Long-Term Trends in Stream Biogeochemistry and Hydrology in Watersheds Underlain with Discontinuous Permafrost in the Boreal Forest of Alaska

Abstract:

In the boreal forest of Alaska, watersheds are commonly underlain with discontinuous permafrost, which has a dominant control on catchment hydrology and the resulting input of dissolved organic matter (DOM) and nutrients into streams. Much of the permafrost is near the point of thawing, which will alter the hydrologic connections between watersheds and streams. Wildfires are also common in the boreal forest of Alaska and can impact streams hydrology and nutrients. Wildfires have been increasing in frequency and severity with recent warming, and have the potential to alter the delivery of DOM and nutrients to streams. Together, permafrost and wildfires interact to regulate hydrology and material inputs into streams.

We have been measuring stream flow since 1980, and water chemistry since 2000 in streams draining sub-catchments with varying extents of underlying permafrost in the Caribou-Poker Creeks Research Watershed in interior Alaska. Across sub-catchments, streams draining watersheds with extensive permafrost have flashier hydrographs compared to streams draining catchments with little permafrost. Coupled to patterns in flow, DOM concentration is greatest in streams draining watersheds with large extents of underlying permafrost, whereas nitrate concentration tends to be highest in streams draining lower permafrost catchments. Following a wildfire in the CPCRW, stream DOM concentration significantly declined compared with a control stream, suggesting that the fire either resulted in a loss of soil organic matter or altered hydrologic flowpaths through catchments. With climatic warming, permafrost thaw and changing fire frequency will interact to determine DOM concentration and composition in streams of the boreal forest, and will have important impacts on the coupling between DOM decomposition and nutrient cycling.