Declining water quality, in addition to hypoxia and eutrophication, may have a
significant impact on the seasonality of biogeochemical parameters throughout the mainstem of the Chesapeake Bay. The carbonate (CO2) system in the Chesapeake Bay experiences seasonal and spatial complexities and is influenced by both natural and anthropogenic variability. Although site-specific studies investigating CO2-system variability exist within the Chesapeake Bay, few studies have investigated the seasonality of the CO2-system throughout the entire mainstem. Additionally, recent comprehensive studies investigating over 50 estuaries along the East Coast of the United States suggest that estuarine systems are heterotrophic and act as sources of CO2 to the atmosphere; this current paradigm does not apply to the mainstem of the Chesapeake Bay. The research presented here will assess the net annual source/sink status of atmospheric CO2 in the mainstem, along with an evaluation of annual net community production and trophic status, which is assessed based on a mass balance of dissolved inorganic carbon (DIC). Discrete observations of DIC and total alkalinity (TA) are collected at 17 stations throughout the mainstem of the Bay on four cruises between November 2016 and July 2017. The latitudinal salinity gradient along the mainstem of the Bay results in elevated DIC and TA concentrations at the mouth of the Bay associated with inflowing Atlantic
Ocean waters. Minimum concentrations of DIC and TA are associated with fresher waters, delivered mainly by the Susquehanna River, at the head of the Bay. The spatial gradients in DIC and TA are observed regardless of season. Spatial variability of the partial pressure of CO2 (pCO2) is observed throughout the surface waters of the estuary, with undersaturation of CO2 with respect to the atmosphere in the upper Bay over the complete seasonal cycle, and supersaturation with respect to atmospheric CO2 in the lower Bay during the warm seasons. The spatial and seasonal distribution of pH and saturation state of aragonite (Ω) are more variable throughout the mainstem, as the seasonality of these parameters are different in each region. The physical (air-sea CO2 exchange and mixing) and biological (photosynthesis and respiration) drivers of CO2-system seasonality is examined throughout the mainstem Bay. In the deep, northern channel of the mainstem, seasonal CO2-system variability is larger than the lower Bay regions that are more directly influenced by exchange with Atlantic Ocean shelf waters. Overall, when averaged over the 2016/2017 seasonal cycle used in this analysis, the mainstem of the Chesapeake Bay is found to be net heterotrophic and a sink of atmospheric CO2.
Friedman J. R., 2019. Seasonal variability of the CO2-system throughout the Chesapeake Bay mainstem. MSc thesis, College of William and Mary in Virginia. Thesis (subscription required).