A climatology of spatial and seasonal patterns of carbonate chemistry parameters on the Northeast U.S. Continental Shelf

The Northeast U.S. Continental Shelf from the North Carolina coast to the Gulf of Maine is a region sensitive to ocean acidification and plays an important role in the U.S. coastal carbon cycle. Understanding the baseline variability of the region’s carbonate chemistry is critical in order to evaluate and plan for ocean acidification’s impacts to the ecosystem and ecosystem services, including the region’s valuable shellfish industry. Seasonal carbonate chemistry climatology grids were constructed for both the surface and bottom waters of the Northeast U.S. Continental Shelf based on high-quality measurements of CO₂ parameters over the last two decades. The climatology utilizes data collected from a variety of research vessel surveys conducted between 1993 and 2017. Data were interpolated to a 0.05 degree spatial resolution grid using a kriging method with both horizontal and bathymetric distance weighting to derive complete seasonal spatial depictions of dissolved inorganic carbon (DIC), total alkalinity (TA), pH and aragonite saturation state (Ω). The gridded climatology of DIC, TA, and Ω data was well correlated to observed values (r² ≥ 0.94 for all three parameters in all seasons), suggesting a robust interpretation of measured data using the kriging method. The seasonal climatology for all carbonate parameters was clearly affected by known large-scale physical and biogeochemical processes in the region. A sensitivity analysis of changes in Ω between seasons indicates that the largest changes were often driven by the balance between respiration and photosynthesis, but physical processes may be important at times. In general, surface waters showed higher seasonal variability than bottom waters. A similar sensitivity analysis suggests that drivers of pH distributions in the region were more complex, where both biological and physical factors played important roles. Saturation states of calcium carbonate may be less sensitive to complex physical forcing (e.g. mixing and dilution) compared to biogeochemical factors in the region, primarily due to opposite effects of concurrent changes in TA and DIC during physical processes. This may be beneficial for using Ω as a simple and useful indicator for biological and ecosystem health of the region under ocean acidification.