Aquakem 250

Oxygen samples were analysed by Winkler titration. Water column and pore water solutes were analysed at Tvärminne Zoological Station (Thermo Scientific Aquakem 250). Quantification limits for solutes were NH₄⁺ 0.0001, NO_x 0.00003, PO₄³⁻ 0.00003 and Si⁴⁺ 0.0007 mmol l⁻¹. Solute fluxes measured in the experimental cores were calculated from the difference in the concentration between start and end samples as mmol m⁻² d⁻¹.While the oxygen concentration inside the experimental cores was allowed to decrease during the dark incubation following respiratory activities of the benthic community, the oxygen concentration never dropped below 56% saturation, and is therefore unlikely to have affected neither faunal behaviour nor chemical processes.

Extracted permafrost filtrates were analyzed for anions by ion chromatography using a Metrohm 881 Compact IC pro with detection by suppressed conductivity. Anion determinations were performed using a Metrosep A Supp 7/250 column with 3.6 mM Na₂CO₃ eluent (0.8 mL/min) at 45°C. A complementary set of determinations were performed targeting organic acid anions using a Metrosep Organic Acids 250/7.8 column with 0.5 mM H₂SO₄ eluent (0.5 mL/min) at 30°C.
Soluble NH₄⁺, NO₃^–, and NO₂^– measurements were carried out using batch-automated spectrophotometry (Aquakem 250, Thermo Fisher Scientific, United States). Ammonium was determined using the Phenate method [United States Environmental Protection Agency (U.S. EPA) method 350.1]. Nitrite was determined by diazotization with sulfanilamide and nitrate was catalytically reduced to nitrite using soluble nitrate reductase in the presence of reduced nicotinamide dinucleotide (NADH; EPA Method 353.1). All samples were analyzed in duplicate. Final DOC, DIN, and SCFA data were corrected for dilution and gravimetric water content.