Field sampling
Sediment was collected on board R/V Electra during 2018 September 20–23 in the Gulf of Finland (Baltic Sea) close to the Tvärminne Zoological Station, Finland. A total of seven stations were visited along coastal gradients in depth and OC (0–4 km, 10–45 m water depth; Figure 1). The stations were divided into four low % OC shallow sites (stations 11, 12, 15, 16; 1.2–2.8 % OC) and three sites with higher % OC and depth (stations 7, 10, 13; 6.2–6.9 % OC), following a station labelling system used during reoccurring monitoring in the Tvärminne region (Table 1). Triplicate sediment cores (labelled A, B, C), retrieved in rinsed acrylic core liners, were collected from each station with a GEMAX twin gravity corer (height: 80 cm, inner diameter: 90 mm). The top 0–2 cm sediment surface layer was sliced into autoclaved 215 ml polypropylene containers (Noax Lab). After slicing, the sediment was directly aseptically homogenized inside the containers and 2 ml sediment transferred into 2 ml cryogenic tubes (VWR) that were immediately flash frozen at -196ºC. The samples were transported on dry ice and stored at -80°C until RNA extraction. The remaining sediment in the 215 ml containers were stored at -20°C for sediment C and N content and pore water chemistry analyses.