DOES TRANSIENT STORAGE IN STREAMS ENHANCE NUTRIENT UPTAKE RATES?: SELECTING THE BEST METRIC FOR ANALYSIS

The temporary storage of stream water in transient storage zones, such as in-channel pools or near-stream flow paths, slows nutrient transport downstream. This temporary storage increases nutrient residence times and the interaction between nutrients and microbial communities in the substrate. A large residence time, coupled with increased interaction with microbes, may lead to enhanced primary productivity, uptake or transformation of nutrients within the stream ecosystem. To measure transient storage, one-dimensional solute transport models simulate conservative solute breakthrough curves. Models characterize the degree of transient storage from best-fit values for α, the storage exchange coefficient, and A_s, the cross-sectional area of the storage zone.

We compiled studies on this hyporheic interaction to evaluate which storage metrics best capture the importance of transient storage along different streams and used these metrics to compare nutrient uptake rates with degree of transient storage. Metrics that ignore velocity do not properly assess transient storage because water velocities limit nutrient interaction with storage zones. Of those metrics that do consider velocity, assessments of transient storage are comparable, but that relationship can be very weak in streams where transient storage is relatively important. Using a more comprehensive metric (%F_med²⁰⁰) for comparing transient storage and nutrient uptake rates does not clarify whether the two are correlated. Evidence suggests transient storage can potentially increase, decrease, or have no effect on nutrient retention in streams.