Technical Note: A comparison of two empirical approaches to estimate in-stream net nutrient uptake

摘要

To establish the relevance of in-stream processes on nutrient export atcatchment scale it is important to accurately estimate whole-reach netnutrient uptake rates that consider both uptake and release processes. Twoempirical approaches have been used in the literature to estimate theserates: (a) the mass balance approach, which considers changes in ambientnutrient loads corrected by groundwater inputs between two stream locationsseparated by a certain distance, and (b) the spiralling approach, which isbased on the patterns of longitudinal variation in ambient nutrientconcentrations along a reach following the nutrient spiralling concept. Inthis study, we compared the estimates of in-stream net nutrient uptake ratesof nitrate (NO₃) and ammonium (NH₄) and the associated uncertaintyobtained with these two approaches at different ambient conditions using adata set of monthly samplings in two contrasting stream reaches during twohydrological years. Overall, the rates calculated with the mass balanceapproach tended to be higher than those calculated with the spirallingapproach only at high ambient nitrogen (N) concentrations. Uncertainty associated withthese estimates also differed between both approaches, especially forNH₄ due to the general lack of significant longitudinal patterns inconcentration. The advantages and disadvantages of each of the approachesare discussed.