57 Sulfide toxicity as key control on anaerobic oxidation of methane in eutrophic coastal sediments Figure 2. Porewater depth profiles of sulfate (SO4 2–), methane (CH4), sulfide (H2S), dissolved iron (Fe2+) and manganese (Mn2+/3+), ammonium (NH4 +) and the sum of nitrate and nitrite (NO x) at our study sites in the Stockholm Archipelago: Site 3 (Sandofjärden), Site 5 (Lilla Värtan) and Site 7 (Skurusundet). The arrow to the left indicates decreasing bottom water (BW) oxygen concentrations from Site 3 to 7. Cmbsf; centimeters below the seafloor. The calculated downward flux of sulfate into the SMTZ (Table 3) was highest at Sites 5 and 7 (1.5 and 1.3 mmol m-2 d-1), but still substantial (0.9 mmol m-2 d-1) at Site 3. The calculated upward flux of methane into the SMTZ at Site 3 was nearly absent, and around 0.5 mmol m-2 d-1 at Sites 5 and 7, which should be regarded as an absolute minimum estimate due to degassing of methane during sampling (Egger et al., 2016; Melaniuk et al., 2022). There was no benthic methane efflux at Site 3. The benthic methane efflux was about seven times larger at Site 7 relative to Site 5 (~ 1 vs. 0.15 mmol m-2 d-1, respectively). 2
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