Land use can affect a stream’s capacity for nutrient processing and storage by changing the amount and sources of nutrients while simultaneously altering biofilm community composition. We conducted nutrient spiraling experiments in impacted and less-impacted tributaries of the Portneuf River, Idaho to determine how land use alters uptake of nitrogen (N), phosphorus (P), and dissolved organic carbon (DOC). We also quantified differences in leaf decomposition as a measure of in-stream carbon availability; lastly, we measured the response of biofilm functional groups to carbon addition in laboratory experiments.
We found average uptake velocities (V_f) an order of magnitude higher for both DOC and P in less-impacted compared to impacted streams, and no difference in V_f for N, perhaps because the entire watershed is limited by N. Leaf decomposition rates decreased with increasing land use intensity. Differences in nutrient uptake and decomposition rates in response to land use may result from shifts in dominant functional groups in biofilms as a response to nutrient availability. The experimental data suggest that competitive effects within the biofilm community may be a mechanism which mediates the ability of streams to retain or export additional nutrients.