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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 07 Oct 2019

Submitted as: research article | 07 Oct 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

Efficient removal of phosphorus and nitrogen in sediments of the eutrophic Stockholm Archipelago, Baltic Sea

Niels A. G. M. van Helmond1,2,a,*, Elizabeth K. Robertson2,3,*, Daniel J. Conley2, Martijn Hermans1, Christoph Humborg4, L. Joëlle Kubeneck1,b, Wytze K. Lenstra1, and Caroline P. Slomp1 Niels A. G. M. van Helmond et al.
  • 1Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Princetonlaan 8a, 3584 CB Utrecht, the Netherlands
  • 2Department of Geology, Lund University, Sölvegatan 12, 223 62 Lund, Sweden
  • 3Department of Marine Sciences, University of Gothenburg, Box 461, 40530 Göteborg, Sweden
  • 4Baltic Sea Centre, Stockholm University, 106 91 Stockholm, Sweden
  • anow at: Department of Microbiology, Institute for Water and Wetland Research, Radboud University, Heyendaalseweg 135, 6525 AJ Nijmegen, the Netherlands
  • bnow at: Institute of Biogeochemistry and Pollutant Dynamics, Department of Environmental Systems Science, ETH Zürich, Universitätstrasse 16, 8092 Zürich, Switzerland
  • *These authors contributed equally to this work.

Abstract. Coastal systems can act as filters for anthropogenic nutrient input into marine environments. Here, we assess the processes controlling the removal of phosphorus (P) and nitrogen (N) for four sites in the eutrophic Stockholm Archipelago. Bottom water concentrations of oxygen and P are inversely correlated. This is attributed to the seasonal release of P from iron (Fe)-oxide-bound P in surface sediments and from degrading organic matter. The abundant presence of sulfide in the pore water, linked to prior deposition of organic-rich sediments in a low oxygen setting (legacy of hypoxia), hinders the formation of a larger Fe-oxide-bound P pool in winter. Burial rates of P are high at all sites (0.03–0.3 mol m−2 y−1), a combined result of high sedimentation rates (0.5 to 3.5 cm yr−1) and high sedimentary P at depth (~ 30 to 50 μmol g−1). Organic P accounts for 30–50 % of reactive P burial. Apart from one site in the inner archipelago, where a vivianite-type Fe(II)-P mineral is likely present at depth, there is little evidence for sink-switching of organic or Fe-oxide bound P to authigenic P minerals. Denitrification is the major benthic nitrate-reducing process at all sites (0.09 to 1.7 mmol m−2 d−1), efficiently removing N as N2. Denitrification rates decrease seaward following the decline in bottom water nitrate and sediment organic carbon. Our results explain how sediments in this eutrophic coastal system can efficiently remove land-derived P and N, regardless of whether the bottom waters are oxic or frequently hypoxic. Hence, management strategies involving artificial reoxygenation are not expected to be successful in removing P and N, emphasizing a need for a focus on nutrient load reductions.

Niels A. G. M. van Helmond et al.
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Niels A. G. M. van Helmond et al.
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