Target2 gmf syringe filter

BW, pore water [wet sediment was centrifuged at 2,200 × g for 15 min and the water filtered through a 0.7- μM Target2™ GMF Syringe Filter (Thermo Scientific™)], and SED was analyzed. pH (pHenomenal, VWR pH electrode, VWR™) was measured for BW and pore water samples. Total Fe, ${\text{PO}}_4^{3-}$ , and ${\text{SO}}_4^{2-}$ concentrations for pore water and BW were measured based on spectrophotometric methods (Broman et al., 2018 (link)). Additionally, ${\text{NO}}_3^{-}$ and ${\text{NO}}_2^{-}$ concentrations for pore water were measured using Hach–Lange cuvette tests LCK339 and LCK341, respectively. Organic matter content (% wt) was measured using the loss on the ignition method as described in Broman et al. (2017b (link)) except that the sample was dried before loss on ignition analysis for 6 days at 40°C instead of 3 days at 80°C.

Three replicate sediment cores were sampled with a Kajak gravity corer at each location to give 24 acrylic cores (inner diameter: 7 cm; length: 60 cm) for each bay and a total of 48 cores per sampling occasion as described in Seidel et al. (2022 (link)). Surface plus bottom water temperatures and bottom water oxygen concentrations were measured in situ (Multiline™ sensor, WTW™) during sampling. Additionally, overlying bottom water samples (BW; 50 ml) were taken from the sediment (SED) cores close to the SED surface and transferred into sterile tubes (Thermo Scientific™) for 16S rRNA gene amplicon sequencing (described below). An additional 15 ml of BW was filtered through a 0.7-μm Target2™ GMF Syringe Filter (Thermo Scientific™) into a pre-acid washed polypropylene tube (Thermo Scientific™) for chemical analyses. The BW was decanted from the core, and a 0–1-cm SED slice was collected in a sterile 50-ml polypropylene tube (Thermo Scientific™). The tube contents were well-mixed, and 15 ml was transferred to a pre-acid washed polypropylene tube (Thermo Scientific™) for subsequent pore water analysis. A further 2-ml reaction tube (Sarstedt, Inc.) was filled with SED for organic matter (OM) analysis. All samples were cooled during transport to the laboratory on the same day where they were frozen at either −80 or −20°C.

Chemistry analyses were conducted on pore waters and the sediment itself. Pore waters were sampled by centrifuging the 15 mL sediment tubes at 2200 × g for 15 min and the supernatant transferred into a new 15 mL acid washed centrifuge tube (Thermo Scientific™). The pore water was filtered through a 0.7 µm Target2™ GMF Syringe Filter (Thermo Scientific™) and pH was measured with a pH electrode (pHenomenal, VWR™). Sulfate, dissolved inorganic phosphate, total iron, nitrate + nitrite (in combination), and organic matter were measured as previously described^{14 (link)} except for organic matter samples that were dried for a total of six days at 40 °C instead of 3 days at 80 °C.