Influence of stoichiometry and suburbanization on in-stream uptake of NH4 and PO4 in headwater, New England streams

The physical, chemical and biological impacts of suburban land use change affect ecosystem structure and functions in headwater streams. These changes, in combination with the altered nutrient inputs, influence the role of headwater streams in mitigating downstream nutrient export. Using in-stream solute additions, we examined the impact of solute stoichiometry on nutrient uptake in two streams with different degrees of watershed urbanization.

We conducted eight nutrient additions using the Tracer Additions for Spiraling Curve Characterization (TASCC) method during the 2011 growing season in two headwater streams in metropolitan Boston, MA, part of the Plum Island Ecosystems (PIE) LTER site. One stream was located in a suburban watershed (25% impervious, 979 persons/km²), the other primarily in a wildlife refuge (8% impervious, 130 persons/km²). A suite of four solute additions were conducted with the objective to increase ambient NH4 and PO4 concentrations by 1) 10x NH4, 2) 10x PO4, 3) 10x NH4 and 10x PO4 together, and 4) 10x NH4 and 100x PO4 together. Using the TASCC method, we calculated dynamic areal uptake rates, dynamic uptake velocities, and ambient uptake lengths to quantify the removal potential of streams with different land uses.

These solute additions show that both land use and solute stoichiometry influence nutrient uptake in stream reaches. Overall, ambient uptake lengths of PO4 and NH4 were lower in the forested reach than in the suburban reach. For the additions with only one solute (just NH4 or just PO4), ambient uptake lengths were shorter for PO4 than NH4in the forested reach, but surprisingly, NH4 uptake lengths were shorter than PO4uptake lengths in the suburban reach. Adding multiple constituents in a single solute addition affected both dynamic uptake rates, as well as estimates of ambient uptake lengths. For the high PO4 addition (10x NH4 and 100x PO4 together), NH4 uptake velocity was more correlated to dynamic PO4 concentration than NH4 concentration in the forested reach, while the suburban reach was equally correlated to both PO4 and NH4 concentrations. In combination with the calculated ambient uptake lengths, this suggests that PO4 is limiting in the forested reach, and that PO4 and NH4 are co-limiting in the suburban reach. Changes to ecosystem function in suburban stream reaches can affect downstream nutrient export by lowering nutrient uptake, although the stoichiometry of nutrient pollution can affect the magnitude of this impact. Quantifying these variables is important for better understanding the role of river networks in mitigating the effect of suburban nutrient pollution on eutrophication in lakes and coastal zones.