Abstract
Abstract. To establish the relevance of in-stream processes on nutrient export at catchment scale it is important to accurately estimate whole-reach net nutrient uptake rates that consider both uptake and release processes. Two empirical approaches have been used in the literature to estimate these rates: (a) the mass balance approach, which considers changes in ambient nutrient loads corrected by groundwater inputs between two stream locations separated by a certain distance, and (b) the spiralling approach, which is based on the patterns of longitudinal variation in ambient nutrient concentrations along a reach following the nutrient spiralling concept. In this study, we compared the estimates of in-stream net nutrient uptake rates of nitrate (NO3) and ammonium (NH4) and the associated uncertainty obtained with these two approaches at different ambient conditions using a data set of monthly samplings in two contrasting stream reaches during two hydrological years. Overall, the rates calculated with the mass balance approach tended to be higher than those calculated with the spiralling approach only at high ambient nitrogen (N) concentrations. Uncertainty associated with these estimates also differed between both approaches, especially for NH4 due to the general lack of significant longitudinal patterns in concentration. The advantages and disadvantages of each of the approaches are discussed.
Highlights
Understanding the relevance of in-stream uptake on nutrient loads has become an important question over the past decades due to the need to establish reliable nutrient budgets at catchment scale and to evaluate the impact of downstream nutrient export on coastal ecosystems (Behrendt and Opitz, 2000; Alexander et al, 2000; Wollheim et al, 2008)
Brookshire et al (2009) found no significant longitudinal patterns in ambient concentration in about 80% of a selection of stream reaches from several biomes, and concluded that in-stream processes may be commonly irrelevant for watershed nutrient balances because in-stream nutrient uptake is rapidly balanced by nutrient release
The aim of this study was to compare the estimates of instream net N uptake rates and the associated uncertainty obtained from the mass balance and the spiralling approaches aforementioned
Summary
Understanding the relevance of in-stream uptake on nutrient loads has become an important question over the past decades due to the need to establish reliable nutrient budgets at catchment scale and to evaluate the impact of downstream nutrient export on coastal ecosystems (Behrendt and Opitz, 2000; Alexander et al, 2000; Wollheim et al, 2008). Brookshire et al (2009) suggested that streams could act as net sinks of nutrients (i.e. positive net uptake rates) under certain environmental conditions that favour denitrification or net biomass growth. Taking all this into consideration it becomes clear that, besides of characterizing stream ecosystems in terms of gross nutrient uptake rates, it is important to estimate net nutrient uptake rates that provide more accurate information on actual nutrient export from a given stream reach and the relevance of in-stream processes at catchment scale
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