The fate of ice sheet-derived organic matter in an oligotrophic Greenland fjord

The Greenland Ice Sheet is melting rapidly, increasing freshwater runoff to the coastal ocean around Greenland. Through this pathway, allochthonous material, including nutrients, sediments, and organic carbon, is transported to coastal waters. The impacts of these inputs on coastal carbon cycling are poorly resolved, and accelerating climate change prompts closer examination of the character and fate of allochthonous material reaching Arctic coasts. In this study, we have taken a closer look at quantity, quality, and transformation of organic matter (OM) in surface waters of an oligotrophic high Arctic fjord influenced by glacial and proglacial runoff. We examined dissolved, suspended, and sinking OM by combining in situ observations along a river-to-sea gradient with experiments quantifying bioavailable carbon fractions, production of transparent exopolymer particles (TEPs), and OM flocculation. We found that dissolved organic carbon (DOC) concentrations in glacial and proglacial river waters were comparatively low (<30 µM), suggesting that these inputs should dilute DOC concentrations in the fjord. At the same time, riverine DOC was at least two times more bioavailable than marine DOC. Non-conservative DOC mixing along the river-to-sea gradients further indicated additional DOC supply, which we hypothesize is due to desorption from inorganic particles.

Much of the riverine particulate OM (POM) was observed to sediment out within the first few kilometers upon entering the fjord, with salt-induced flocculation and, to some extent, TEPs formation contributing to efficient aggregation and sinking. The sinking POM flux included a distinct contribution from chlorophyll-containing particles, indicating that freshwater inputs enhance downward export of phytoplankton biomass. The coexistence of this export with low but steady chlorophyll standing stocks in the water column implies concurrent primary production that persists even under turbid low-light conditions.

Overall, our results highlight the complexity of coastal carbon cycling in a changing Arctic and demonstrate that glacial river plumes act as reaction zones for rapid and multidirectional transformations of OM. By resolving interactions among freshwater inputs, particle dynamics, and multiple OM pools along river-to-sea gradients, this study advances understanding of how increasing land-ocean connectivity reshapes carbon cycling and ecosystem functioning in the coastal Arctic.