Modulation of CO2 Air-Sea Exchange by Along-Channel Winds in Chesapeake Bay

Estuaries are proposed to play an important role in the global carbon budget, but there are considerable spatial and temporal uncertainties in their atmospheric fluxes of CO₂. Along-estuary measurements have shown that CO₂ partial pressure, from which estuary outgassing is derived, decreases rapidly away from the freshwater end member. Current estimates of estuarine contribution to CO₂ flux into the atmosphere may be biased high due to measurements of surface CO₂ partial pressure being made primarily in low salinity regions of estuaries. Additionally, diurnal and seasonal variations have not been resolved. Here, we use a model to develop a better understanding of how circulation and mixing affect net air-sea CO₂ flux across an entire estuary. Analyses are done on a yearlong simulation of the Chesapeake Bay using the Regional Ocean Modeling System (ROMS), which includes a simple biogeochemical model for first order inorganic carbon dynamics. Results indicate spatial distribution of annual mean CO₂ air-sea flux is dominated by residual estuarine circulation. In the lower bay, stratification prevents outgassing of CO₂ from deep waters. Outgassing is greatest in the upper bay, where stratification is weak and there is bottom convergence in residual circulation. This signal is modulated by ventilation along the mid-bay, where along-channel winds can cause upwelling on either side of the channel, allowing outgassing of CO₂ from deep waters, especially in times of strong stratification and high respiration rates. The effects of this modulation are compared for northward versus southward winds and between seasons. Understanding the impacts of estuarine physics on air-sea fluxes of CO₂ aids in our ability to define the role of estuaries in the global carbon budget.