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 (C-anth) interact with these drivers of carbonate chemistry to determine seasonally resolved rates of acidification in these habitats. Three independent simulations of C-anth accumulation from 1765 to 2100 were run using two previously published methods and one novel method for C-anth 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 pH(T) and pCO(2) is increased (while that of Omega(aragonite) is decreased) and acidification rates are accelerated when compared with open-ocean estimates, highlighting how feedbacks between local metabolism and C-anth 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 C-anth estimation uncertainties manifest in shallow coastal waters and limit our ability to predict impacts to coastal organisms and ecosystems from anthropogenic CO2 emissions.