Modeling nutrient in-stream processes at the watershed scale using Nutrient Spiralling metrics

摘要

One of the fundamental problems of using large-scale biogeochemicalmodels is the uncertainty involved in aggregating the components offine-scale deterministic models in watershed applications, and inextrapolating the results of field-scale measurements to largerspatial scales. Although spatial or temporal lumping may reduce theproblem, information obtained during fine-scale research may not applyto lumped categories. Thus, the use of knowledge gained throughfine-scale studies to predict coarse-scale phenomena is notstraightforward. In this study, we used the nutrient uptake metricsdefined in the Nutrient Spiralling concept to formulate the equationsgoverning total phosphorus in-stream fate in a deterministic, watershed-scalebiogeochemical model. Once the model was calibrated, fitted phosphorusretention metrics where put in context of global patterns of phosphorusretention variability. For this purpose, we calculated power regressionsbetween phosphorus retention metrics, streamflow, and phosphorus concentrationin water using published data from 66 streams worldwide, including bothpristine and nutrient enriched streams.Performance of the calibrated model confirmed that the Nutrient Spirallingformulation is a convenient simplification of the biogeochemicaltransformations involved in total phosphorus in-stream fate. Thus,this approach may be helpful even for customary deterministic applicationsworking at short time steps. The calibrated phosphorus retention metricswere comparable to field estimates from the study watershed, and showed highcoherence with global patterns of retention metrics from streams of the world.In this sense, the fitted phosphorus retention metrics were similar to fieldvalues measured in other nutrient enriched streams. Analysis of the bibliographicaldata supports the view that nutrient enriched streams have lower phosphorusretention efficiency than pristine streams, and that this efficiency lossis maintained in a wide discharge range. This implies that both small andlarger streams may be impacted by human activities in terms of nutrientretention capacity, suggesting that larger rivers located in human populatedareas can exert considerable influence on phosphorus exports from watersheds.The role of biological activity in this efficiency loss showed by nutrientenriched streams remained uncertain, because the phosphorus mass transfercoefficient did not show consistent relationships with streamflow and phosphorusconcentration in water. The heterogeneity of the compiled data and the possiblerole of additional inorganic processes on phosphorus in-stream dynamics mayexplain this. We suggest that more research on phosphorus dynamics at thereach scale is needed, specially in large, human impacted watercourses.