High-frequency continuous measurements reveal strong diel and seasonal cycling of pCO.sub.2 and CO.sub.2 flux in a mesohaline reach of the Chesapeake Bay

We estimated hourly air-water gas transfer velocities (k.sub.600) for carbon dioxide in the Rhode River, a mesohaline sub-estuary 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 pCO.sub.2 in the water and in the overlying atmosphere, we determined the direction and magnitude of CO.sub.2 flux at hourly intervals across a 3-year record (1 July 2018 to 1 July 2021). Continuous year-round measurements enabled us to document strong seasonal cycling, whereby the Rhode River is primarily autotrophic during cold-water months (December-May) and largely net heterotrophic in warm-water months (June-November). Although there is inter-annual variability in CO.sub.2 flux in the Rhode River, the annual mean condition is near carbon neutral. Measurement at high temporal resolution across multiple years revealed that CO.sub.2 flux and apparent trophic status can reverse during a single 24 h period. pCO.sub.2 and CO.sub.2 flux are mediated by temperature effects on biological activity and are inverse to temperature-dependent physical solubility of CO.sub.2 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.