Feedbacks Between Estuarine Metabolism and Anthropogenic CO2 Accelerate Local Rates of Ocean Acidification and Hasten Threshold Exceedances

Attribution of the ocean acidification (OA) signal in estuarine carbonate system observations is necessary for quantifying the impacts of global anthropogenic CO2 emissions on water quality, and informing managers of the efficacy of potential mitigation options. We present an analysis of observational data to characterize dynamics and drivers of seasonal carbonate system variability in two seagrass habitats of Puget Sound, WA, USA, and estimate how carbon accumulations due to anthropogenic CO2 emissions (Canth) interact with these drivers of carbonate chemistry to determine seasonally resolved rates of acidification in these habitats. Three independent simulations of Canth accumulation from 1765 to 2100 were run using two previously published methods and one novel method for Canth estimation. Our results revealed persistent seasonal differences in the magnitude of carbonate system responses to anthropogenic CO2 emissions caused by seasonal metabolic changes to the buffering capacity of estuarine waters. The seasonal variability of pHT and pCO2 is increased (while that of Ωaragonite is decreased) and acidification rates are accelerated when compared with open-ocean estimates, highlighting how feedbacks between local metabolism and Canth can control the susceptibility of estuarine habitats to OA impacts. The changes in seasonal variability can shorten the timeline to exceedance of established physiological thresholds for endemic organisms and existing Washington State water quality criteria for pH. We highlight how Canth estimation uncertainties manifest in shallow coastal waters and limit our ability to predict impacts to coastal organisms and ecosystems from anthropogenic CO2 emissions.

Impact/Purpose

This study provides the first estimates of how seasonal pH, aragonite saturation state, and pCO2 dynamics are altered by local production, respiration, and exchange with the atmosphere in estuarine habitats on the west coast of the United States. Our study utilized a combination of high-resolution observations, mechanistic modeling, multiple techniques for estimating anthropogenic carbon loading into the system, and hindcasting/forecasting models to determine the impacts of altered local biological rates with changing atmospheric exchange rates in two seagrass habitats in Puget Sound, WA, USA. The interaction of local metabolism and anthropogenic carbon in source waters to the study sites causes the rates of acidification in these habitats to outpace rates published for the open-ocean. Our study highlights the spatial and temporal complexity of coastal acidification impacts on ecologically and economically-vital ecosystems. These findings directly contribute to ongoing discussions in the research and management community regarding how to quantify the role of local drivers’ impact on attainment of water quality standards, and the potential role of seagrass habitats as chemical refugia for coastal acidification. Water quality impacts due to eutrophication-enhanced coastal acidification have been studied in a variety of coastal systems, including the Chesapeake Bay, the Gulf of Mexico, and the northeast coast of the United States, but little work has been done to understand the dynamics of this process in west coast estuaries. This is despite the large documented impacts of coastal acidification on the productivity of the west coast shellfish aquaculture industry, where over half of surveyed industry stakeholders have expressed that coastal acidification is currently affecting their business. We discuss how the future success of estuarine organisms sensitive to coastal acidification will likely depend on their resilience to increasingly extreme carbonate chemistry, as well as the spatial and temporal overlap of sensitive life history stages with our estimates of maximal coastal acidification impacts. This study is the second publication from the RARE project with Region 10.

Citation

Pacella, S., Cheryl A. Brown, R. Labiosa, B. Hales, TChris MochonCollura, W. Evans, AND G. Waldbusser. Feedbacks Between Estuarine Metabolism and Anthropogenic CO2 Accelerate Local Rates of Ocean Acidification and Hasten Threshold Exceedances. American Geophysical Union, Washington, DC, 129(3):e2023JC020313, (2024). [DOI: 10.1029/2023JC020313]

DOI: Feedbacks Between Estuarine Metabolism and Anthropogenic CO2 Accelerate Local Rates of Ocean Acidification and Hasten Threshold Exceedances