Wetlands are important regulators of biogeochemical cycling at the interface of terrestrial-aquatic systems and are known for their ability to remove excess nutrients (nitrogen, N; phosphorus; P) from stormwater runoff. However, as surrounding urban areas increase in size and density, changes in the source and composition of stormwater runoff may influence the nutrient removal capacity of urban wetlands. Given the large variability of N and P concentrations in urban stormwater, we may be misinterpreting the nutrient processing capacity of these systems after significant runoff events. Our objectives were to (1) measure ambient elemental stoichiometry of natural and urban wetlands and (2) quantify nutrient uptake capacity across a gradient of surrounding impervious cover. Wetlands (n=18) in two coastal, humid climates in Portland, Oregon and Valdivia, Chile were subjected to pulsed additions of elevated nitrate (NO3) and phosphate (PO4) at 2 to 4 times above ambient conditions to simulate stormwater runoff events. We observed hydraulic behavior and residence time to model breakthrough curves for each wetland using a conservative tracer (chloride, Cl-). Surface water subsamples were collected over the curve to measure uptake velocity (vf), nutrient uptake per unit area (Ut), and maximum nutrient uptake (Umax).
Results/Conclusions
Variable N and P concentrations were observed across all wetlands with strong phosphorus limitation (N:P molar ratio: 50-2900) in both natural and urban settings. Urban wetlands were characterized by high NO3 and low ammonium (NH4) concentrations which may reflect elevated nutrient loading from upstream and increased nitrification in oxygenated surface waters. Level of surrounding urbanization did not correlate with nutrient uptake rate; highest uptake measurements were observed in both high and low urbanized wetlands. NO3 and PO4 areal uptake (Ut) were positively correlated with nutrient concentrations across all sites (r2 = 0.35 , log-log relationship). NO3 and PO4 maximum uptake (Umax) were also positively correlated with N:P molar ratio (r2 = 0.73) with PO4 uptake between 400 - 5600 µg m-2 d-1 and NO3 uptake between 20 - 360 mg m-2 d-1. These results suggest wetlands with severe P limitation experience rapid nutrient uptake that reach saturation, after which wetlands switch from nutrient sinks to sources. However, nutrient subsidies in stormwater runoff may alleviate system nutrient limitation and stimulate short-term nutrient removal. These experiments provide insights into significant short-term nutrient removal by urban wetlands, and the need to integrate this information into network-scale urban biogeochemical models.