Removal of phosphorus and nitrogen in sediments of the eutrophic Stockholm archipelago, Baltic Sea
<p>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 (<span class...
| Main Authors: | , , , , , , , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2020-05-01
|
| Series: | Biogeosciences |
| Online Access: | https://www.biogeosciences.net/17/2745/2020/bg-17-2745-2020.pdf |
| Summary: | <p>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
(<span class="inline-formula">O<sub>2</sub></span>) 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 (<span class="inline-formula">∼4</span> to
8 mmol m<span class="inline-formula"><sup>−2</sup></span> d<span class="inline-formula"><sup>−1</sup></span>), linked to prior deposition of organic-rich
sediments in a low-<span class="inline-formula">O<sub>2</sub></span> 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 <span class="inline-formula">O<sub>2</sub></span> concentrations prevail in summer. Burial rates
of P are high at all sites (0.03–0.3 mol m<span class="inline-formula"><sup>−2</sup></span> yr<span class="inline-formula"><sup>−1</sup></span>), a combined
result of high sedimentation rates (0.5 to 3.5 cm yr<span class="inline-formula"><sup>−1</sup></span>) and high
sedimentary P at depth (<span class="inline-formula">∼30</span> to 50 <span class="inline-formula">µ</span>mol g<span class="inline-formula"><sup>−1</sup></span>).
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<span class="inline-formula"><sup>−2</sup></span> d<span class="inline-formula"><sup>−1</sup></span>) 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.</p> |
|---|---|
| ISSN: | 1726-4170 1726-4189 |