Sensitivity analysis of a pulse nutrient addition technique for estimating nutrient uptake in large streams

Laurence Lin, J. R. Webster

Limnol. Oceanogr. Methods 10:718-727 (2012) | DOI: 10.4319/lom.2012.10.718

ABSTRACT: The constant nutrient addition technique has been used extensively to measure nutrient uptake in streams. However, this technique is impractical for large streams, and the pulse nutrient addition (PNA) has been suggested as an alternative. We developed a computer model to simulate Monod kinetics nutrient uptake in large rivers and used this model to evaluate the sensitivity of the PNA technique. We parameterize our model using the average hydrogeomorphological estimates from a pulse release study of ammonium in the Snake River, WY, and used this study to demonstrate how data from a field experiment can be effectively analyzed using a simulation model. To evaluate the sensitivity of the PNA technique, we manipulated the hydrogeomorphology and uptake kinetics of our stream model, simulated a pulse ammonium addition, and measured the downstream response in our model as if it were a field experiment, while ammonium areal uptake at ambient concentration was kept unchanged in the model. Ammonium uptake estimates by the PNA technique were different from the uptake in our model and these differences were nonrandom. The difference was greatest when velocity was high and there was little solute spread, either in the water column or from exchange with transient storage. The difference was also high when the half saturation coefficient for uptake was low. Our estimates of ammonium uptake under the assumption of Monod kinetics were higher than those under the assumption of first-order kinetics based on direct calculation from the experimental data.