Supra-Permafrost Groundwater is a Source of Highly Biodegradable Dissolved Organic Matter to the Arctic Ocean

Supra-permafrost groundwater (SPGW) flowing through seasonally thawed soils is an important source of dissolved organic matter (DOM) to the Arctic Ocean, yet few studies have investigated the quality of this DOM. We sampled SPGW, runoff, and rivers near Simpson Lagoon, Alaska, during spring ice breakup, summer open water, and fall freeze-up seasons. Through incubation experiments, we compared biodegradable DOC (BDOC) across sources and seasons and linked these results with DOM composition using Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS). DOM composition was distinctly different between SPGW and rivers and shifted throughout the year. SPGW contained an order of magnitude more dissolved organic carbon (DOC) than rivers, with average concentrations increasing from breakup (22 ± 5 mg C L^-1) to freeze-up (110 ± 42 mg C L^-1). SPGW also contained three times or more BDOC than rivers, averaging 18 ± 1% during breakup and declining to 3.4 ± 0.7% during freeze-up. BDOC was positively correlated with condensed aromatic and polyphenolic DOM formula classes that are often associated with recalcitrant organic matter. Thus, SPGW DOM is highly aromatic yet biodegradable and is likely an important energy source to Arctic coastal waters in summer when river inputs are low.