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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]

Ocean warming and deoxygenation are already modifying the habitats of many aerobic organisms. Benthic habitat in the Northeast Pacific is sensitive to deoxygenation, as low oxygen concentrations occur naturally in continental shelf bottom waters. Here, we examine the potential impacts of deoxygenation and ocean warming on the habitat distribution of Pacific halibut (Hippoglossus stenolepis), one of the most commercially important groundfish in North America. We combine fisheries‐independent Pacific halibut survey data (1998–2020) with oceanographic measurements and a regional ocean circulation model to investigate current and future (end of 21st century) influences of deoxygenation and warming on optimal Pacific halibut habitat. We use the observations and model output to derive a metabolic index of Pacific halibut‐specific suitable habitat. Our results show high Pacific halibut counts in regions where the metabolic index is greatest and demonstrate that interannual variability in Pacific halibut abundance is coherent with the Pacific Decadal Oscillation. Working with model projections, we examine potential future changes in suitable Pacific halibut habitat by the end of the century under a high carbon dioxide emissions scenario. These projections indicate that suitable Pacific halibut habitat may largely disappear off the coast of Washington state, retreating approximately 5° latitude northward. In bottom waters along coastal British Columbia and Alaska continental shelf, Pacific halibut habitat is projected to decrease by about 50%. Such habitat changes may potentially drive a northward shift in Pacific halibut, with significant implications for commercial fisheries. [ABSTRACT FROM AUTHOR]

Estuarine systems host a rich diversity of marine life that is vulnerable to changes in ocean chemistry due to addition of anthropogenic carbon. However, the detection and impact of secular carbon trends in these systems is complicated by heightened natural variability as compared to open‐ocean regimes. We investigate biogeochemical changes between the pre‐industrial (PI) and modern periods using a high‐resolution, three‐dimensional, biophysical model of the Salish Sea, a representative Northeast Pacific coastal system. While the seasonal amplitude of the air‐sea difference in pCO2 has increased on average since pre‐industrial times, the net CO2 source has changed little. Our simulations show that inorganic carbon has increased throughout the model domain by 29–39 mmol m−3 (28–38 µmol kg−1) from the pre‐industrial to present. While this increase is modest in a global context, the region's naturally high inorganic carbon content and the low buffering capacity of the local carbonate system amplify the resultant effects. Notably, this increased carbon drives the estuary toward system‐wide undersaturation of aragonite, negatively impacting shell‐forming organisms. Undersaturation events were rare during the pre‐industrial experiment, with 10%–25% of the domain undersaturated by volume throughout the year, while under present‐day conditions, the majority (55%–75%) of the system experiences corrosive, undersaturated conditions year‐round. These results are extended using recent global coastal observations to show that estuaries throughout the Pacific Rim have already undergone a similar saturation state regime shift. Plain Language Summary: The coastal ocean hosts rich ecosystems which are critical to marine organisms and people and sensitive to human‐caused climate change. We used a detailed computer simulation to understand the carbon cycle in a complex, productive North Pacific coastal system that is fed by a large un‐dammed river, has connecting fjords, and narrows that have strong tides. We determine how much extra carbon has entered the system since the beginning of the industrial revolution. This increase is important because extra carbon in the ocean makes it harder for animals to build shells and structures. We find that large chemical changes have occurred. In fact, we are surprised to find that an important threshold under which shells dissolve has already been crossed in most of the region. We consider observations from other global coastal oceans and find that this same threshold may have been crossed throughout coastal oceans in much of the Pacific, and that this shift will likely become prominent in the Atlantic Ocean by 2080. Key Points: On average, dissolved inorganic carbon has increased by 29–39 mmol m−3 in the Salish Sea, causing a shift to majority aragonite undersaturation by volumeModern aragonite saturation conditions, though variable, are typically outside of the range of pre‐industrial values throughout the domainMuch of the coastal Pacific Rim has similar carbonate chemistry conditions, and comparable shifts in aragonite saturation may have occurred [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]

The ocean absorbs anthropogenic carbon, slowing atmospheric CO2 increase but driving ocean acidification. Long‐term changes in the carbon system are typically assessed from single‐point time series or from hydrographic sections spaced by decades. Using higher resolution observations (1–3 year−1) from the Line P time series, we investigate processes modulating trends in the carbon system of the northeast subarctic Pacific. Dissolved inorganic carbon (DIC) and apparent oxygen utilization (AOU) from 1990 to 2019 reveal substantial trends over most of the upper water column along the 1,500 km coastal to open ocean transect. At the surface, an increasing trend in salinity‐normalized DIC (sDIC33) (+0.5 ± 0.4 μmol kg−1 yr−1) is associated with a decrease in pH (0.01–0.02 decade−1) and a decrease in aragonite saturation state (0.04–0.08 decade−1). These observed trends are driven by anthropogenic CO2 uptake, partially offset by trends in surface salinity or temperature. Stratification associated with recent marine heat waves appears to have caused anomalously low surface pCO2. sDIC33 trends of similar magnitude were found below the seasonal thermocline on the 26.7–26.8 isopycnals (150–300 m), which are ventilated in the western Pacific. Roughly, a third (20%–50%) of the subsurface sDIC33 trend is driven by increased remineralization, likely caused by long‐term decreases in ventilation in the western Pacific. Bidecadal oscillations in the ventilation of the 26.7–26.8 isopycnals arising from the Lunar Nodal Cycle cause oscillations in sDIC33 and AOU at the offshore end of our transect. We trace the oscillations to alternating periods of higher anthropogenic carbon uptake or higher carbon remineralization. Plain Language Summary: The ocean takes up anthropogenic carbon and slows the rate at which atmospheric carbon dioxide is increasing. Using 30 years of data from the NE Pacific, we show that the surface ocean is indeed absorbing anthropogenic carbon and undergoing ocean acidification in this region. We find that carbon is also increasing in the subsurface waters. This excess carbon was absorbed in the western Pacific, where dense waters sink from the surface to the subsurface and transport anthropogenic carbon and oxygen eastward. This western Pacific breathing process appears to be weakening in time. If this trend continues, less anthropogenic carbon will be taken up and stored in subsurface waters in the future. Despite this reduction, some carbon will still naturally accumulate as the available oxygen is consumed and natural carbon dioxide is produced. Using data from three decades, we demonstrate the impact of climatic events and natural variability on the carbon system. At the surface, recent marine heat waves have reduced the amount of carbon absorbed by the ocean. In the subsurface waters, we observed that anthropogenic carbon increased in decades with stronger ventilation, while natural carbon generated by respiration of organic matter increased during periods of weaker ventilation. Key Points: Local uptake of anthropogenic CO2 decreases surface pH in the NE Pacific but the trend appears modulated by recent temporal variabilityDeclining ventilation in the NW Pacific appears responsible for 20%–50% of the inorganic carbon increase in NE Pacific intermediate waterThe signature of tidally driven bidecadal oscillations propagates east to Line P producing fluctuations in intermediate water preformed DIC [ABSTRACT FROM AUTHOR]

Anthropogenic climate change is causing our oceans to lose oxygen and become more acidic at an unprecedented rate, threatening marine ecosystems and their associated animals. In deep‐sea environments, where conditions have typically changed over geological timescales, the associated animals, adapted to these stable conditions, are expected to be highly vulnerable to any change or direct human impact. Our study coalesces one of the longest deep‐sea observational oceanographic time series, reaching back to the 1960s, with a modern visual survey that characterizes almost two vertical kilometers of benthic seamount ecosystems. Based on our new and rigorous analysis of the Line P oceanographic monitoring data, the upper 3,000 m of the Northeast Pacific (NEP) has lost 15% of its oxygen in the last 60 years. Over that time, the oxygen minimum zone (OMZ), ranging between approximately 480 and 1,700 m, has expanded at a rate of 3.0 ± 0.7 m/year (due to deepening at the bottom). Additionally, carbonate saturation horizons above the OMZ have been shoaling at a rate of 1–2 m/year since the 1980s. Based on our visual surveys of four NEP seamounts, these deep‐sea features support ecologically important taxa typified by long life spans, slow growth rates, and limited mobility, including habitat‐forming cold water corals and sponges, echinoderms, and fish. By examining the changing conditions within the narrow realized bathymetric niches for a subset of vulnerable populations, we resolve chemical trends that are rapid in comparison to the life span of the taxa and detrimental to their survival. If these trends continue as they have over the last three to six decades, they threaten to diminish regional seamount ecosystem diversity and cause local extinctions. This study highlights the importance of mitigating direct human impacts as species continue to suffer environmental changes beyond our immediate control. [ABSTRACT FROM AUTHOR]

We present a coupled three‐dimensional biological‐physical model for the Salish Sea and evaluate it by comparison to nitrate, silicate, and chlorophyll observations. It accurately reproduces nitrate concentrations with Willmott skill scores, root‐mean‐square error, and bias ranging from 0.84–0.95, 4.02–6.5 μM, and −2.33–1.84 μM, respectively, compared to three independent discrete sample data sets. A prominent feature of the model output is a tidal jet emanating from Discovery Passage producing a downstream plume of elevated surface nitrate. The signal is present from April to September, when surface nitrate is otherwise drawn down. It has a weak but statistically significant correlation to Discovery Passage tidal velocity (R=0.37, p<0.01). Within the turbulent jet and associated plume, the average rate of vertical nitrate supply due to mixing and advection across a depth of roughly 6 m is 0.46 μmol m−2 s−1 between 15 May and 20 August 2015, compared to 0.10 μmol m−2 s−1 for the northern Strait of Georgia as a whole. Close to Discovery Passage, where velocities and shear are strongest, the majority of the vertical nitrate flux is due to mixing. As velocities weaken downstream, vertical advection becomes more important relative to mixing, but vertical velocities also decrease. The tidal pulses out of Discovery Passage drive waves that contribute net upward nitrate flux as far south as Cape Lazo, 40 km away. The nitrate supply drives new production, consistent with existing observations. Similar dynamics have been described in many other tidally influenced coastal systems. Plain Language Summary: A three‐dimensional coupled physical‐biological model has been developed that simulates currents, nutrients, and phytoplankton in the Salish Sea. Comparisons with observations show that the model successfully represents key aspects of the nitrogen cycle in the Strait of Georgia, including the seasonal cycle and regional differences. The model is used to investigate mechanisms through which tidal currents lead to nutrient supply to the surface layer, where phytoplankton have enough light to grow, in the northern Strait of Georgia. Strong tidal flow through the narrow Discovery Passage leads to turbulence that mixes deep nutrients to the surface. Tidal currents then advect the surface nutrient plume further into the Strait of Georgia. Additional mixing and vertical motion also occur downstream, increasing the amount of nutrients supplied to the surface. Key Points: A physical‐biological model of the Salish Sea represents spring blooming and seasonal and spatial patterns in nutrients and chlorophyllA tidal jet drives mixing and transport, producing a surface nitrate plume, fueling significant summertime primary productionA wave associated with the jet propagates southward along the Vancouver Island coast and makes a smaller contribution to mixing and transport of nitrate [ABSTRACT FROM AUTHOR]

Under physically isolated conditions, net community production (NCP) can be accurately estimated from the rate of oxygen evasion to the atmosphere derived from local mixed layer oxygen/argon measurements. We use a simple box model to demonstrate that, when physical inputs are negligible, the sea‐to‐air flux of biological oxygen (bioflux) represents the average NCP exponentially weighted over the past several residence times of oxygen in the mixed layer. This new weighting scheme shows that there is no apparent lag between bioflux and exponentially weighted time‐averaged NCP. Furthermore, a strict steady state assumption is unnecessary to this relationship. However, this widely used O2/Ar method is not effective in dynamic coastal zones where low oxygen water upwells to the surface. Yet these zones are highly productive and their episodic productivity needs to be quantified. We use a quasi‐2‐D version of the Regional Ocean Modeling System, including oxygen and argon as prognostic variables, to explore the application of this method and the relationship between NCP and bioflux in a coastal upwelling system. We show that bioflux is an accurate measure of NCP over large regions of time and space. Bioflux is most biased near the shore following upwelling favorable winds, where bioflux is sometimes negative (flux from the atmosphere to the ocean) and even positive bioflux values can severely underestimate NCP. Assessing a range of model variables that are easily observed in the field, we show that sea surface temperature is the most effective at identifying bioflux measurements that are likely to be biased. [ABSTRACT FROM AUTHOR]

The distribution of dissolved (<0.4 μm) iron (Fe) across the continental shelf and slope of Queen Charlotte Sound on the west coast of Canada was examined to estimate the potential of these waters as a source of Fe to the Fe-limited waters of the subarctic northeast Pacific. Iron profiles obtained in shelf, slope, and offshore waters demonstrate decreasing concentrations of Fe with distance from the continent. Within 50 m of the shelf sediments dissolved Fe concentrations were 5.3 ± 0.3 nM. This signal was detected, although attenuated by 80%, along the isopycnal surface at offshore stations 40-50 km seaward of the shelf break, strongly suggesting cross-shelf transport of an Fe-rich plume originating in low dissolved oxygen (<3 ml L-1, <130 μmol kg-1) waters in subsurface water over the continental shelf. Several physical mechanisms that may cause these Fe-enriched waters to advect offshore in this region (i.e., tidal currents and Ekman transport in the bottom boundary layer, coastal downwelling/relaxation from upwelling, and the formation of anticyclonic, westward-propagating, coastal eddies) are discussed. We suggest that strong tidal currents over broad continental shelves may play a key role in Fe supply to ocean basins. [ABSTRACT FROM AUTHOR]

We present continuous, high-resolution measurements of surface dimethylsulfide (DMS), pCO2, and O2/Ar obtained in coastal waters off British Columbia, Canada, using membrane inlet mass spectrometry (MIMS). Sampled underway at a frequency of twice per minute (every ~160 m at 10 knots cruising speed), our data reveal fine-scale structure in gas variability and its covariance with a number of hydrographic parameters. All parameters exhibited large ranges (pCO2, 200-747 ppm; DMS, <1-29 nM; chl a, <0.1-33 µg L-1), highlighting the dynamic nature of the study area. A strong anticorrelation between pCO2 and O2/Ar was observed across the survey region, with the distributions of these gases influenced by biology and its interplay with physical processes. In contrast, DMS levels, which varied dramatically over short distances, showed no significant correlations with any single variable for the full, high-resolution data set. However, when measurements were binned to a much coarser spatial resolution, we found a linear relationship between surface DMS and the chlorophyll/mixed layer depth ratio. The slope of this relationship differed significantly from that previously derived from open ocean data. We used several statistical techniques to estimate the spatial variability of gases and hydrographic parameters and the inherent sampling errors associated with low-frequency sampling approaches. These analyses emphasize the importance of high-resolution sampling in coastal areas, particularly for DMS. [ABSTRACT FROM AUTHOR]

Diel vertical migration (DVM) has been hypothesized to actively transport organic material out of the euphoric layer, thus forming a novel part of the "biological pump." However, quantifying DVM is made difficult by observational limitations. Conventionally, the difference between night and day biomass from net trawls in the surface has been assumed to be a consequence of species that have migrated up from their deep daytime depths. However, some of this difference might be an artifact of visual net avoidance. Here, we present a method that facilitates quantification of zooplankton that are migrating, those that are not migrating, and those able to avoid net capture. The algorithm is applied to an extensive data set gathered in the Azores Front region. Results indicate that DVM, and thus active carbon transport, calculated in the traditional manner would overestimate the true value by ∼50%. [ABSTRACT FROM AUTHOR]