49 Sulfide toxicity as key control on anaerobic oxidation of methane in eutrophic coastal sediments Figure 1. Sampling locations within the Stockholm Archipelago selected for this study. Bottom water redox conditions at each site are color coded with a green triangle (oxic), an orange circle (hypoxic) and a red square (euxinic). This figure was generated using Ocean Data View (Schlitzer, 2015) with permission (changes were made) from Copyright 2023 Ocean Data View https://creativecommons.org/licenses/by/4.0/. Porewater analysis Methane concentrations were determined with a gas chromatograph equipped with a flame ionization detector after addition of nitrogen headspace of 10 mL and equilibration of the gas and water phase for a week (van Helmond et al., 2020). Porewater sulfide and ammonium concentrations were measured spectrophotometrically with phenylenediamine and ferric chloride (Cline, 1969) and indophenol-blue (Solorzano, 1969), respectively. Iron and Mn were measured by Inductively Coupled Plasma-Optimal Emission Spectroscopy (ICP-OES), nitrate and nitrite were determined with a discrete analyzer and sulfate by ion chromatography. Details are described in the supplement. Flux estimates The downward and upward fluxes of sulfate and methane, respectively, into the sulfate-methane transition zone (SMTZ) and the diffusive fluxes of methane across the sediment-water interface were calculated using Fick’s first law of diffusion: 2
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