Recent methodological advances for estimating community metabolism with dissolved oxygen (DO) stable isotopes allow the quantification of daily averaged photosynthesis, respiration, and gas exchange rates under transient field conditions.  During summer and fall of 2004-2007, we applied these and more traditional methods to reaches of the Saint John River, NB, Canada with nutrient gradients in regulated and unregulated reaches to determine the effects of nutrient and flow stressors on riverine metabolism. Community metabolism was measured using a) open-system, single station, diel measurement of DO and stable oxygen isotopes, b) a dynamic O₂ and δ¹⁸O-O₂ model that quantifies average photosynthesis, respiration, and gas exchange rates, and c) experimental domes to measure diel DO of the benthos.  Diel patterns of dissolved oxygen exist in the Saint John River, and nutrient loading and daily desiccation caused by discharge regulation shifted metabolism to a more respiration-dominated community.  The most heavily depleted δ¹⁸O-O₂ sites occurred within the regulated reach and were dominated by respiration processes.  The direct measurement of community metabolism via DO isotopes allows assessment of river ecosystem health, providing a new assessment tool that can be used to compare ecosystems across seasons and landscapes.