Tuesday, May 19, 2009
Information regarding variation in nutrient uptake between stream channels and surface transient storage (STS; eddies, sides of pools, etc.) is needed to improve modeling of river network nutrient cycling. We measured STS hydrologic residence time and nutrient uptake in six 1st-5th-order streams in a coastal watershed. In one 3rd-order stream, we distinguished between activity in the main channel (thalweg) and in STS zones by collecting zone-specific samples of dissolved inorganic nitrogen, N2, and N2O during 15N additions. Data from a whole-reach 15NO3- tracer addition show enrichment of 15N2 in STS compared to the channel, suggesting that STS zones were denitrification hotspots. However, differences between main channel and STS chemistry were relatively small because STS residence time was short. To facilitate measurement of nitrogen dynamics, we partially isolated STS zones from the main channel to increase water residence time while maintaining water circulation. Results indicate that a high rate of denitrification was occurring in some STS zones. Discriminating main channel and STS uptake rates will improve understanding of how nutrient removal varies among stream types and sizes.