Using Biogeochemical Tracers to Quantify Catchment Resilience to Nutrient Loading
Résumé
Agriculture and urbanization have reshaped the flow of carbon and nutrients through ecosystems at a global scale, disrupting fundamental ecosystem processes that support human life. Because nutrient excess affects ecosystems in developed and developing countries, we need tools for quantifying landscape nutrient removal capacity to focus and guide sustainable management efforts. In this study, we tested how a suite of biogeochemical and hydrological tracers could be used to affordably estimate the resilience of catchments to nutrient loading. We hypothesized that the ratio of water residence time and biogeochemical reaction time (the Damköhler number or Da) would control nutrient resilience because Da predicts how much of a nutrient can be processed as it passes through the soil, aquifer, hyporheic zone, and stream network. Using a 10-year public dataset from northwestern France, we calculated nutrient removal capacity with nitrate inputs and outputs for 49 catchments. We sampled each catchment 3 times over a 4-year period (November 2015, March 2016, and June 2018), analyzing water for biogeochemical tracers including rare earth elements, nitrate isotopes, and dissolved gases. We found that annual runoff ratios, a proxy of residence time, were strongly correlated with nutrient removal, as were several biogeochemical proxies. We conclude that affordable biogeochemical analyses and increasingly available public datasets could be combined to improve management of catchments in the Anthropocene.