Dissolved Fe across the Weddell Sea and Drake Passage: impact of DFe on nutrient uptake

R. Middag,I. Neven,H. J. W. De Baar,P. Laan,M. B. Klunder,J. Van Ooijen

doi:10.5194/bg-11-651-2014

R. Middag, I. Neven + Show 4 more

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https://doi.org/10.5194/bg-11-651-2014

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Abstract

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.

Highlights

It is well established that phytoplankton growth in the high nutrients low chlorophyll (HNLC) Southern Ocean is primarily limited by low Fe concentrations (De Baar et al, 1990; Buma et al, 1991; De Baar et al, 1995, 1999; Measures and Vink, 2001; Boyd et al, 2007; Sedwick et al, 2008; Pollard et al, 2009; Smetacek et al, 2012), likely in combination with light limitation
This station is situated on the Scotia Ridge and several water masses were observed that have been in contact with the bottom sediments, explaining the higher concentrations of dissolved Fe (DFe) compared to our stations (De Jong et al, 2011)
In the Weddell Sea, no clear influence of the shelf was observed in the seasonal nutrient drawdown, it should be noted that no winter water was observed north of 64◦ S, and no removal values could be reported close to the shelf

Summary

Introduction

It is well established that phytoplankton growth in the high nutrients low chlorophyll (HNLC) Southern Ocean is primarily limited by low Fe concentrations (De Baar et al, 1990; Buma et al, 1991; De Baar et al, 1995, 1999; Measures and Vink, 2001; Boyd et al, 2007; Sedwick et al, 2008; Pollard et al, 2009; Smetacek et al, 2012), likely in combination with light limitation Klunder et al.: Dissolved Fe across the Weddell Sea and Drake Passage Stations Biovolume/cell. Small (> 5000 μm3) (< 1000 μm3) (%) Greenwich Meridian

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