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Bottom waters of the northeast Pacific continental shelf naturally experience localized hypoxic conditions, with significant influences on food webs and biogeochemical cycling. In August 2021, extreme hypoxia was detected from several measurement platforms along the southern British Columbia continental shelf, with oxygen concentration <60 μmol kg−1, and a difference from the seasonal climatology of more than 2 standard deviations. Early and intense remote upwelling and local density shifts were associated with an anomalously strong spring phytoplankton bloom, which likely stimulated localized respiration of subsurface organic matter. This event was concurrent with unsuitable habitat for Pacific halibut and calcite and aragonite undersaturation throughout most of the water column. The drivers of this extreme low oxygen event could be enhanced under future climate change, with potentially significant impacts on marine ecology and biogeochemistry. Plain Language Summary: Most marine organisms consume oxygen, and are therefore impacted when seawater oxygen concentrations reach low values. Extreme low oxygen concentrations are rare in the coastal waters of British Columbia, Canada. However, unusually strong oxygen depletion was observed off the coast of Vancouver Island during summer 2021. Unusually strong, early season upwelling winds along the California coast impacted Vancouver Island by causing nutrient‐rich water to be mixed into the surface, stimulating a large and earlier than usual spring phytoplankton bloom. Decomposition of the bloom‐derived organic carbon consumed local subsurface oxygen throughout the spring and summer. These subtle changes in timing and intensity of seasonal processes likely caused this low oxygen event, which was also associated with high concentrations of inorganic carbon, leading to ocean acidification. Such extreme low oxygen events, even if short‐lived, can have a significant impact on marine ecosystems, restricting the habitat available for groundfish species, such as Pacific halibut, and impacting the formation of carbonate shells by various organisms. The drivers of extreme low oxygen events are projected to intensify as climate change progresses. Key Points: Widespread extreme hypoxia was observed throughout the water column along the southern British Columbia continental shelf in summer 2021Early and intense upwelling followed by a strong biological response contributed to oxygen depletion in subsurface watersMultiple indices suggest that this extreme event was concurrent with unsuitable habitat conditions for groundfish and calcifying organisms [ABSTRACT FROM AUTHOR]

We measured the carbonate system (between 2015 and 2018) in an isolated and a well‐connected temperate estuary, both known for shellfish growth. We evaluated end‐member model estimates of inorganic carbon, alkalinity, pH, mineral saturation states (Ωa), and pH sensitivity (βDIC). We find winter conditions are estimated within observational uncertainty. Spring‐summer primary productivity elevates observed pH and Ωa above theoretical lines, beyond uncertainty. Both estuaries are sensitive in winter and likely to experience rapid pH changes with increased inorganic carbon inputs. Summer pH sensitivity is reduced by productivity and is least sensitive in the midsalinity region. We estimate carbon increased by up to 49 μmol kg−1, since the pre‐industrial period resulting in significant decreases in pH (0.2) and Ωa (0.5). The largest pH decrease occurred outside the minimum buffer zone, at higher salinities where carbon increase was greatest. The largest pH decrease occurred in winter, but the largest Ωa decrease occured in summer. Plain Language Summary: The ocean has absorbed about a third of human carbon dioxide emissions, increasing its acidity. Increasing acidity negatively affects marine organisms such as shellfish and fish that build calcium carbonate structures because they must spend more energy building and maintaining these structures. Most farmed and wild shellfish live in estuarine environments where acidification has not been widely studied. We made complete carbon system measurements over several years and across all seasons, in two distinct estuaries with significant aquaculture, where no previous carbon data existed. We estimated key acidification parameters and evaluated our ability to predict these parameters using only salinity, once properties in the freshest and saltiest water in the systems were well defined in all seasons. Our simple "mixing model" estimates winter conditions well, but during summer, phytoplankton blooms take up carbon and make conditions more favorable than predicted. We found that both estuaries are sensitive to future increases in carbon and are likely to experience rapid changes in chemistry. We estimate that human activity has already caused significant increases in inorganic carbon and associated acidification. The largest change critical to shell‐building organisms has occurred during the summer, and at higher salinities that are typical of grow sites. Key Points: End‐member mixing estimates winter pH and mineral saturation states in temperate estuaries within measurement uncertaintySeasonal productivity keeps pH and mineral saturation states above theoretical mixing curves by up to ∼1 pH and ∼2.5 Ωa, respectivelySince the preindustrial period nearshore estuarine pH declined by 0.05–0.2 with largest decline outside the minimum buffer zone [ABSTRACT FROM AUTHOR]