LAUSR

Oxygen flux measurements between oyster reefs and the overlying water column approximate total ecosystem metabolism, representing a potentially valuable reef monitoring tool. In this study, seasonal oxygen flux measurements were made over an intertidal Crassostrea virginica oyster reef on the Virginia (USA) coast using the non-invasive aquatic eddy covariance (AEC) technique. Reef respiration (R) ranged from − 276 mmol m −2  day −1 in the summer to − 55 mmol m −2  day −1 in the winter, likely due to temperature effects on oyster filtering and sediment microbial activity. Reef gross primary production (GPP) varied less seasonally, resulting in net ecosystem metabolism (NEM) that was highly heterotrophic in the summer (− 141 mmol m −2  day −1 ) and nearly balanced in the winter (− 11 mmol m −2  day −1 ). Measurements of reef sediment chl a indicated higher concentrations of benthic microalgae than surrounding bare mudflat, while photosynthesis-irradiance curves utilizing 15-min flux averages confirmed light as a dominant short-term driver of microalgal production. Metabolic values were compared with past AEC results from this reef, creating a 4-year record that included a significant oyster die-off. Over this time span, R was closely coupled to GPP, indicating rapid internal cycling of carbon, while reef primary production was primarily attributed to sediment, rather than epiphytic, microalgae. Both R and GPP substantially decreased following the oyster die-off. These results illustrate that oyster reefs are highly dynamic environments, with complex processes that act on numerous time scales ranging from minutes to years. Consequently, AEC metabolism measurements can aid in oyster reef monitoring.