Seasonal and spatial variability in surface pCO2 and air–water CO2 flux in the Chesapeake Bay

Abstract

AbstractInteractions between riverine inputs, internal cycling, and oceanic exchange result in dynamic variations in the partial pressure of carbon dioxide (pCO2) in large estuaries. Here, we report the first bay‐wide, annual‐scale observations of surface pCO2 and air–water CO2 flux along the main stem of the 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 (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 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 mesohaline middle bay was a net CO2 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 CO2 source (3.7 ± 3.3 × 109 mol C) during the dry hydrologic (calendar) year 2016, our observations showed higher river discharge could decrease CO2 efflux. In contrast to many other estuaries worldwide that are strong sources of CO2 to the atmosphere, the Chesapeake Bay and potentially other large estuaries are very weak CO2 sources in dry years, and could even turn into a CO2 sink in wet years.