ABSTRACT: Oyster reef restoration is pursued at numerous places worldwide, yet little is known about how ecosystem function changes as these reefs develop. In this study, we used the non-invasive aquatic eddy covariance technique to measure the in situ oxygen flux of 4 intertidal sites on the Virginia (USA) coast: a natural oyster reef, 2 restoration reefs, and a mudflat. Oyster densities of the 3 reefs were 350, 295, and 186 oysters m^-2. Mean summer values of nighttime oxygen flux (Flux_DARK) were -488, -428, -300, and -56 mmol m^-2 d^-1 over the natural reef, 2 restoration reefs, and mudflat, respectively. All 4 sites had smaller daytime vs. nighttime oxygen uptake, with mean summer differences between Flux_DARK and Flux_LIGHT of 360, 250, 239, and 27 mmol m^-2 d^-1. Light was an important short-term driver of daytime reef oxygen flux due to its effect on microalgal photosynthesis. All 3 oyster reefs were significantly heterotrophic, with summer net ecosystem metabolism values ranging from -157 to -298 mmol m^-2 d^-1. Flux_DARK values were similar across the 3 reefs when normalized by density, with values between -1.8 and -2.3 mmol m^-2 d^-1 oyster^-1. This study represents the most detailed measurements to date of in situ oyster reef oxygen flux, and shows that benthic microalgae can significantly impact reef oxygen dynamics. Flux_DARK values scale with oyster density, and consequently can be used as a metric for reef restoration success.