Citation:

Pacella, S., W. Fairchild, E. Stets, S. Alin, B. Carter, AND R. Feely. Acidification dynamics of United States estuarine waters as determined by trends in coastal stream and ocean chemistry. Coastal and Estuarine Research Federation 27th Biennial Conference, Portland, OR, November 12 - 16, 2023.

Impact/Purpose:

This presentation will discuss the first national-level assessment of United States estuaries' response to acidification from anthropogenic changes in both coastal ocean and coastal streams. This work leveraged large datasets organized by EPA, USGS, and NOAA to quantify trends in coastal stream and coastal ocean CO2 chemistry since the 1970s for 178 estuaries nationwide, and characterized how estuarine acidification dynamics have responded at the national scale.  Included in these calculations is a novel application of a metric which quantifies the sensitivity of estuarine waters to further CO2 addition, which now enables direct comparison of the acidification trajectories and vulnerabilities of all 178 estuaries around the conterminous United States.  Results of this study also enable future work to characterize how U.S. estuaries' carbon emissions have changed since the 1970s, as well the suitability of estuarine habitats for nature-based carbon removal applications.

Description:

Human activities have caused continental-scale changes to United States freshwater acidity and alkalinity, but no study has yet quantified how these changes interact with coastal ocean chemistry to control the dynamics and trends of acidification in estuaries of the United States.   The goal of this study was to quantify and contrast acidification pressures in conterminous U.S. estuaries by leveraging existing datasets of coastal stream and ocean chemistry.  Coastal stream CO2 chemistry trends were characterized for 178 stations around the U.S. using pHT, alkalinity, and water temperature data from the Water Quality Portal (WQP).  Coastal ocean CO2 chemistry data in proximity to the stream stations was obtained from the Coastal Ocean Data Analysis Product in North America (CODAP-NA, Version 2021).  The resulting dataset characterized the CO2 chemistry of ~86% of stream discharge and ~93% of the drainage area of the conterminous U.S.  Site-specific interpolations of estuarine CO2 chemistry (e.g. pHT and pCO2) were calculated using the WQP coastal stream and CODAP-NA coastal ocean data, including the carbonate system sensitivity factors which describe the pHT response of waters to further additions of CO2 (i.e. acidification sensitivity).  Our analysis found 1). a wide range of acidification sensitivities in U.S. coastal streams, 2). this range was much greater than that of acidification sensitivities in U.S. coastal ocean waters, and 3). coastal stream acidification sensitivities control most of the vulnerability of U.S. estuaries to further acidification.  Streams and estuaries in the New England and Mid-Atlantic regions had the highest acidification sensitivities, while those in the Gulf of Mexico had the lowest sensitivities.  Analysis of decadal trends in coastal stream acidity showed 63% of total stream discharge to the conterminous U.S. coastal zone with decadal decreases in acidity (increasing pHT).  Since the 1970s, decreasing acidity in many of these coastal streams has buffered against the atmospheric CO2-induced acidification of coastal ocean waters across ~70% of the estuarine salinity spectrum, up to salinities of approximately 23.  Ocean acidification outcompetes trends in river alkalinization and decreasing acidity at higher salinities, however, with a net decrease in estuarine pHT and increased acidification sensitivities.  Further work is needed to identify the mechanisms underlying these trends in stream CO2 chemistry, but past work on inland freshwater systems suggests decreasing acidification pressures may be related to the same mechanisms as freshwater salinization syndrome – including impacts of the U.S. Clean Air Act and stream alkalinization due to changes in land use.

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