Nutrient Attenuation in Streams: A Simplified Model to Explain Field Observations

Abstract

A parsimonious nutrient-periphyton model quantifying uptake and recycling below a steady nutrient point source was tested in a shallow, stony-bed river during summer low flow. Close to the source DIN and DRP concentrations decreased linearly with distance, the DIN:DRP uptake ratio was constant (although twice the Redfield ratio), and rates of photosynthesis and algal biomass were high—consistent with model predictions. Further downstream, DIN concentrations were near the detection limit (but DRP was measurable), and rates of photosynthesis and algal biomass were low—also consistent with model predictions. However, two model features were not supported by observations. First, concentrations of organic N and P remained constant, whereas the model predicts increasing concentrations. Second, observations showed that N and P recycling were decoupled, whereas the model assumes close coupling. Although the model was reliable close to the source, it over-simplified recycling in the test stream. The model could be improved by separating dissolved from particulate organics and including recycling from settled particulates and pH-mediated P release from sediment, although this would increase complexity and reduce parsimony.