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. Coastal systems can act as filters for anthropogenicnutrient input into marine environments. Here, we assess the processescontrolling the removal of phosphorus (P) and nitrogen (N) for four sites inthe eutrophic Stockholm archipelago. Bottom water concentrations of oxygen( O2 ) and P are inversely correlated. This is attributed to the seasonalrelease of P from iron-oxide-bound (Fe-oxide-bound) P in surface sediments and fromdegrading organic matter. The abundant presence of sulfide in the pore waterand its high upward flux towards the sediment surface ( ∼4 to8 mmol m −2  d −1 ), linked to prior deposition of organic-richsediments in a low- O2 setting (“legacy of hypoxia”), hinder theformation of a larger Fe-oxide-bound P pool in winter. This is mostpronounced at sites where water column mixing is naturally relatively lowand where low bottom water O2 concentrations prevail in summer. Burial ratesof P are high at all sites (0.03–0.3 mol m −2  yr −1 ), a combinedresult of high sedimentation rates (0.5 to 3.5 cm yr −1 ) and highsedimentary P at depth ( ∼30 to 50  µ mol g −1 ).Sedimentary P is dominated by Fe-bound P and organic P at the sedimentsurface and by organic P, authigenic Ca-P and detrital P at depth. Apartfrom one site in the inner archipelago, where a vivianite-type Fe(II)-Pmineral is likely present at depth, there is little evidence forsink 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 theinner to outer archipelago. Our results explain how sediments in thiseutrophic coastal system can remove P through burial at a relatively highrate, regardless of whether the bottom waters are oxic or (frequently)hypoxic. Our results suggest that benthic N processes undergo annual cyclesof removal and recycling in response to hypoxic conditions. Further nutrientload reductions are expected to contribute to the recovery of the eutrophicStockholm archipelago from hypoxia. Based on the dominant pathways of P andN removal identified in this study, it is expected that the sediments willcontinue to remove part of the P and N loads.

Removal of phosphorus and nitrogen in sediments of the eutrophic Stockholm archipelago, Baltic Sea