Decadal-Scale Acidification Trends in Adjacent North Carolina Estuaries: Competing Role of Anthropogenic CO2 and Riverine Alkalinity Loads

Abstract

Decadal-scale pH trends for the open ocean are largely monotonic and controlled by anthropogenic CO 2 invasion. In estuaries, though, such long-term pH trends are often obscured by a variety of other factors, including changes in net metabolism, temperature, estuarine mixing, and riverine hydrogeochemistry. In this study, we mine an extensive biogeochemical database in two North Carolina estuaries, the Neuse River estuary (NeuseRE) and New River estuary (NewRE), in an effort to deconvolute decadal-scale trends in pH and associated processes. By applying a Generalized Additive Mixed Model (GAMM), we show that temporal changes in NewRE pH were insignificant, while pH decreased significantly throughout much of the NeuseRE. In both estuaries, variations in pH were accompanied by increasing river discharge, and were independent of rising temperature. Decreases in bottom-water pH in the NeuseRE coincided with elevated primary production in surface waters, highlighting the importance of eutrophication on long-term acidification trends. Next, we used a simple mixing model to illustrate the impact of changing river discharge on estuarine carbonate chemistry. We found that increased riverine alkalinity loads to the NewRE likely buffered the impact of CO 2-intrusion-induced acidification. In the NeuseRE, however, elevated dissolved inorganic carbon loads further decreased the buffering capacity, exacerbating the effects of CO 2-intrusion-driven acidification. Taken together, the findings of this study show that future trajectories in estuarine pH will be shaped by complex interactions among global-scale changes in climate, regional-scale changes in precipitation patterns, and local-scale changes in estuarine biogeochemistry.

Climate change rapidly warms and acidifies Australian estuaries

“…This decline is faster than that projected by IPCC (0.00625 pH units year −1 , pH 7.7) for the year 2100 under "business as usual" emission scenarios for the open ocean 26 . pH is dependent on temperature and should decrease as temperature increases; however, the magnitude of pH increase as seen in this study far exceeds that expected by the effects of temperature on pH 27 . These pH changes also cannot be explained by increased rainfall as Eastern Australia has dried over the last decade 21 .…”

“…Decadal pH recordings in Neuse River estuary, USA show pH declines of~0.02 units year −1 . Those declines were dependent on pCO 2 concentrations, catchment conditions and primary production 27 . Van Dam and Wang 27 suggest that the acidification of the Neuse River estuary is largely dependent on localised conditions.…”

“…Those declines were dependent on pCO 2 concentrations, catchment conditions and primary production 27 . Van Dam and Wang 27 suggest that the acidification of the Neuse River estuary is largely dependent on localised conditions. Our data show wide-scale acidification of all estuary types, suggesting that in addition to local influences, acidification processes are pervasive and are not confined to individual estuaries.…”

“…These pH changes also cannot be explained by increased rainfall as Eastern Australia has dried over the last decade 21 . While pCO 2 changes in the atmosphere contribute to pH change in estuaries, these dynamic systems are influenced by complex interactions between catchment characteristics, cover of submerged vegetation and season 27 . These factors are partially correlated with greater air temperatures, less seagrass cover and greater summer rainfall in lower latitudes (northern coast).…”

Acidification in the U.S. Southeast: Causes, Potential Consequences and the Role of the Southeast Ocean and Coastal Acidification Network

“…Historically, only four southeast estuarine systems are characterized as highly eutrophic, but more than half of the estuaries currently exhibiting low to moderate eutrophic conditions are expected to worsen (Bricker et al, 2008). However, studies show eutrophicationbased acidification may be limited to stratified estuaries (Feely et al, 2010;Cai et al, 2011;Van Dam and Wang, 2019) which could be important because eutrophication can potentially offset acidification in well-mixed systems (Nixon et al, 2015).…”