The Western Antarctic Peninsula (WAP) displays high but variable productivity and is also undergoing rapid change. Long-term studies of phytoplankton communities and primary production have suggested transient limitation by the micronutrient iron (Fe), but to date no data have been available to test this hypothesis. Here, we present the first spatially extensive, multi-year measurements of dissolved and particulate trace metals in surface waters to investigate the key sources and sinks of Fe in the central WAP shelf. Surface samples of dissolved and particulate metals were collected throughout the 700 x 200 km grid of the Palmer Long-Term Ecological Research program in three consecutive austral summers (2010-2012). Iron concentrations varied widely. Both dissolved and particulate Fe were high in coastal waters (up to 8 nmol kg-1 and 42 nmol kg-1 , respectively). In contrast, very low Fe concentrations (< 0.1 nmol kg-1) were widespread in mid-to outer-shelf surface waters, especially in the northern half of the sampling grid, suggesting possible Fe limitation of primary production on the shelf. Sea ice and dust inputs of Fe were minor, although their relative importance increased with distance from shore due to the larger near-shore sources. Sedimentary inputs were inferred from manganese distributions; these were more significant in the northern portion of the grid, and showed interannual variation in intensity. Overall, the interannual distribution of Fe was most closely correlated to that of meteoric water (glacial melt and precipitation). Although the Fe concentrations and relative contributions of dissolved and particulate Fe attributed to meltwater were variable throughout the sampling region, increasing glacial meltwater flux can be expected to increase the delivery of Fe to surface waters of the coastal WAP in the future.

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The Baltic Sea is characterized by the largest area of hypoxic (oxygen (O 2 ) < 2 mg L -1 ) bottom 2 -depleted waters largely depends on episodic Major Baltic Inflows from the adjacent North Sea. In 2014 and 2015, two such inflows led to a strong rise in O 2 and decline in phosphate (HPO 4 2-) in waters below 125 m depth in the Eastern Gotland Basin. This provided the opportunity to assess the impact of such re-oxygenation events on the cycles of manganese (Mn), iron (Fe) and phosphorus (P) in the sediment for the first time. We demonstrate that the re-oxygenation induced the activity of sulphur (S)-oxidising bacteria, known as Beggiatoaceae in the surface sediment where a thin oxic and suboxic layer developed. At the two deepest sites, strong enrichments of total Mn and to a lesser extent Fe oxides and P were observed in this surface layer. A combination of sequential sediment extractions and synchrotron-based X-ray spectroscopy revealed evidence for the abundant presence of P-bearing rhodochrosite and Mn(II)phosphates. In contrast to what is typically assumed, the formation of Fe oxides in the surface sediment was limited. We attribute this lack of Fe oxide formation to the high flux of reductants, such as sulphide, from deeper sediments which allows Fe(II) in the form of FeS to be preserved and restricts the penetration of O 2 into the sediment. We estimate that enhanced P sequestration in surface sediments accounts for only ~5% of water column HPO 4 2removal in the Eastern Gotland Basin linked to the recent inflows. The remaining HPO 4 2was transported to adjacent areas in the Baltic Sea. Our results highlight that the benthic O 2 demand arising from the accumulation of organic-rich sediments over several decades, the legacy of hypoxia, has major implications for the biogeochemical response of euxinic basins to re-oxygenation. In particular, P sequestration in the sediment in association with Fe oxides is limited. This implies that artificial ventilation projects that aim at removing water column HPO 4 2and thereby improving water quality in the Baltic Sea will likely not have the desired effect.

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