Dissolved Organic Matter Discharge from the Six Largest Arctic Rivers: Chemical Composition and Seasonal Variability

The vulnerability of the Arctic to climate change has been realized due to disproportionately large increases in surface air temperatures. Effects of this temperature shift are widespread in the Arctic but likely include changes to the hydrological cycle and permafrost thaw, which have implications for the transport of organic carbon from the watersheds into rivers. The focus of this research was to describe the seasonal variability of the chemical composition of dissolved organic matter (DOM) in the six largest Arctic rivers (Yukon, Mackenzie, Ob, Yenisei, Lena and Kolyma) using optical properties (UV-Vis Absorbance and Fluorescence) and lignin phenol analysis. We also investigated differences between rivers and how watershed characteristics influence DOM composition. Dissolved organic carbon (DOC) concentrations followed the hydrograph with highest concentrations measured during snowmelt. The six rivers studied here shared a similar seasonal pattern and chemical composition. There were, however, large differences between rivers in terms of total carbon discharge reflecting the differences in watershed characteristics such as climate, catchment size, river discharge, slope, elevation, soil types, and permafrost distribution. Carbon and lignin flux estimates were highest from the large Siberian rivers (Lena, Yenisei), with a large proportion of permafrost. Although the Kolyma and Mackenzie rivers had the smallest carbon and lignin flux estimates; the Kolyma exported about twice the amount of organic material annually. Multiple linear regressions of the carbon (kg DOC km2 yr-1) export indicate the most important predictors are maximum elevation, temperature and percentage of continuous permafrost (Adj R2=0.45, p=0.05). Similarly, the most important predictors of lignin (kg lignin km2 yr-1) export were slope, maximum elevation, temperature and percentage of continuous permafrost (Adj R2=0.89, p<<0.001). The chemical composition of DOM during peak flow indicates a dominance of freshly leached material with elevated aromaticity, larger molecular weight, and elevated lignin yields relative to baseflow DOM. The seasonal difference in DOM quality likely reflects increased microbial processing of soil and river organics during summer. Other factors influencing the concentration and composition of river DOM include flow path (surface run off versus percolation) and sorption to different mineral soils.