Organic Alkalinity Distributions and Characteristics in Two Gulf of Maine Estuaries

The contribution of organic matter to the alkalinity of seawater, termed organic alkalinity (Org-Alk), has become a topic of increasing scrutiny. A growing body of evidence shows the presence of substantial Org-Alk, particularly in coastal waters; however, the sources and characteristics of this Org-Alk are not well documented. Terrestrial organic matter delivered via rivers is a significant component of Org-Alk in coastal systems. We have surveyed two Gulf of Maine estuaries with contrasting river endmember properties. Our seasonal (fall and spring) surveys included the two freshwater endmembers, the contiguous estuarine mixing zones, and surrounding coastal ocean waters. Samples from these surveys were directly analyzed for Org-Alk by titration, and Org-Alk was also estimated from comparisons of directly-measured total alkalinity and total alkalinity calculated from dissolved inorganic carbon and pH. Results show a strong river source of Org-Alk with complex mixing in the estuary. Titrations of estuarine samples, conducted to characterize the pKa of Org-Alk, agreed with previous findings that important Org-Alk functional groups are titrated near pH 4.5. This represents a challenge to alkalinity measurements that rely on a well-defined TA equivalence point. Org-Alk has both acid and base components, and Org-Alk contributions of organics with pKa ≈ 4.5 are ambiguous with respect to the current definition of alkalinity, complicating calculations of Org-Alk contributions to TA.