 2014; McGillicuddy et al, 2015; Hatta et al, 2017), even in early spring due to “early season depletion” (Sedwick et al, 2011) This early depletion was confirmed by additional shipboard DFe measurements made in the spring of 2013, with DFe in the upper mixed layer as low as 0.02–0.08 nM (Gerringa et al, 2015) when the polynya was still forming. It has been established that apart from local sea ice melt at the margins of the polynya and lateral transport from land, upward fluxes from the sediment are the most important DFe source to surface waters of the Ross Sea (Marsay et al, 2014; Gerringa et al, 2015; Hatta et al, 2017)
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