Riverine Dissolved Organic Carbon And Freshwater Export In The Eastern Gulf Of Alaska

The coastal zone of southeast Alaska contains thousands of streams and rivers that drain one of the wettest, carbon-rich, and most topographically varied regions in North America. Watersheds draining temperate rainforests, peatlands, glaciers, and three large rivers that flow from the drier interior of the Yukon Territory and British Columbia discharge water and dissolved organic carbon (DOC) into southeast Alaskan coastal waters. This area, which we have designated the southeast Alaska drainage basin (SEAKDB), discharges about twice as much water as the Columbia or Yukon Rivers. An understanding of the timing, location, and source of water and DOC guides research to better understand the influence of terrestrial outputs on the adjacent marine systems. Additionally, a spatially extensive understanding of riverine DOC flux will improve our understanding of lateral losses related to terrestrial carbon cycling. We estimate 1.17 Tg C yr(-1) of DOC enters the adjacent marine system along with 430 km(2) of freshwater that influences estuary, shelf, and Gulf of Alaska hydrology. We estimate that 23% to 66% of the DOC entering coastal waters is bioavailable and may influence metabolism and productivity within the marine system. The combination of the large and spatially distributed water and DOC input, long and complex shoreline, large enclosed estuarine volume, and bounded nearshore coastal currents suggests that the physiographic structure of southeast Alaska may have a significant impact on the metabolism of riverine DOC in coastal marine ecosystems.Plain Language Summary The coast of southeast Alaska drains coastal rainforests, glaciers, and interior basins in Alaska, British Columbia, and the Yukon. High precipitation, glacier melt, and large interior watersheds combine to discharge an annual volume of freshwater greater than that of the Columbia or Yukon Rivers, which drain basins 3.5 and 4.5 times larger. This large amount of water entering the Alexander Archipelago estuaries and coastal shelf creates currents and density gradients that influence how the river discharge and organic matter in it enter the Gulf of Alaska in the northern Pacific Ocean. Along with the large amount of water there is 1.17 Tg of dissolved organic carbon (DOC), which can be taken up by bacteria and other microbes and turned into carbon dioxide or food that can be incorporated into the marine food web. A portion may settle into marine sediments to be stored for long periods of time. The amount and type of DOC and its entry into the marine ecosystem are controlled by watershed characteristics within the southeastern coast of Alaska. A better understanding of where and when DOC and freshwater enter the ocean may help us understand its importance to marine processes.