River Inflow Dominates Methane Emissions in an Arctic Coastal System.

We present a year‐round time series of dissolved methane (CH4), along with targeted observations during ice melt of CH4 and carbon dioxide (CO2) in a river and estuary adjacent to Cambridge Bay, Nunavut, Canada. During the freshet, CH4 concentrations in the river and ice‐covered estuary were up to 240,000% saturation and 19,000% saturation, respectively, but quickly dropped by >100‐fold following ice melt. Observations with a robotic kayak revealed that river‐derived CH4 and CO2 were transported to the estuary and rapidly ventilated to the atmosphere once ice cover retreated. We estimate that river discharge accounts for >95% of annual CH4 sea‐to‐air emissions from the estuary. These results demonstrate the importance of resolving seasonal dynamics in order to estimate greenhouse gas emissions from polar systems. Plain Language Summary: The primary cause of recent global climate change is increasing concentrations of heat‐trapping greenhouse gases in the atmosphere. Ongoing rapid Arctic climate change is affecting the annual cycle of sea ice formation and retreat; however, most published studies of greenhouse gases in Arctic waters have been conducted during ice‐free, summertime conditions. In order to characterize seasonal variability in greenhouse gas distributions, we collected year‐round measurements of the greenhouse gas methane (CH4) in a coastal Arctic system near Cambridge Bay, Nunavut, Canada. We found that during the ice melt season, river water contains methane concentrations up to 2,000 times higher than the wintertime methane concentrations in the coastal ocean. We utilized a novel robotic kayak to conduct high‐resolution mapping of greenhouse gas distributions during ice melt. From these data, we demonstrate that the river water containing elevated levels of methane and carbon dioxide (CO2) flowed into the coastal ocean, and when ice cover melted, these greenhouse gases were rapidly emitted into the atmosphere. We estimate that in this system, more than 95% of all annual methane emissions from the estuary are driven by river inflow. Key Points: Methane concentrations in an Arctic estuary show strong seasonality; river inflow at the start of the freshet drives elevated concentrationsObservations with a novel robotic kayak demonstrate that methane and carbon dioxide in the estuary are rapidly ventilated following ice meltRiver discharge is estimated to account for >95% of annual methane emissions from the estuary [ABSTRACT FROM AUTHOR]