Multidecadal fCO2 Increase Along the United States Southeast Coastal Margin

Coastal margins could be hotspots for acidification due to terrestrial‐influenced CO2 sources. Currently there are no long‐term (>20 years) records from biologically important coastal environments that could demonstrate sea surface CO2 fugacity (fCO2) and pH trends. Here, multidecadal fCO2 trends are calculated from underway and moored time series observations along the United States southeast coastal margin, also referred to as the South Atlantic Bight (SAB). fCO2 trends across the SAB, derived from ∼26 years of cruises and ∼9.5 years from a moored time series, range from 3.0 to 4.5 µatm yr−1, and are greater than the open ocean increases. The pH decline related to the fCO2 increases could be as much as −0.004 yr−1; a rate greater than that expected from atmospheric‐influenced pH alone. We provide evidence that fCO2 increases and pH decreases on an ocean margin can be faster than those predicted for the open ocean from atmospheric influence alone. We conclude that a substantial fCO2 increase across the marginal SAB is due to both increasing temperature on the middle and outer shelves, but to lateral land‐ocean interactions in the coastal zone and on inner shelf. Key Points Long‐term fCO2 in coastal South Atlantic Bight is increasing at a rate greater than open ocean The fCO2 increase is likely a combination of increases from terrestrial sources, with little to no influence from temperature The related pH decrease on this coastal margin is likely greater than that of the open ocean