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A zero-order approach to estimate nutrient uptake length using tracer addition

Nutrient uptake in streams is often quantified by uptake length (Sw). Nutrient addition experiment is a cheap way to estimate Sw compared to isotope tracers. The current approach to estimate uptake length using linear regression of log-transformed nutrient concentration (called the 1st order approach) assumes the nutrient uptake follows 1st order kinetics, which is invalid under nutrient-saturated conditions. Theoretic analysis using one dimensional solute transport model confirmed that the 1st order approach consistently overestimate the actual Sw by up to 4.6-fold. The overestimation is even greater for pulse tracer injection in which nutrient-saturated condition is inevitable due to large amount of nutrient addition. To overcome this limitation, we have developed an alternative, a zero-order approach, to estimate Sw under nutrient saturated condition as it mimics Michaelis-Menten kinetics closely when nutrient is not limiting. Our theoretic analysis confirms the zero-order approach provides much more accurate estimate of Sw than the commonly used 1st order approach. We conclude that the zero-order approach should be adapted whenever possible to estimate nutrient uptake lengths in nutrient addition experiments, especially for pulse addition experiments.

Chuanhui Gu (Primary Presenter/Author), Appalachian State University,;

William P. Anderson, Jr. ( Co-Presenter/Co-Author), Appalachian State University,;

Laura Heinen ( Co-Presenter/Co-Author), Appalachian State University,;