Remineralization of soil-derived dissolved organic carbon in the high Arctic fjord (Kongsfjorden)

Ongoing climate change has a significant impact on marine and terrestrial polar ecosystems. The increased melting, and retreat of glaciers, as well as the permafrost thawing, intensify the transport of dissolved organic matter from land to Arctic fjords. It has been estimated that the permafrost surface layer contains as much as 1035 ± 150 Pg of organic carbon (Hugelius et al., 2014), so even a small release can significantly change the carbon loads reaching the fjords. Although there are quantitative estimations of the dissolved organic carbon (DOC) delivered from land, the fate of the soil-derived DOC in fjords remains highly unknown. It is still unclear to what extent this DOC pool is bioavailable and how fast can it be remineralized. Therefore, the following research objectives have been formulated: (1) to quantify the shares of labile, semi-labile, and refractory fractions in the soil-derived DOC, and (2) to estimate remineralization rate constants and half-life times for different bioavailable fractions of DOC. This has been done through the 180-days-lasting incubation experiments of the DOC released from soils (soil leachates) mixed with the seawater from adjacent fjord. At the beginning and after 1, 2, 3, 5, 9, 19, 29, 65, 90, and 180 days of incubation, the individual samples were collected to measure DOC concentrations. For the study site, the catchments of two rivers in Kongsfjorden (West Spitsbergen, Svalbard) - Bayelva River and Londonelva River were selected. Bayelva River is 4 km long, with a glacierised catchment area of 32 km², which is almost entirely underlain by permafrost with a seasonal active layer (Killingtveit, 2004). Londonelva River is located on Blomstradøya (small island in Kongsfjorden) and is characterized by a small (0.7 km²) de-glacierised catchment area. The results indicate that the soil leachates contain a lot of DOC (420 μmol L^-1 in the Bayelva catchment and 2730 μmol L^-1 in the Londonelva catchment), which is highly bioavailable - 61% and 66% of DOC was remineralized during the incubation experiment, respectively. This high bioavailability of terrestrial DOC indicates that its supply has the potential to play an essential role in sustaining the bacterial loop in the fjord and, through the CO₂ release, to amplify ocean acidification in the coastal zone. The obtained results contribute to a better understanding of the processes shaping the carbon cycling in the Arctic fjords (and likely in other polar regions).