Oceanic uptake of anthropogenic carbon dioxide (CO2) from the atmosphere has changed ocean biogeochemistry and threatened the health of organisms through a process known as ocean acidification (OA). Such large-scale changes affect ecosystem functions and can have effects on societal uses, fisheries resources, and economies. In many large estuaries, anthropogenic CO2-induced acidification is enhanced by strong stratification, long water residence times, eutrophication, and a weak acid–base buffer capacity. In this article, we review how a variety of processes influence aquatic acid–base properties in estuarine waters, including river–ocean mixing, upwelling, air–water gas exchange, biological production and subsequent respiration, anaerobic respiration, calcium carbonate (CaCO3) dissolution, and benthic inputs. We emphasize the spatial and temporal dynamics of partial pressure of CO2 ( pCO2), pH, and calcium carbonate mineral saturation states. Examples from three large estuaries—Chesapeake Bay, the Salish Sea, and Prince William Sound—are used to illustrate how natural and anthropogenic processes and climate change may manifest differently across estuaries, as well as the biological implications of OA on coastal calcifiers. Expected final online publication date for the Annual Review of Marine Science, Volume 13 is January 4, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Increasing levels of nutrients, persistent hypoxia, harmful algal blooms, and increased frequency of fish kills are degrading the ecological health of the Salish Sea. An improved version of a diagnostic hydrodynamic and biogeochemical model (nutrients, phytoplankton, carbon, dissolved oxygen, and pH) of the Salish Sea has been developed with the ability to simulate characteristic circulation and water quality features. Sensitivity tests were conducted to assess the responsiveness of the system to land-based (rivers and wastewater sources) nutrient loading. The influence of Fraser River on the magnitude of estuarine exchange with the Pacific Ocean and nearshore habitat was examined given that it contributes nearly half of the total freshwater discharged to the Salish Sea. A large region of hypoxia in Hood Canal that extends over 30-40 km during its peak was reproduced and attributed primarily to the existence of a two-layer classic fjord-type circulation and a nearly stagnant deep bottom layer that occupies nearly 60% of the water column. Nitrate mass in the euphotic zone from land-based and oceanic sources is depleted to near-zero limiting levels during summer. Under such conditions, the Salish Sea is responsive to changes in nutrient loads entering the euphotic zone directly. A hypothetical scenario involving the elimination of land-based nutrient sources results in notable water-quality improvement, featuring a reduction in algal biomass (≈5.4%), reduction in sediment oxygen demand (≈17.1%), and significant reduction in hypoxic area (≈39%) and exposure in area-days to bottom layer hypoxia (≈62%) within the Salish Sea. KHANGAONKAR ET AL.4735

While ocean acidification was first investigated as a global phenomenon, coastal acidification has received significant attention in recent years, as its impacts have been felt by different socio-economic sectors (e.g., high mortality of shellfish larvae in aquaculture farms). As a region that connects land and ocean, the Salish Sea (consisting of Puget Sound and the Straits of Juan de Fuca and Georgia) receives inputs from many different sources (rivers, wastewater treatment plants, industrial waste treatment facilities, etc.), making these coastal waters vulnerable to acidification. Moreover, the lowering of pH in the Northeast Pacific Ocean also affects the Salish Sea, as more acidic waters get transported into the bottom waters of the straits and estuaries. Here, we use a numerical ocean model of the Salish Sea to improve our understanding of the carbonate system in Puget Sound; in particular, we studied the sensitivity of carbonate variables (e.g., dissolved inorganic carbon, total alkalinity, pH, saturation state of aragonite) to ocean and freshwater inputs. The model is an updated version of our FVCOM-ICM framework, with new carbonate-system and sediment modules. Sensitivity experiments altering concentrations at the open boundaries and freshwater sources indicate that not only ocean conditions entering the Strait of Juan de Fuca, but also the dilution of carbonate variables by freshwater sources, are key drivers of the carbonate system in Puget Sound.

Ocean acidification (OA) is projected to have profound impacts on marine ecosystems and resources, especially in estuarine habitats. Here, we describe biological risks under current levels of exposure to anthropogenic OA in the Salish Sea, an estuarine system that already experiences inherently low pH and aragonite saturation state ( ar ) conditions. We used the Pacific Northwest National Laboratory and Washington State Department of Ecology Salish Sea biogeochemical model (SSM) informed by a selection of OA-related biological thresholds of ecologically and economically important calcifiers, pteropods, and Dungeness crabs. The SSM was implemented to assess current exposure and associated risk due to reduced ar and pH conditions with respect to the magnitude, duration, and severity of exposure below the biological thresholds in the Salish Sea in comparison to the pre-industrial era. We further investigated the individual effects of atmospheric CO 2 uptake and nutrient-driven eutrophication on changes in chemical exposure since pre-industrial times. Our model predicts average decreases in ar and pH since pre-industrial times of about 0.11 and 0.06, respectively, in the top 100 m of the water column of the Salish Sea. These decreases predispose pelagic calcifiers to increased duration, intensity, and severity of exposure. For pteropods, present-day exposure is below the thresholds related to sublethal effects across the entire Salish Sea basin, while mortality threshold exposure occurs on a spatially limited basis. The greatest risk for larval Dungeness crabs is associated with spatially limited exposures to low calcite saturation state in the South Sound in the springtime, triggering an increase in internal dissolution. The main anthropogenic driver behind the predicted impacts is atmospheric CO 2 uptake, while nutrient-driven eutrophication plays only a marginal role over spatially and temporally limited scales. Reduction of CO 2 emissions can help sustain biological species vital for ecosystem functions and society.

No abstract