The effects of transient storage on carbon uptake in a sub-arctic stream in interior Alaska
Abstract
The fate of dissolved organic carbon (DOC) in streams is largely controlled by organic matter and nutrient inputs from the catchment, biotic uptake, and hydrologic retention within transient storage zones (hyporheic zone, pools, eddies). The hyporheic zone can be an important site for processing of organic matter and, thus hydrologic residence time in transient storage is an important factor regulating carbon uptake. Our research examined transient storage and carbon uptake in a sub-arctic stream to understand the mechanisms affecting hyporheic carbon cycling in streams underlain by warming permafrost. The research was conducted in a stream draining a low permafrost catchment in the Caribou-Poker Creeks Research Watershed in interior Alaska, in which permafrost underlies ~5% of the catchment. Short-term steady-state solute injections of a conservative tracer and acetate were performed in the summers of 2008 through 2010 to capture variation in soil thaw and stream discharge. Transient storage was described with the one-dimensional transport with inflow and storage (OTIS) model. Carbon uptake was determined by calculating the mass transfer coefficient (Vf; mm/min), which describes the vertical velocity of carbon across the stream/sediment interface. Residence time in transient storage increased with discharge, but was not related to thaw depth suggesting the hyporheic zone is relatively shallow compared to the thaw bulb beneath the stream. The stream is highly incised with thick vegetation along the banks. High discharge may result in increased lateral hydrologic exchange with flowpaths through the stream bank. Stream flow frequently cuts into and under the bank up to 50 cm (average width = 80 cm) creating “lateral pockets,” and vegetation roots create pools. During high flows these surface features may increase surface storage and enhance lateral exchange. The carbon mass transfer coefficient (mm/min) decreased with discharge revealing more efficient carbon uptake at lower discharge when hydrologic residence time was lowest. When stream discharge is low, hydrologic exchange may be largely with the subsurface where carbon demand is high and residence time is short. At high discharge, exchange seems to increase with lateral surface storage zones where carbon demand may be lower but residence time is longer. This pattern is contrary to what was expected and suggests storage zones shift between low and high discharge and that these storage zones have differing capacities to process organic matter. This study shows that carbon uptake in streams underlain with discontinuous permafrost is highly influenced by discharge. With climate warming, permafrost thaw will alter aquatic-terrestrial linkages, thereby changing the hydrology of streams. Long-term stream flow changes will affect the balance between surface and sub-surface storage, which will have important implications for carbon and nutrient cycling.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.H24C..07R
- Keywords:
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- 0428 BIOGEOSCIENCES / Carbon cycling;
- 0702 CRYOSPHERE / Permafrost;
- 1615 GLOBAL CHANGE / Biogeochemical cycles;
- processes;
- and modeling;
- 1830 HYDROLOGY / Groundwater/surface water interaction