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Puget Sound (Washington, USA) is a large estuary, known for its profitable shellfish aquaculture industry. However, in the past decade, scientists have observed strong acidification, hypoxia, and temperature anomalies in Puget Sound. These co-occurring environmental stressors are a threat to marine ecosystems and shellfish aquaculture. Our research assesses how environmental variability in Puget Sound impacts two ecologically and economically important bivalves, the purple-hinge rock scallop (Crassodoma gigantea) and Mediterranean mussel (Mytilus galloprovincialis). Our study examines the effect of depth and seasonality on the physiology of these two important bivalves to gain insight into ideal grow-out conditions in an aquaculture setting, improving the yield and quality of this sustainable protein source. To do this, we used Hood Canal (located in Puget Sound) as a natural multiple-stressor laboratory, which allowed us to study acclimatization capacity of shellfish in their natural habitat and provide the aquaculture industry information about differences in growth rate, shell strength, and nutritional sources across depths and seasons. Bivalves were outplanted at two depths (5 and 30 m) and collected after 3.5 and 7.5 months. To maximize mussel and scallop growth potential in an aquaculture setting, our results suggest outplanting at 5 m depth, with more favorable oxygen and pH levels. Mussel shell integrity can be improved by placing out at 5 m, regardless of season, however, there were no notable differences in shell strength between depths in scallops. For both species, δ13C values were lowest at 5 m in the winter and δ15N was highest at 30 m regardless of season. Puget Sound's combination of naturally and anthropogenically acidified conditions is already proving to be a challenge for shellfish farmers. Our study provides crucial information to farmers to optimize aquaculture grow-out as we begin to navigate the impacts of climate change. [Display omitted] • High acclimatization capability in mussels and scallops • Growth rates, δ13C, δ15N, and shell strength differed between seasons and depths. • Mussels and scallops had higher growth rates at 5 m than 30 m. • Shell strength changed with depth in mussels but not in the scallops. • Differences in nutritional sources between depths are higher in winter than spring. [ABSTRACT FROM AUTHOR]

Inorganic carbon chemistry data from the surface and subsurface waters of the West Coast of North America have been compared with similar data from the northern Gulf of Mexico to demonstrate how future changes in CO 2 emissions will affect chemical changes in coastal waters affected by respiration-induced hypoxia ([O 2 ] ≤ ~ 60 µmol kg −1 ). In surface waters, the percentage change in the carbon parameters due to increasing CO 2 emissions are very similar for both regions even though the absolute decrease in aragonite saturation is much higher in the warmer waters of the Gulf of Mexico. However, in subsurface waters the changes are enhanced due to differences in the initial oxygen concentration and the changes in the buffer capacity (i.e., increasing Revelle Factor) with increasing respiration from the oxidation of organic matter, with the largest impacts on pH and CO 2 partial pressure ( p CO 2 ) occurring in the colder West Coast waters. As anthropogenic CO 2 concentrations begin to build up in subsurface waters, increased atmospheric CO 2 will expose organisms to hypercapnic conditions ( p CO 2 >1000 µatm) within subsurface depths. Since the maintenance of the extracellular pH appears as the first line of defense against external stresses, many biological response studies have been focused on pCO 2 -induced hypercapnia. The extent of subsurface exposure will occur sooner and be more widespread in colder waters due to their capacity to hold more dissolved oxygen and the accompanying weaker acid-base buffer capacity. Under present conditions, organisms in the West Coast are exposed to hypercapnic conditions when oxygen concentrations are near 100 µmol kg −1 but will experience hypercapnia at oxygen concentrations of 260 µmol kg −1 by year 2100 under the highest elevated-CO 2 conditions. Hypercapnia does not occur at present in the Gulf of Mexico but will occur at oxygen concentrations of 170 µmol kg −1 by the end of the century under similar conditions. The aragonite saturation horizon is currently above the hypoxic zone in the West Coast. With increasing atmospheric CO 2 , it is expected to shoal up close to surface waters under the IPCC Representative Concentration Pathway (RCP) 8.5 in West Coast waters, while aragonite saturation state will exhibit steeper gradients in the Gulf of Mexico. This study demonstrates how different biological thresholds (e.g., hypoxia, CaCO 3 undersaturation, hypercapnia) will vary asymmetrically because of local initial conditions that are affected differently with increasing atmospheric CO 2 . The direction of change in amplitude of hypercapnia will be similar in both ecosystems, exposing both biological communities from the West Coast and Gulf of Mexico to intensification of stressful conditions. However, the region of lower Revelle factors (i.e., the Gulf of Mexico), currently provides an adequate refuge habitat that might no longer be the case under the most severe RCP scenarios. [ABSTRACT FROM AUTHOR]

Estuaries are recognized as one of the habitats most vulnerable to coastal ocean acidification due to seasonal extremes and prolonged duration of acidified conditions. This is combined with co-occurring environmental stressors such as increased temperature and low dissolved oxygen. Despite this, evidence of biological impacts of ocean acidification in estuarine habitats is largely lacking. By combining physical, biogeochemical, and biological time-series observations over relevant seasonal-to-interannual time scales, this study is the first to describe both the spatial and temporal variation of biological response in the pteropod Limacina helicina to estuarine acidification in association with other stressors. Using clustering and principal component analyses, sampling sites were grouped according to their distribution of physical and biogeochemical variables over space and time. This identified the most exposed habitats and time intervals corresponding to the most severe negative biological impacts across three seasons and three years. We developed a cumulative stress index as a means of integrating spatial-temporal OA variation over the organismal life history. Our findings show that over the 2014–2016 study period, the severity of low aragonite saturation state combined with the duration of exposure contributed to overall cumulative stress and resulted in severe shell dissolution. Seasonally-variable estuaries such as the Salish Sea (Washington, U.S.A.) predispose sensitive organisms to more severe acidified conditions than those of coastal and open-ocean habitats, yet the sensitive organisms persist. We suggest potential environmental factors and compensatory mechanisms that allow pelagic calcifiers to inhabit less favorable habitats and partially offset associated stressors, for instance through food supply, increased temperature, and adaptation of their life history. The novel metric of cumulative stress developed here can be applied to other estuarine environments with similar physical and chemical dynamics, providing a new tool for monitoring biological response in estuaries under pressure from accelerating global change. Unlabelled Image • Spatial and temporal variation in estuarine acidification cause severe biological responses. • Extreme low saturation state and duration of exposure cause pteropod shell dissolution. • Changing estuarine conditions cause cumulative stress that was used to generate stress index. • Compensatory mechanisms allow pelagic calcifiers to persist in extreme OA estuarine habitats. [ABSTRACT FROM AUTHOR]

Ocean acidification (OA) along the US West Coast is intensifying faster than observed in the global ocean. This is particularly true in nearshore regions (<200 m) that experience a lower buffering capacity while at the same time providing important habitats for ecologically and economically significant species. While the literature on the effects of OA from laboratory experiments is voluminous, there is little understanding of present-day OA in-situ effects on marine life. Dungeness crab (Metacarcinus magister) is perennially one of the most valuable commercial and recreational fisheries. We focused on establishing OA-related vulnerability of larval crustacean based on mineralogical and elemental carapace to external and internal carapace dissolution by using a combination of different methods ranging from scanning electron microscopy, energy dispersive X-ray spectroscopy, elemental mapping and X-ray diffraction. By integrating carapace features with the chemical observations and biogeochemical model hindcast, we identify the occurrence of external carapace dissolution related to the steepest Ω calcite gradients (∆Ω cal,60) in the water column. Dissolution features are observed across the carapace, pereopods (legs), and around the calcified areas surrounding neuritic canals of mechanoreceptors. The carapace dissolution is the most extensive in the coastal habitats under prolonged (1-month) long exposure, as demonstrated by the use of the model hindcast. Such dissolution has a potential to destabilize mechanoreceptors with important sensory and behavioral functions, a pathway of sensitivity to OA. Carapace dissolution is negatively related to crab larval width, demonstrating a basis for energetic trade-offs. Using a retrospective prediction from a regression models, we estimate an 8.3% increase in external carapace dissolution over the last two decades and identified a set of affected OA-related sublethal pathways to inform future risk assessment studies of Dungeness crabs. Unlabelled Image • Coastal habitats with the steepest ocean acidification gradients are most detrimental for larval Dungeness crabs. • Severe carapace dissolution was observed in larval Dungeness crabs along the US west coast. • Mechanoreceptors with important sensory and behavioral functions were destabilized. • Dissolution is negatively related to the growth, demonstrating energetic trade-offs. • 10% dissolution increase over the last two decades estimated due to atmospheric CO 2. [ABSTRACT FROM AUTHOR]