High frequency pCO₂ measurements reveal net neutral CO₂ flux in a mesohaline reach of the Chesapeake Bay. Net off-gassing to the atmosphere begins in June when water temperatures rise above ~26ºC, continuing through November when temperatures fall below ~10ºC. Dissolved CO₂ concentrations follow day–night cycles and are especially pronounced in warm waters. From December through May, the river is largely an uninterrupted sink for CO₂ (i.e. CO₂ is drawn out of the atmosphere into the river).