Removal of phosphorus and nitrogen in sediments of the eutrophic Stockholm archipelago, Baltic Sea
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Published:2020-05-24
Issue:10
Volume:17
Page:2745-2766
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ISSN:1726-4189
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Container-title:Biogeosciences
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language:en
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Short-container-title:Biogeosciences
Author:
van Helmond Niels A. G. M., Robertson Elizabeth K.ORCID, Conley Daniel J.ORCID, Hermans Martijn, Humborg Christoph, Kubeneck L. Joëlle, Lenstra Wytze K.ORCID, Slomp Caroline P.ORCID
Abstract
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
(O2) and P are inversely correlated. This is attributed to the seasonal
release of P from iron-oxide-bound (Fe-oxide-bound) P in surface sediments and from
degrading organic matter. The abundant presence of sulfide in the pore water
and its high upward flux towards the sediment surface (∼4 to
8 mmol m−2 d−1), linked to prior deposition of organic-rich
sediments in a low-O2 setting (“legacy of hypoxia”), hinder the
formation of a larger Fe-oxide-bound P pool in winter. This is most
pronounced at sites where water column mixing is naturally relatively low
and where low bottom water O2 concentrations prevail in summer. Burial rates
of P are high at all sites (0.03–0.3 mol m−2 yr−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).
Sedimentary P is dominated by Fe-bound P and organic P at the sediment
surface and by organic P, authigenic Ca-P and detrital P at depth. 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) with rates decreasing seaward from the
inner to outer archipelago. Our results explain how sediments in this
eutrophic coastal system can remove P through burial at a relatively high
rate, regardless of whether the bottom waters are oxic or (frequently)
hypoxic. Our results suggest that benthic N processes undergo annual cycles
of removal and recycling in response to hypoxic conditions. Further nutrient
load reductions are expected to contribute to the recovery of the eutrophic
Stockholm archipelago from hypoxia. Based on the dominant pathways of P and
N removal identified in this study, it is expected that the sediments will
continue to remove part of the P and N loads.
Funder
Nederlandse Organisatie voor Wetenschappelijk Onderzoek European Research Council Havs- och Vattenmyndigheten
Publisher
Copernicus GmbH
Subject
Earth-Surface Processes,Ecology, Evolution, Behavior and Systematics
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