Seasonal and Spatial Variability in Surface pCO2 and Air-water CO2 Flux in the Chesapeake Bay

Interactions between riverine inputs, internal cycling, and oceanic exchange result in dynamic variations in the surface water partial pressure of carbon dioxide (pCO₂) in estuaries. Here, we report the first bay-wide, annual-scale observations of surface pCO₂ and air-water CO₂ flux along the main stem of Chesapeake Bay, revealing large annual variations in pCO₂ (43–3408 µatm) and a spatial-dependence of pCO₂ on internal and external drivers. The low salinity upper bay was a net source of CO₂ to the atmosphere supported by inputs of CO₂-rich Susquehanna River water and the respiration of allochthonous organic matter, but this region was also characterized by low pCO₂ during spring and fall phytoplankton blooms. pCO₂ decreased downstream due to CO₂ ventilation supported by long water residence times, stratification, mixing with low pCO₂ water masses, and carbon removal by biological uptake. The net effect of biogeochemical modification downstream led to pCO₂ nearly in equilibrium with the atmosphere in waters of salinity 12, but the mesohaline and polyhaline middle and lower bay were net CO₂ sinks (-5.15 and -5.70 mmol m^-2 d^-1), respectively. Although the main stem bay oscillated between a weak sink and source of CO₂ (-2.7 x 10⁹ to 0.4 x 10⁹ mol C y^-1) during the dry hydrologic year 2016, our calculations suggest that the CO₂ flux could be up to (-10 to -14) x 10⁹ mol C y^-1 during a wet year. In contrast to many other estuaries worldwide, Chesapeake Bay and other large estuaries could be absorbing CO₂ from the atmosphere.