High frequency, continuous measurements reveal strong diel and seasonal cycling of pCO₂ and CO₂ flux in a mesohaline reach of the Chesapeake Bay

Abstract. We estimated hourly air-water gas transfer velocities (k₆₀₀) for carbon dioxide in the Rhode River, a mesohaline subestuary of the Chesapeake Bay. Gas transfer velocities were calculated from estuary-specific parameterizations developed explicitly for shallow, microtidal estuaries in the Mid-Atlantic region of the United States, using standardized wind speed measurements. Combining the gas transfer velocity with continuous measurements of pCO₂ in the water and in the overlying atmosphere, we determined the direction and magnitude of CO₂ flux at hourly intervals across a 3-year record (01 July 2018 to 01 July 2021). Continuous year-round measurements enabled us to document strong seasonal cycling whereby the Rhode River is net autotrophic during cold-water months (Dec–May), and largely net heterotrophic in warm-water months (Jun–Nov). Although there is inter-annual variability in CO₂ flux in the Rhode River, the annual mean condition is near carbon neutral. Measurement at high temporal resolution across multiple years revealed that CO₂ flux can reverse during a single 24-hour period. pCO₂ and CO₂ flux are mediated by temperature effects on biological activity and are inverse to temperature-dependent physical solubility of CO₂ in water. Biological/biogeochemical carbon fixation and mineralization are rapid and extensive, so sufficient sampling frequency is crucial to capture unbiased extremes and central tendencies of these estuarine ecosystems.