High-Frequency CO₂ System Variability Over the Winter-to-Spring Transition in a Coastal Plain Estuary

Understanding the vulnerability of estuarine ecosystems to anthropogenic impacts requires a quantitative assessment of the dynamic drivers of change to the carbonate (CO₂) system. Here we present new high-frequency pH data from a moored sensor. These data are combined with discrete observations to create continuous time series of total dissolved inorganic carbon (TCO₂), CO₂ partial pressure (pCO₂), and carbonate saturation state. We present two deployments over the winter-to-spring transition in the lower York River (where it meets the Chesapeake Bay mainstem) in 2016/2017 and 2017/2018. TCO₂ budgets with daily resolution are constructed, and contributions from circulation, air-sea CO₂ exchange, and biology are quantified. We find that TCO₂ is most strongly influenced by circulation and biological processes; pCO₂ and pH also respond strongly to changes in temperature. The system transitions from autotrophic to heterotrophic conditions multiple times during both deployments; the conventional view of a spring bloom and subsequent summer production followed by autumn and winter respiration may not apply to this region. Despite the dominance of respiration in winter and early spring, surface waters were undersaturated with respect to atmospheric CO₂ for the majority of both deployments with mean fluxes ranging from −9 to −5 mmol C·m⁻²·day⁻¹. Deployments a year apart indicate that the seasonal transition in the CO₂ system differs significantly from one year to the next and highlights the necessity of sustained monitoring in dynamic nearshore environments.