Measurement uncertainty in stream nutrient uptake: Detecting land-use impacts on tropical streams

摘要

Stream nutrient uptake is an important ecosystem service, because it supplies human needs such as water purification. Uncertainty in nutrient uptake measurement determines, in combination with effect magnitude, whether effects of land use, stream restoration or climate change can be detected by this method. However, measurement uncertainty for nutrient uptake metrics is rarely reported. Here, we compared whole-stream phosphate and nitrate uptake among three pristine and three agricultural tropical Cerrado streams. We quantified uncertainty in uptake metrics estimated by kinetic nutrient-addition experiments and evaluated their potential to detect land-use effects. Ambient phosphate and nitrate uptake lengths (S-w) ranged from 11 to 106 m and from 29 to 357 m, respectively, in pristine streams and from 41 to 586 m and from 76 to 1372 m in agricultural streams. Moderate measurement uncertainty in S-W allowed for the detection of land-use impacts with this metric. However, derived nutrient uptake metrics, such as ambient uptake rates and uptake velocities, as well as parameters of whole-stream saturation kinetics, were associated with high measurement uncertainties that prevented the detection of potential land-use effects. Sensitivity analyses suggested that avoiding high enrichment levels in kinetic addition experiments and increasing sampling effort for plateau nutrient and tracer concentration at the first and last sampling stations of investigated stream reaches are the most promising strategies to reduce measurement uncertainty. Analyzing nutrient uptake without considering measurement uncertainty might lead to poor interpretation in case studies and meta-analyses, such as incorrect evaluations of human impacts or comparisons among systems.