“Spring break” of Dissolved Organic Matter above the Arctic Circle: New Hints on Source and Composition

Abstract:

The Arctic Ocean, on a volume basis, receives the largest input of terrestrial organic matter of all ocean basins. These inputs come from the most important stock of soil organic carbon (OC) on Earth, estimated around 1700 Pg OC, which is well preserved in permafrost (from patchy to continuous). Arctic watersheds are experiencing unprecedented climate warming, and future warming is projected to be stronger at high latitudes. As a result, we can expect an increase in active layer depth and a decrease of permafrost extent in the near future. These shifts will affect the riverine contributions of terrestrial organic matter to the coastal and oceanic carbon pools. Until recently, few data existed about dissolved organic carbon (DOC) fluxes in Siberian rivers, particularly during the spring freshet. Further, there is still a dearth of information about the source and degradation state of this DOC in Arctic rivers through their hydrographs. To address this issue, we intensively sampled two spring flood periods in a small Canadian river (Great Whale river) and the largest Arctic river (Yenisei) at their outlet in order to get information on the source and state (“freshness”) of mobilized DOC during these active flood periods. Combining geographical information data (GIS) and biogeochemical analysis (elemental, isotopic and molecular) we were able to discern dynamic ecosystem linkages. DOC concentration increased 4-9 fold in each river from low flow to peak flow. Molecular characteristics of this DOC are extremely variable during the flood event, ranging from old, altered DOC to fresh, labile DOC. We observed a partition of this quality between DOC and POC, with fresher DOC coming mostly from the leaching of softwoods during peak flow. Snow cover is variable along the period with snow-free area of the watershed imparting the greatest influence on DOC composition within the river. These results confirm that DOC dynamics during the spring flood are complex and much different than during low flow conditions. As export of DOC in this period can reach up to 80% of annual DOC export, measurements acquired during this study will bring crucial information on not only watershed-aquatic system linkages, but also on global carbon cycling.