Ocean acidification (OA) is increasing predictably in the global ocean as rising levels of atmospheric carbon dioxide lead to higher oceanic concentrations of inorganic carbon. The Gulf of Maine (GOM) is a seasonally varying region of confluence for many processes that further affect the carbonate system including freshwater influences and high productivity, particularly near the coast where local processes impart a strong influence. Two main regions within the GOM currently experience carbonate conditions that are suboptimal for many organisms—the nearshore and subsurface deep shelf. OA trends over the past 15 years have been masked in the GOM by recent warming and changes to the regional circulation that locally supply more Gulf Stream waters. The region is home to many commercially important shellfish that are vulnerable to OA conditions, as well as to the human populations whose dependence on shellfish species in the fishery has continued to increase over the past decade. Through a review of the sensitivity of the regional marine ecosystem inhabitants, we identified a critical threshold of 1.5 for the aragonite saturation state (Ω a ). A combination of regional high-resolution simulations that include coastal processes were used to project OA conditions for the GOM into 2050. By 2050, the Ω a declines everywhere in the GOM with most pronounced impacts near the coast, in subsurface waters, and associated with freshening. Under the RCP 8.5 projected climate scenario, the entire GOM will experience conditions below the critical Ω a threshold of 1.5 for most of the year by 2050. Despite these declines, the projected warming in the GOM imparts a partial compensatory effect to Ω a by elevating saturation states considerably above what would result from acidification alone and preserving some important fisheries locations, including much of Georges Bank, above the critical threshold.

The objective of this study was to assess experimentally the potential impact of anthropogenic pH perturbation (ApHP) on concentrations of dimethyl sulfide (DMS) and dimethylsulfoniopropionate (DMSP), as well as processes governing the microbial cycling of sulfur compounds. A summer planktonic community from surface waters of the Lower St. Lawrence Estuary was monitored in microcosms over 12 days under three pCO 2 targets: 1 × pCO 2 (775 µatm), 2 × pCO 2 (1,850 µatm), and 3 × pCO 2 (2,700 µatm). A mixed phytoplankton bloom comprised of diatoms and unidentified flagellates developed over the course of the experiment. The magnitude and timing of biomass buildup, measured by chlorophyll a concentration, changed in the 3 × pCO 2 treatment, reaching about half the peak chlorophyll a concentration measured in the 1 × pCO 2 treatment, with a 2-day lag. Doubling and tripling the pCO 2 resulted in a 15% and 40% decline in average concentrations of DMS compared to the control. Results from 35 S-DMSP d uptake assays indicated that neither concentrations nor microbial scavenging efficiency of dissolved DMSP was affected by increased pCO 2 . However, our results show a reduction of the mean microbial yield of DMS by 34% and 61% in the 2 × pCO 2 and 3 × pCO 2 treatments, respectively. DMS concentrations correlated positively with microbial yields of DMS (Spearman’s ρ = 0.65; P < 0.001), suggesting that the impact of ApHP on concentrations of DMS in diatom-dominated systems may be strongly linked with alterations of the microbial breakdown of dissolved DMSP. Findings from this study provide further empirical evidence of the sensitivity of the microbial DMSP switch under ApHP. Because even small modifications in microbial regulatory mechanisms of DMSP can elicit changes in atmospheric chemistry via dampened efflux of DMS, results from this study may contribute to a better comprehension of Earth’s future climate.

Studying carbon dioxide in the ocean helps to understand how the ocean will be impacted by climate change and respond to increasing fossil fuel emissions. The marine carbonate system is not well characterized in the Arctic, where challenging logistics and extreme conditions limit observations of atmospheric CO 2 flux and ocean acidification. Here, we present a high-resolution marine carbon system data set covering the complete cycle of sea-ice growth and melt in an Arctic estuary (Nunavut, Canada). This data set was collected through three consecutive yearlong deployments of sensors for pH and partial pressure of CO 2 in seawater ( p CO 2sw ) on a cabled underwater observatory. The sensors were remarkably stable compared to discrete samples: While corrections for offsets were required in some instances, we did not observe significant drift over the deployment periods. Our observations revealed a strong seasonality in this marine carbon system. Prior to sea-ice formation, air–sea gas exchange and respiration were the dominant processes, leading to increasing p CO 2sw and reduced aragonite saturation state (Ω Ar ). During sea-ice growth, water column respiration and brine rejection (possibly enriched in dissolved inorganic carbon, relative to alkalinity, due to ikaite precipitation in sea ice) drove p CO 2sw to supersaturation and lowered Ω Ar to < 1. Shortly after polar sunrise, the ecosystem became net autotrophic, returning p CO 2sw to undersaturation. The biological community responsible for this early switch to autotrophy (well before ice algae or phytoplankton blooms) requires further investigation. After sea-ice melt initiated, an under-ice phytoplankton bloom strongly reduced aqueous carbon (chlorophyll- a max of 2.4 µg L –1 ), returning Ω Ar to > 1 after 4.5 months of undersaturation. Based on simple extrapolations of anthropogenic carbon inventories, we suspect that this seasonal undersaturation would not have occurred naturally. At ice breakup, the sensor platform recorded low p CO 2sw (230 µatm), suggesting a strong CO 2 sink during the open water season.

Environmental change can have a significant impact on biogeochemical cycles at high latitudes and be particularly important in ecologically valuable fjord ecosystems. Seasonality in biogeochemical cycling in a sub-Arctic fjord of northern Norway (Kaldfjorden) was investigated from October 2016 to September 2018. Monthly changes in total inorganic carbon (C T ), alkalinity (A T ), major nutrients and calcium carbonate saturation (Ω) were driven by freshwater discharge, biological production and mixing with subsurface carbon-rich coastal water. Stable oxygen isotope ratios indicated that meteoric water (snow melt, river runoff, precipitation) had stratified and freshened surface waters, contributing to 81% of the monthly C T deficit in the surface layer. The timing and magnitude of freshwater inputs played an important role in Ω variability, reducing A T and C T by dilution. This dilution effect was strongly counteracted by the opposing effect of primary production that dominated surface water Ω seasonality. The spring phytoplankton bloom rapidly depleted nitrate and C T to drive highest Ω (~2.3) in surface waters. Calcification reduced A T and C T , which accounted for 21% of the monthly decrease in Ω during a coccolithophore bloom. Freshwater runoff contributed C T , A T and silicates of terrestrial origin to the fjord. Lowest surface water Ω (~1.6) resulted from organic matter remineralisation and mixing into subsurface water during winter and spring. Surface waters were undersaturated with respect to atmospheric CO 2 , resulting in modest uptake of –0.32 ± 0.03 mol C m –2 yr –1 . Net community production estimated from carbon drawdown was 14 ± 2 g C m –2 yr –1 during the productive season. Kaldfjorden currently functions as an atmospheric CO 2 sink of 3.9 ± 0.3 g C m –2 yr –1 . Time-series data are vital to better understand the processes and natural variability affecting biogeochemical cycling in dynamic coastal regions and thus better predict the impact of future changes on important fjord ecosystems.

Ocean acidification (OA) will have a predominately negative impact on marine animals sensitive to changes in carbonate chemistry. Coastal upwelling regions, such as the Northwest coast of North America, are likely among the first ecosystems to experience the effects of OA as these areas already experience high pH variability and naturally low pH extremes. Over the past decade, pH off the Olympic coast of Washington has declined an order of magnitude faster than predicted by accepted conservative climate change models. Resource managers are concerned about the potential loss of intertidal biodiversity likely to accompany OA, but as of yet, there are little pH sensitivity data available for the vast majority of taxa found on the Olympic coast. The intertidal zone of Olympic National Park is particularly understudied due to its remote wilderness setting, habitat complexity, and exceptional biodiversity. Recently developed methodological approaches address these challenges in determining organism vulnerability by utilizing experimental evidence and expert opinion. Here, we use such an approach to determine intertidal organism sensitivity to pH for over 700 marine invertebrate and algal species found on the Olympic coast. Our results reinforce OA vulnerability paradigms for intertidal taxa that build structures from calcium carbonate, but also introduce knowledge gaps for many understudied species. We furthermore use our assessment to identify how rocky intertidal communities at four long-term monitoring sites on the Olympic coast could be affected by OA given their community composition.

Understanding larval bivalve responses to variable regimes of seawater carbonate chemistry requires realistic quantification of physiological stress. Based on a degree-day modeling approach, we developed a new metric, the ocean acidification stress index for shellfish (OASIS), for this purpose. OASIS integrates over the entire larval period the instantaneous stress associated with deviations from published sensitivity thresholds to aragonite saturation state (Ω Ar ) while experiencing variable carbonate chemistry. We measured survival to D-hinge and pre-settlement stage of four Pacific oyster ( Crassostrea gigas ) cohorts with different histories of carbonate chemistry exposure at the Whiskey Creek Hatchery, Netarts Bay, OR, to test the utility of OASIS as a stress metric and document the effects of buffering seawater in mitigating acute and chronic exposure to ocean acidification. Each cohort was divided into four groups and reared under the following conditions: 1) stable, buffered seawater for the entire larval period; 2) stable, buffered seawater for the first 48 hours, then naturally variable, unbuffered seawater; 3) stable, unbuffered seawater for the first 48 hours, then buffered seawater; and 4) stable, unbuffered seawater for the first 48 hours, then naturally variable, unbuffered seawater. Patterns in Netarts Bay carbonate chemistry were dominated by seasonal upwelling at the time of the experimental work, resulting in naturally highly variable Ω Ar for the larvae raised in the unbuffered treatments. Two of the four cohorts showed strongly positive responses to buffering in survival to 48 hours; three of the four, in survival to pre-settlement. OASIS accurately predicted survival for two of the three cohorts tested (the fourth excluded due to other environmental factors), suggesting that this new metric could be used to better understand larval bivalve survival in naturally variable environments. OASIS may also be useful to an array of diverse stakeholders with increasing access to highly resolved temporal measurements of carbonate chemistry.

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.

A pilot study of sampling, using monthly marine flights over spatially distributed stations, was conducted with the aim to characterize the carbonate system in Puget Sound over a full year-long period. Surface waters of Puget Sound were found to be under-saturated with respect to aragonite during October–March, and super-saturated during April–September. Highest pCO 2 and lowest pH occurred during the corrosive October–March period. Lowest pCO 2 and highest pH occurred during the super-saturated April–September period. The monthly variations in pCO 2 , pH, and aragonite saturation state closely followed the variations in monthly average chlorophyll a . Super-saturated conditions during April–September are likely strongly influenced by photosynthetic uptake of CO 2 during the phytoplankton growing season. The relationship between phytoplankton production, the carbonate system, and aragonite saturation state suggests that long-term trends in eutrophication processes may contribute to trends in ocean acidification in Puget Sound.

Information on ecosystem sensitivity to global change can help guide management decisions. Here, we characterize the sensitivity of the Puget Sound ecosystem to ocean acidification by estimating, at a number of taxonomic levels, the direct sensitivity of its species. We compare sensitivity estimates based on species mineralogy and on published literature from laboratory experiments and field studies. We generated information on the former by building a database of species in Puget Sound with mineralogy estimates for all CaCO 3 -forming species. For the latter, we relied on a recently developed database and meta-analysis on temperate species responses to increased CO 2 . In general, species sensitivity estimates based on the published literature suggest that calcifying species are more sensitive to increased CO 2 than non-calcifying species. However, this generalization is incomplete, as non-calcifying species also show direct sensitivity to high CO 2 conditions. We did not find a strong link between mineral solubility and the sensitivity of species survival to changes in carbonate chemistry, suggesting that, at coarse scales, mineralogy plays a lesser role to other physiological sensitivities. Summarizing species sensitivity at the family level resulted in higher sensitivity scalar scores than at the class level, suggesting that grouping results at the class level may overestimate species sensitivity. This result raises caution about the use of broad generalizations on species response to ocean acidification, particularly when developing summary information for specific locations. While we have much to learn about species response to ocean acidification and how to generalize ecosystem response, this study on Puget Sound suggests that detailed information on species performance under elevated carbon dioxide conditions, summarized at the lowest taxonomic level possible, is more valuable than information on species mineralogy.

Ocean acidification is intensifying and hypoxia is projected to expand in the California Current large marine ecosystem as a result of processes associated with the global emission of CO 2 . Observed changes in the California Current outpace those in many other areas of the ocean, underscoring the pressing need to adopt management approaches that can accommodate uncertainty and the complicated dynamics forced by accelerating change. We argue that changes occurring in the California Current large marine ecosystem provide opportunities and incentives to adopt an integrated, systems-level approach to resource management to preserve existing ecosystem services and forestall abrupt change. Practical options already exist to maximize the benefits of management actions and ameliorate impending change in the California Current, for instance, adding ocean acidification and hypoxia to design criteria for marine protected areas, including consideration of ocean acidification and hypoxia in fisheries management decisions, and fully enforcing existing laws and regulations that govern water quality and land use and development.

Morphological and phylogenetic analyses suggest that the ability to precipitate carbonates evolved several times in marine invertebrates in the past 600 million years. Over the past decade, there has been a profusion of genomic, transcriptomic, and proteomic analyses of calcifying representatives from three metazoan phyla: Cnidaria, Echinodermata, and Mollusca. Based on this information, we compared proteins intimately associated with precipitated calcium carbonate in these three phyla. Specifically, we used a cluster analysis and gene ontology approach to compare ∼1500 proteins, from over 100 studies, extracted from calcium carbonates in stony corals, in bivalve and gastropod mollusks, and in adult and larval sea urchins to identify common motifs and differences. Our analysis suggests that there are few sequence similarities across all three phyla, supporting the independent evolution of biomineralization. However, there are core sets of conserved motifs in all three phyla we examined. These motifs include acidic proteins that appear to be responsible for the nucleation reaction as well as inhibition; structural and adhesion proteins that determine spatial patterning; and signaling proteins that modify enzymatic activities. Based on this analysis and the fossil record, we propose that biomineralization is an extremely robust and highly controlled process in metazoans that can withstand extremes in pH predicted for the coming century, similar to their persistence through the Paleocene-Eocene Thermal Maximum (∼55 Mya).