The fate of terrestrial organic carbon (OC) in the global ocean is largely unknown and it is speculated to be rapidly degraded in the coastal waters. Arctic marine waters, especially Hudson Bay, have a disproportionally large terrestrial dissolved organic carbon (tDOC) input compared to the global ocean, which is increasing due to climate change. The findings of these first studies of microbial degradation of terrestrial OC in Hudson Bay have revealed the presence of high Apparent Oxygen Utilization in a subsurface, winter-ventilated water mass that cannot be explained by respiration of settling marine-produced OC as currently understood. I hypothesize that tDOC deposited into coastal waters ultimately gets degraded (consuming oxygen) under the ice cover. The findings from incubation experiments that 20-50% of the tDOC deposited into Hudson Bay by southern rivers in late winter is biodegradable within a few weeks are consistent with this hypothesis.