Monday, August 6, 2007 - 1:50 PMCOS 15-2: Impact of channel geometry and periphyton growth on nutrient uptake and location of biogeochemical hotspots: Insights from a field-scale flume
Cailin Huyck Orr1, Jeffery J. Clark2, Jacques C Finlay1, and Peter R. Wilcock3. (1) University of Minnesota, (2) Lawrence University, (3) Johns Hopkins University
We investigated the impact of channel geometry on nutrient retention under a series of controlled conditions in a field-scale flume. Using a combination of conservative tracer, soluble reactive phosphorous (SRP) and nitrate additions, we studied the effects of two bed morphologies (plane bed and alternate bar) and two sediment mixtures (gravel and a sand-gravel mix), and variable periphyton abundance on nutrient retention. The addition of surface features, both alternate bar morphology and artificial structures, increased water transient storage area more than the addition of sand decreased it. Across treatments, a greater percent of the overall transient storage area was accounted for in surface features than in hyporheic flow. However, water exchange rates were much greater with clean gravel and a large volume (50%) of surface water exchanged with the bed over a 50 m test reach. SRP uptake rates varied widely (0-147 μg*m-2*min-1) across experimental conditions and changes in bed porosity had larger impacts on uptake rates than the addition of surface features, even though surface features increased transient storage. Nitrate uptake was only measurable concurrent with the highest SRP uptake rates. Periphyton growth had much greater impact on uptake than changes in geomorphic configuration and uptake was highest during periods of biomass accumulation. Bed porosity and hyporheic exchange were greatly reduced by periphyton accumulation and clogging. Biomass accumulation essentially isolated subsurface and surface water flow paths. This has implications for seasonal nutrient dynamics as periphyton growth has the potential to cut off surface water from biogeochemical transformations occurring at the sediment-water interface.