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.