A Stoichiometric Approach to Nutrient Retention in Stream Ecosystems

We examined patterns of variability in stream retention response among nutrients based on results from multiple nutrient additions conducted in 11 European streams covering a wide range of environmental conditions. Based on previous observations, we argued that streams were not equally efficient at retaining different nutrients (i.e., N and P) or different ionic forms of the same element (i.e., NO3 and NH4) and generally exhibited higher efficiency for the nutrient or ion that was available in lower proportion. In this study we focussed on mass transfer coefficients for NH4, NO3, and PO4 (i.e., stream nutrient demand, Vf in m/s) as the most appropriate stream retention metric for comparing among and within streams, and among different nutrient forms. Stream nutrient demand was significantly different depending on the nutrient under consideration. Ammonium demand was the largest, followed by phosphate demand, and nitrate was the lowest. Using ratios between stream demand for pairs of nutrients (VfNO3:VfNH4, VfNH4:VfPO4, and VFNO3:VFPO4), we found consistent patterns with chemical parameters as well as with parameters of ecosystem metabolism which supported our hypothesis. Consideration of different nutrients and their relative proportions proved useful to expand our understanding of stream ecosystems in terms of nutrient dynamics.