Seasonal Variability of the CO 2 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·year) and net heterotrophic throughout the water column (net community production of −0.48‐mol C m·year), (2) a sink of 0.38‐mol C m·year for atmospheric CO2, and (3) significantly seasonally and spatially variable with respect to biologically driven changes in DIC. Plain Language Summary Water quality in the Chesapeake Bay, the largest estuary in the continental United States, has been extensively monitored for over 30 years, yet relatively less is known about the cycling of carbon in these waters. The data collected in this study demonstrate considerable seasonal and spatial variability of dissolved carbon dioxide (CO2) in the Chesapeake Bay mainstem. Much of this variability is driven by the physical setting: Waters have lower salinity in the northern Bay due to riverine inputs and higher salinity in the southern Bay due to exchange with the Atlantic Ocean. Changes in salinity driven by estuarine circulation patterns throughout the mainstem have a large influence on the seasonal and spatial variability of CO2, as do biological processes. In surface waters of the mainstem, photosynthesis is greater than respiration over a complete seasonal cycle. In the years studied, there is also large spatial variability with respect to the uptake of atmospheric CO2. Through the combination of changes in salinity and biological processes, the mainstem of the bay acts as a net sink of atmospheric CO2.