Impact of seasonal changes in stream metabolism on nitrate concentrations in an urban stream

Nitrate (NO₃ ⁻) dynamics in urban streams differ from many natural streams due to stormwater runoff, sewage inputs, decreased groundwater discharge, often limited hyporheic exchange, increased primary productivity, and limited carbon input. We investigated NO₃ ⁻ dynamics in a first-order urban stream in Syracuse, NY, which has urbanized headwaters and a geomorphologically natural downstream section. Twice-monthly water sampling, NO₃ ⁻ injection tests, NO₃ ⁻ isotopic analysis, filamentous algae mat density, and riparian shading were used to identify processes regulating NO₃ ⁻ dynamics in the stream over a 12-month period. The urban headwater reach had low NO₃ ⁻ (0.006–0.2 mg N/L) in the spring through fall, with a minimum uptake length of 900 m, no canopy cover, and high algae mat density. The downstream natural reach (100% canopy cover during the summer and low algae mat density) had nitrate concentrations between 0.6 and 1.2 mg N/L from winter to summer, which decreased during autumn leaf-off. In the urban reach, autotrophic uptake by filamentous green algae is a major NO₃ ⁻ sink in summer. In the natural reach, the addition of organic matter to the stream at leaf-off led to a decrease in NO₃ ⁻ concentration followed by an increase in NO₃ ⁻ concentration in winter as gross primary productivity decreased. This study shows that the balance between autotrophy and heterotrophy in urban streams is variable and depends on an interplay of drivers such as temperature, light, and carbon inputs that are mediated by the riparian ecosystem.