Seasonal Variability of the CO₂ System in a Large Coastal Plain Estuary

The Chesapeake Bay, a large coastal plain estuary, has been studied extensively in terms of its water quality, and yet, comparatively less is known about its carbonate system. Here we present discrete observations of dissolved inorganic carbon (DIC) and total alkalinity from four seasonal cruises in 2016–2017. These new observations are used to characterize the regional CO₂ system and to construct a DIC budget of the mainstem. In all seasons, elevated DIC concentrations were observed at the mouth of the bay associated with inflowing Atlantic Ocean waters, while minimum concentrations of DIC were associated with fresher waters at the head of the bay. Significant spatial variability of the partial pressure of CO₂ was observed throughout the mainstem, with net uptake of atmospheric CO₂ during each season in the upper mainstem and weak seasonal outgassing of CO₂ near the outflow to the Atlantic Ocean. During the time frame of this study, the Chesapeake Bay mainstem was (1) net autotrophic in the mixed layer (net community production of 0.31-mol C m⁻²·year⁻¹) and net heterotrophic throughout the water column (net community production of −0.48-mol C m⁻²·year⁻¹), (2) a sink of 0.38-mol C m⁻²·year⁻¹ for atmospheric CO₂, and (3) significantly seasonally and spatially variable with respect to biologically driven changes in DIC.