pH is a key index for ocean biogeochemical and acidification studies. However, the spatial patterns and drivers of pH in various coastal regions remain insufficiently understood. In this study, we present summertime distributions of the subsurface water pH and the processes controlling them along the North American Atlantic Ocean Margins, including the northern Gulf of Mexico (GoMx), South Atlantic Bight (SAB), Mid-Atlantic Bight (MAB), and Gulf of Maine (GoMe). Our findings reveal a continuous low pH band on the slope, associated with the oxygen minimum and CO₂ maximum zones, along the entire North American Atlantic coast, ranging from the bottom layer (600-1000 m) in the GoMx and SAB to the middle layer (300-500 m) off the MAB and GoMe. We also observed unique low pH features on various shelves: a low pH with relatively high aragonite saturation state (W_arag) associated with hypoxia in the northern GoMx, an onshore upwelling of the low pH slope water in the SAB, a seasonal low pH feature on the MAB shelf bottom associated with the Cold Pool water, and an inflow of low pH slope water into the bottom of the GoMe’s bottom layer.
Our analysis highlights that net biological respiration predominately drives the sub-regional subsurface pH distributions on shelf waters. However, on a larger latitudinal scale, the initial wintertime CO₂—higher in cold northern waters—plays a significant role in shaping the subsequent warm-season pH and W_arag. In particular, we discuss this mechanism in controlling pH and W_arag in the Cold Pool water in the MAB and, further north, in bottom waters in the GoMe and beyond. In these waters, while oxygen is relatively high and pH is only moderately low, W_arag is particularly low, in stark contrast to the very low pH and moderately high W_arag waters found in the northern GoMx hypoxia zone. Our analysis provides new insights and lays a foundation for interpreting future pH changes in response to processes such as water masses shifting, ocean warming, and anthropogenic carbon uptake in coastal oceans.

coastal ocean pH, carbonate saturation state, ocean acidification