Stream ecosystem processes such as nutrient cycling may vary with stream position in the watershed. Using a scaling approach, we examined the relationship between stream size and nutrient uptake length, which represents the mean distance that a dissolved solute travels prior to removal from the water column. Ammonium uptake length increased proportionally with stream size measured as specific discharge (discharge/stream width) with a scaling exponent = 1.01. In contrast, the scaling exponent for nitrate (NO₃⁻) was 1.19 and for soluble reactive phosphorus (SRP) was 1.35, suggesting that uptake lengths for these nutrients increased more rapidly than increases in specific discharge. Additionally, the ratio of nitrogen (N) uptake length to SRP uptake length declined with stream size; there was lower demand for SRP relative to N as stream size increased. Ammonium and NO₃⁻ uptake velocity positively related with stream metabolism, while SRP did not. Finally, we related the scaling of uptake length and specific discharge to that of stream length using Hack's law and downstream hydraulic geometry. Ammonium uptake length increased less than proportionally with distance from the headwaters, suggesting a strong role for larger streams and rivers in regulating nutrient transport

Nitrogen (N) pollution of freshwater, estuarine, and marine ecosystems is widespread and has numerous environmental and economic impacts. A portion of this excess N comes from urban watersheds comprised of natural and engineered ecosystems which can alter downstream N export. Studies of urban N cycling have focused on either specific ecosystems or on watershed-scale mass balances. Comparisons of specific N transformations across ecosystems are required to contextualize rates from individual studies. Here we reviewed urban N cycling in terrestrial, aquatic, and engineered ecosystems, and compared N processing in these urban ecosystem types to native reference ecosystems. We found that net N mineralization and net nitrification rates were enhanced in urban forests and riparian zones relative to reference ecosystems. Denitrification was highly variable across urban ecosystem types, but no significant differences were found between urban and reference denitrification rates. When focusing on urban streams, ammonium uptake was more rapid than nitrate uptake in urban streams. Additionally, reduction of stormwater runoff coupled with potential decreases in N concentration suggests that green infrastructure may reduce downstream N export. Despite multiple environmental stressors in urban environments, ecosystems within urban watersheds can process and transform N at rates similar to or higher than reference ecosystems.

Lay Abstract Air–water gas exchange governs the rate at which atmospheric gases flow into and out of aquatic ecosystems. Knowing this rate is necessary to calculate river photosynthesis and respiration, but there are few data from large rivers, and there are no data that include whitewater rapids. We studied the Colorado River, Grand Canyon; this river has flat reaches separated by extremely large, steep, whitewater rapids. We measured gas transfer velocity (the height of the water column that can exchange all of its gas per hour) by measuring how quickly the river gained oxygen as it flowed over the first 7 major rapids. The Colorado River has low oxygen concentration as it flows out of Glen Canyon Dam, located 25 kilometers upriver from Lees Ferry. We found that gas transfer velocity increased as river slope increased. Gas transfer velocity was low in flat reaches but was up to 800 times higher in rapids, which were the highest rates ever measured in a river. Based on the rate of change of oxygen concentration per meter of river drop, we estimated gas transfer velocity for the remainder of the Colorado River in Grand Canyon. Gas exchange varied 5‐fold depending on the slope of the immediate reach. Gas transfer velocity was higher for the Colorado River than for other aquatic ecosystems because of its large rapids. Our approach of scaling gas transfer velocity to the entire river will allow comparing gas transfer velocity across rivers that have variable river slopes, such as the Colorado River, Grand Canyon.

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