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 partial pressure of carbon dioxide (pCO(2)) in large estuaries. Here, we report the first bay-wide, annual-scale observations of surfacepCO(2)and air-water CO(2)flux along the main stem of the Chesapeake Bay, revealing large annual variations inpCO(2)(43-3408 mu atm) and a spatial-dependence ofpCO(2)on internal and external drivers. The low salinity upper bay was a net source of CO(2)to the atmosphere (31.2 mmol m(-2)d(-1)) supported by inputs of CO2-rich Susquehanna River water and the respiration of allochthonous organic matter, but part of this region was also characterized by lowpCO(2)during spring and fall phytoplankton blooms.pCO(2)decreased downstream due to CO(2)ventilation supported by long water residence times, stratification, mixing with lowpCO(2)water masses, and carbon removal by biological uptake. The mesohaline middle bay was a net CO(2)sink (-5.8 mmol m(-2)d(-1)) and the polyhaline lower bay was nearly in equilibrium with the atmosphere (1.0 mmol m(-2)d(-1)). Although the main stem of the bay was a weak CO(2)source (3.7 +/- 3.3 x 10(9) mol C) during the dry hydrologic (calendar) year 2016, our observations showed higher river discharge could decrease CO(2)efflux. In contrast to many other estuaries worldwide that are strong sources of CO(2)to the atmosphere, the Chesapeake Bay and potentially other large estuaries are very weak CO(2)sources in dry years, and could even turn into a CO(2)sink in wet years.