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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. Plain Language Summary: Quantifying the impacts of ocean acidification on estuarine water quality is problematic due to the many natural and human processes affecting acidification of these systems. However, understanding the role of these human processes is essential in order to understand the spatial and temporal patterns of human‐driven acidification. This study combines observations and model projections to assess how seasonal acidification dynamics in two estuarine seagrass habitats will be altered by rising atmospheric CO2 levels. We characterize how the interaction of local carbon cycling and rising atmospheric CO2 controls the sensitivity of each seagrass habitat to acidification impacts. The resulting spatial and temporal variability of acidification sensitivity creates hotspots for exceedances of organismal thresholds and existing water quality criteria. Our study constrains the dynamics of anthropogenic CO2 in estuarine waters more than past studies, but we discuss how further work is necessary to improve confidence in estuarine acidification forecasts more broadly. Key Points: Estuarine metabolism heavily modifies local seasonal ocean acidification (OA) rates, revealing a potentially large biological feedback mechanismVariable estuarine carbon cycling creates OA hotspots for exceedances of species' chemical tolerances and existing water quality criteriaReducing anthropogenic CO2 estimation uncertainties in estuarine waters is needed to improve OA forecasts and inform OA management options [ABSTRACT FROM AUTHOR]

Agua Hedionda Lagoon (AHL), a tidal estuary located on the southern California coast, supports a diverse ecosystem while serving numerous recreation activities, a marine fish hatchery, a shellfish hatchery, and the largest desalination plant in the western hemisphere. In this work, a 1-year time series of carbon dioxide data is used to establish baseline average dissolved inorganic carbon conditions in AHL. Based on a mass balance model of the outer basin of the lagoon, we propose that AHL is a source of inorganic carbon to the adjacent ocean, through advective export, at a rate of 5.9 × 106 mol C year−1, and a source of CO2 to the atmosphere of 0.21 × 106 mol C year−1 (1 mol C m−2 year−1), implying a net heterotrophic system on the order of 6.0 × 106 mol C year−1 (30 mol C m−2 year−1). Although variable with a range throughout the year of 80% about the mean, the ecosystem remained persistently heterotrophic, reaching peak rates during the summer season. Using results from the mass balance, the annual cycle of selected properties of the aqueous CO2 system (pH, pCO2, and CaCO3 saturation state) were mathematically decomposed in order to examine the relative contribution of drivers including advection, ecosystem metabolism, and temperature that act to balance their observed annual cycle. Important findings of this study include the identification of advection as a prime driver of biogeochemical variability and the establishment of a data-based estimate of mean flushing time for AHL. [ABSTRACT FROM AUTHOR]

The impacts of ocean acidification (OA) on coastal water quality have been subject to intensive research in the past decade, but how emissions-driven OA combines with human modifications of coastal river inputs to affect estuarine acidification dynamics is less well understood. This study presents a methodology for quantifying the synergistic water quality impacts of OA and riverine acidification on biologically-relevant timescales through a case study from a small, temperate estuary influenced by coastal upwelling and watershed development. We characterized the dynamics and drivers of carbonate chemistry in Tillamook Bay, OR (USA), along with its coastal ocean and riverine end-members, through a series of synoptic samplings and continuous water quality monitoring from July 2017 to July 2018. Synoptic river sampling showed acidification and increased CO2 content in areas with higher proportions of watershed anthropogenic land use. We propagated the impacts of 1). the observed riverine acidification, and 2). modeled OA changes to incoming coastal ocean waters across the full estuarine salinity spectrum and quantified changes in estuarine carbonate chemistry at a 15- minute temporal resolution. The largest magnitude of acidification (-1.4 pHT units) was found in oligo- and mesohaline portions of the estuary due to the poor buffering characteristics of these waters, and was primarily driven by acidified riverine inputs. Despite this, emissions-driven OA is responsible for over 94% of anthropogenic carbon loading to Tillamook Bay and the dominant driver of acidification across most of the estuary due to its large tidal prism and relatively small river discharges. This dominance of ocean-sourced anthropogenic carbon challenges the efficacy of local management actions to ameliorate estuarine acidification impacts. Despite the relatively large acidification effects experienced in Tillamook Bay (-0.16 to -0.23 pHT units) as compared with typical open ocean change (approximately -0.1 pHT units), observations of estuarine pHT would meet existing state standards for pHT. Our analytical framework addresses pressing needs for water quality assessment and coastal resilience strategies to differentiate the impacts of anthropogenic acidification from natural variability in dynamic estuarine systems. [ABSTRACT FROM AUTHOR]

A model of microelectrode measurements of porewater oxygen and pH from six stations in North Atlantic predicts that 35% of the calcium carbonate rain dissolves in the sediments at all stations. The dissolution is induced by the addition of metabolic CO2. Although there are irregularities in the stoichiometry of respiration and the saturation state of the bottom waters, the dissolution rate is restrained between 3% and 30% per day.