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

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.

Quality of phytoplankton deposition structures bacterial communities at the water‐sediment interface

“…We also found that cyano‐favoured taxa tended to correlate with high and NOx concentrations, while the opposite was true for diatom‐favoured taxa. accumulation over time in the microcosms most likely resulted from the OM degradation above and/or at the sediment surface, as shown to occur in the Baltic Sea (Schneider, 2011; van Helmond et al, 2020). The strong positive correlation between and NOx (Figure D) supports previous findings highlighting that phosphate availability can control the nitrification activity in marine sediment environments (Dang et al, 2013).…”

Recovery from multi‐millennial natural coastal hypoxia in the Stockholm Archipelago, Baltic Sea, terminated by modern human activity

“… * Based on present‐day water column monitoring curated by the Swedish Meteorological and Hydrological Institute (SMHI; van Helmond et al 2020). …”

“…Here, we study sedimentary records from three locations in the Stockholm Archipelago at an unprecedented temporal resolution. Extensive present‐day water column monitoring curated by the Swedish Meteorological and Hydrological Institute (SMHI; van Helmond et al 2020) combined with the vicinity of the largest city in Scandinavia, makes the selected study area uniquely suited to address the role of humans in the development of (pre‐modern) hypoxia in the coastal zone of the Baltic Sea. In addition, in order to address the role of climate, we have generated the first detailed biomarker‐based sea surface temperature (SST) record for the coastal zone of the Baltic Sea.…”

Baltic Earth Assessment Report on the biogeochemistry of the Baltic Sea

“…These wastewater discharges depend on the population, their diets and the efficiency of wastewater treatments (Van Puijenbroek et al, 2015). The land-use activities that drive N and P loads are mainly associated with agricultural activities (Reusch et al, 2018), and these activities vary greatly across the Baltic Sea basin with much greater N inputs and losses in the south compared to the northern part of the drainage basin (Andersen et al, 2016).…”

“…Under these conditions, ammonium released during remineralization may be nitrified, facilitating nitrate reduction processes in the upper sediment column (principally denitrification, leading to loss of fixed N as N2 gas; and dissimilatory nitrate reduction to ammonium (DNRA), which retains fixed N). Denitrification dominates in Baltic Sea sediments (van Helmond et al 2020;Hietanen and Kuparinen, 2008;Jäntti et al, 2011) although DNRA may be more active under certain conditions (Bonaglia et al, 2017;Jäntti and Hietanen, 2012). Anaerobic ammonium oxidation (anammox), leading to loss of fixed N as N2 gas, has been found to be insignificant in Baltic Proper sediments (Hylén et al, unpublished results).…”