Surface accumulation of low molecular weight dissolved organic matter in surface waters and horizontal off-shelf spreading of nutrients and humic-like fluorescence in the Chukchi Sea of the Arctic Ocean.

Polar regions play unique roles in global overturning circulation, carbon cycling, and climate change. In this study, seawater dissolved organic matter (DOM) was characterized for the Chukchi Sea in the Arctic Ocean in the summer season. The seawater generally contains high concentrations of dissolved organic carbon (DOC, up to 92 μM C) and tyrosine-like fluorescence (up to 0.21 RU), and it was enriched with heteroatomic molecular formula with nitrogen-containing and sulfur-containing formulas counting 2246 (~41% of total identified molecular formula) and 1838 (~34%), respectively. Significant correlations were observed between salinity and the absorption coefficient at 254 nm, between chlorophyll-a and DOC as well as the tyrosine-like component, C270/302 (Cex/em maxima), and between biological index and two protein-like components, C275/338 and C305/344 . A comparison between surface waters and close-to-seafloor deep waters suggested a trend of the accumulation of low molecular weight (LMW) fraction (~54-74%, nominal average molecular weight Mn  < ~350 Da) in the surface waters. Another interesting finding from spatial data was an obvious horizontal off-shelf spreading of nutrients and humic-like fluorescence. This study sheds novel insights of DOM characteristics and dynamics in the highly productive polar sea.