48 Chapter 2 MATERIALS AND METHODS Sampling Sampling was carried out on board the R/V Electra from June 11 to 13, 2019, at three locations in the Stockholm Archipelago with contrasting bottom water redox conditions, that were selected based on extensive environmental monitoring by the Swedish Meteorological and Hydrological Institute (SMHI; https://sharkweb. smhi.se/hamta-data/): Site 3 (Sandöfjarden – oxic), Site 5 (Lilla Värtan – seasonally hypoxic), and Site 7 (Skurusundet - euxinic) (Figure 1). Prior to sediment retrieval, a CTD instrument (SBE 911plus, Sea-bird Scientific, USA) was deployed to determine in-situ water column characteristics, such as dissolved oxygen concentrations, temperature and salinity. Site coordinates and characteristics are provided in Table 1 and 2. At each study site, nine sediment cores were collected with a Gemini gravity corer, using transparent PVC core liners of 80 cm in length and an inner diameter of 8 cm. Three cores were sieved over a 0.5 mm mesh size, after which macrofauna was determined at genus level. One core was transferred to the onboard laboratory for high-resolution depth profiling of oxygen, pH and sulfide using microelectrodes. Samples for bottom and porewater methane were collected from another core directly after its retrieval with cutoff syringes via pre-drilled holes covered with tape prior to coring. One core was sliced in a glove bag under a nitrogen atmosphere. Firstly, two bottom water samples were taken from the overlying water, after which sediment slices were collected in 50 mL conical tubes, centrifuged and subsampled for a range of solutes. One core was sliced under ambient atmospheric conditions to determine the water content of the sediment. Details on core handling, resolution and porewater extraction are described in the supplements. The last two cores were sliced under a nitrogen atmosphere for microbiological analyses as detailed below.
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