54 Chapter 2 RESULTS AND DISCUSSION Sulfate-dependent anaerobic oxidation of methane is the dominating process for benthic methane removal. At the time of sampling, water column characteristics at the three study sites were coherent with historical environmental monitoring data by SMHI (Table 1 and 2, and Supplementary Figure 1) (Beulig et al., 2019). Oxygen penetration into the sediment followed the trend in ambient bottom water redox conditions, with the deepest oxygen penetration at Site 3 and no oxygen penetration at Site 7 (Supplementary Table 3). In fact, for the latter, sulfide (10 µmol L-1) was detected in the bottom water (Table 2 and Supplementary Table 3). Macrofauna (polychaetes and bivalves) were only observed at Site 3. While bottom water pH, salinity and temperature were similar across sites, the different bottom water redox conditions were reflected in full profile-averaged sedimentary TOC contents, with the highest TOC observed at the euxinic site, Site 7 (Table 2 and Supplementary Table 3). However, taken together, the three sites are geochemically rather similar, and comparable to other sites in the Stockholm Archipelago that were previously geochemically characterized, i.e. organic-rich sediments, sulfide accumulation in the porewaters and a shallow SMTZ (Sawicka & Brüchert, 2017; van Helmond et al., 2020). Table 1. Geographical coordinates and characteristics of each sampling site. Site Coordinates (DD°MM’sS”) Water depth* Bottom water redox conditions* Location in the Archipelago** Site 3 (Sandöfjarden) 59°24”33”N 18°31”14”E 64 m Oxic Intermediate Site 5 (Lilla Värtan) 59°22”08”N 18°05”35”E 21 m Seasonally hypoxic Inner Site7 (Skurusundet) 59°17”55”N 18°13”46”E 27 m Generally euxinic Intermediate *Based on monitoring data from the Swedish Meteorological and Hydrological Institute in 2019 (https://sharkweb.smhi.se/hamta-data/). **Following a previously described classification system (Almroth-Rosell et al., 2016).
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