55 Sulfide toxicity as key control on anaerobic oxidation of methane in eutrophic coastal sediments Table 2. Characterization of sampling sites. BW, bottom water; TOC, total organic carbon. Site Oxygen (µmol L-1) and pH at sedimentwater interface* Oxygen penetration depth (mm)* BW salinity BW temperature (°C) Macrofauna Average TOC (%)* Site 3 (Sandöfjarden) 150 7.4 5.3 5.8 2.1 Marenzelleria sp. and M. balthica 5.1 Site 5 (Lilla Värtan) 55 7.3 1.4 4.9 2.3 none 5.9 Site 7 (Skurusundet) 0** 7.1 0 5.3 2.8 none 8.5 *Full profiles in Supplementary Table 3. **At Site 7, sulfide (10 µmol L-1) was present in the bottom water. The observed shallow SMTZ is a common feature of eutrophic coastal sediments (Wallenius et al., 2021). Porewater sulfate decreased with depth at all sites (Figure 2, Supplementary Table 3). At Site 3, it took ~ 30 cm below the seafloor (cmbsf) before sulfate was depleted, whereas at Sites 5 and 7 this removal occurred already around 10 cmbsf (Figure 2). Methane concentrations increased slightly with depth to a maximum of ca. 100 µmol L-1 at Site 3. At Sites 5 and 7, in contrast, concentrations of methane increased strongly with depth, reaching values of ~ 2 mmol L-1. Porewater sulfide was only present at low concentrations (< 200 µmol L-1) and in a confined zone (10-30 cmbsf) at Site 3. At Sites 5 and 7, however, sulfide concentrations increased rapidly with depth, with the strongest increase and highest concentrations (>1 mmol L-1) at Site 7. Additionally, sediments in Site 7 had a higher approximated annual sulfide exposure (0.88 mmol year-1) in comparison to Site 5 (0.39 mmol year1) and Site 3 (0.08 mmol year-1) (Supplementary Table 3). The shallow SMTZ is the combined result of low salinity, hence low sulfate concentrations, and high rates of organic matter deposition and degradation, culminating in a vertical compression of the redox zonation (Canfield & Thamdrup, 2009). The somewhat deeper SMTZ at Site 3 can be explained by its ambient redox conditions. Oxygen is perennially available throughout the water column, leading to more aerobic degradation of 2
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