Dissolved Fe across the Weddell Sea and Drake Passage: impact of DFe on nutrient uptake
- 1Department of Biological Oceanography, Royal Dutch Institute for Sea Research, Landsdiep 4,1797 SZ Texel, the Netherlands
- 2Department of Ocean Ecosystems, University of Groningen, Kerklaan 30 ,9751 NN Haren, the Netherlands
- 3University of Otago, Department of Chemistry, NIWA/UO Research Centre for Oceanography, P.O. Box 56, Dunedin 9054, New Zealand
- 4Department of Plant Physiology, University of Groningen, Kerklaan 30, 9751 NN Haren, the Netherlands
- 5Department of Chemical Oceanography, Royal Dutch Institute for Sea Research, Landsdiep 4,1797 SZ Texel,the Netherlands
- *now at: Dutch board for the authorisation of biocides and plant protection produces, Wageningen, the Netherlands
Abstract. This manuscript reports the first full depth distributions of dissolved iron (DFe) over a high-resolution Weddell Sea and Drake Passage transect. Very low dissolved DFe concentrations (0.01–0.1 nM range) were observed in the surface waters of the Weddell Sea, and within the Drake Passage polar regime. Locally, enrichment in surface DFe was observed, likely due to recent ice melt (Weddell Sea) or dust deposition (Drake Passage). As expected, in low DFe regions, usually a small silicate drawdown compared to the nitrate drawdown was observed. However, the difference in drawdown between these nutrients appeared not related to DFe availability in the western Weddell Sea. In this region with relatively small diatoms, no relationship between N : P and N : Si removal ratios and DFe was observed. In comparison, along the Greenwich Meridian (Klunder et al., 2011a), where diatoms are significantly larger, the N : P and N : Si removal ratios did increase with increasing DFe. These findings confirm the important role of DFe in biologically mediated nutrient cycles in the Southern Ocean and imply DFe availability might play a role in shaping phytoplankton communities and constraining cell sizes.
Over the shelf around the Antarctic Peninsula, higher DFe concentrations (>1.5 nM) were observed. These elevated concentrations of Fe were transported into Drake Passage along isopycnal surfaces. Near the South American continent, high (>2 nM) DFe concentrations were caused by fluvial/glacial input of DFe.
On the Weddell Sea side of the Peninsula region, formation of deep water (by downslope convection) caused relatively high Fe (0.6–0.8 nM) concentrations in the bottom waters relative to the water masses at mid-depth (0.2–0.4 nM). During transit of Weddell Sea Bottom Water to the Drake Passage, through the Scotia Sea, additional DFe is taken up from seafloor sources, resulting in highest bottom water concentrations in the southernmost part of the Drake Passage in excess of 1 nM. The Weddell Sea Deep Water concentrations (∼0.32 nM) were consistent with the lowest DFe concentrations observed in Antarctic bottom water in the Atlantic Ocean.