The ocean is a key facet of climate stability and Earth's carbon cycle. Given the immense scale of atmospheric carbon dioxide removal (CDR) necessary to achieve international climate goals, ocean-based CDR approaches (or "ocean CDR") warrant serious consideration. Ocean CDR is a still nascent, but cautiously promising, area of scientific research with several approaches under evaluation. Here, we examine one of the most promising approaches, ocean alkalinity enhancement, to highlight and exemplify challenges and opportunities of an emerging ocean CDR industry in the blue economy.

Predicting the potential for species adaption to climate change is challenged by the need to identify the physiological mechanisms that underpin species vulnerability. Here, we investigated the sensitivity to ocean acidification in marine mussels during early development, and specifically the trochophore stage. Using RNA and DNA sequencing and in situ RNA hybridization, we identified developmental processes associated with abnormal development and rapid adaptation to low pH. Trochophores exposed to low pH seawater exhibited 43 differentially expressed genes. Gene annotation and in situ hybridization of differentially expressed genes point to pH sensitivity of (1) shell field development and (2) cellular stress response. Five genes within these two processes exhibited shifts in allele frequencies indicative of a potential for rapid adaptation. This case study contributes direct evidence that protecting species' existing genetic diversity is a critical management action to facilitate species resilience to climate change.

Phenotypic plasticity is expected to facilitate the persistence of natural populations as global change progresses. The attributes of fluctuating environments that favor the evolution of plasticity have received extensive theoretical investigation, yet empirical validation of these findings is still in its infancy. Here, we combine high-resolution environmental data with a laboratory-based experiment to explore the influence of habitat pH fluctuation dynamics on the plasticity of gene expression in two populations of the Mediterranean mussel, Mytilus galloprovincialis. We linked differences in the magnitude and predictability of pH fluctuations in two habitats to population-specific gene expression profiles in ambient and stressful pH treatments. Our results demonstrate population-based differentiation in gene expression plasticity, whereby mussels native to a habitat exhibiting a large magnitude of pH fluctuations with low predictability display reduced phenotypic plasticity between experimentally imposed pH treatments. This work validates recent theoretical findings on evolution in fluctuating environments, suggesting that the predictability of fluctuating selection pressures may play a predominant role in shaping the phenotypic variation observed across natural populations.

Climate change refugia in the terrestrial biosphere are areas where species are protected from global environmental change and arise from natural heterogeneity in landscapes and climate. Within the marine realm, ocean acidification, or the global decline in seawater pH, remains a pervasive threat to organisms and ecosystems. Natural variability in seawater carbon dioxide (CO2) chemistry, however, presents an opportunity to identify ocean acidification refugia (OAR) for marine species. Here, we review the literature to examine the impacts of variable CO2 chemistry on biological responses to ocean acidification and develop a framework of definitions and criteria that connects current OAR research to management goals. Under the concept of managing vulnerability, the most likely mechanisms by which OAR can mitigate ocean acidification impacts are by reducing exposure to harmful conditions or enhancing adaptive capacity. While local management options, such as OAR, show some promise, they present unique challenges, and reducing global anthropogenic CO2 emissions must remain a priority.

Global climate change has intensified the need to assess the capacity for natural populations to adapt to abrupt shifts in the environment. Reductions in seawater pH constitute a conspicuous stressor associated with increasing atmospheric carbon dioxide that is affecting ecosystems throughout the world’s oceans. Here, we quantify the phenotypic and genetic modifications associated with rapid adaptation to reduced seawater pH in the marine mussel, Mytilus galloprovincialis . We reared a genetically diverse larval population in ambient and extreme low pH conditions (pH T 8.1 and 7.4) and tracked changes in the larval size and allele frequency distributions through settlement. Additionally, we separated larvae by size to link a fitness-related trait to its underlying genetic background in each treatment. Both phenotypic and genetic data show that M. galloprovincialis can evolve in response to a decrease in seawater pH. This process is polygenic and characterized by genotype-environment interactions, suggesting the role of cryptic genetic variation in adaptation to future climate change. Holistically, this work provides insight into the processes underpinning rapid evolution, and demonstrates the importance of maintaining standing variation within natural populations to bolster species’ adaptive capacity as global change progresses.

Global climate change has intensified the need to assess the capacity for natural populations to adapt to abrupt shifts in the environment. Reductions in seawater pH constitute a conspicuous global change stressor that is affecting marine ecosystems globally. Here, we quantify the phenotypic and genetic modifications associated with rapid adaptation to reduced seawater pH in the Mediterranean mussel, Mytilus galloprovincialis. We reared a genetically diverse larval population in two pH treatments (pHT 8.1 and 7.4) and tracked changes in the shell-size distribution and genetic variation through settlement. Additionally, we identified differences in the signatures of selection on shell growth in each pH environment. Both phenotypic and genetic data show that standing variation can facilitate adaptation to declines in seawater pH. This work provides insight into the processes underpinning rapid evolution, and demonstrates the importance of maintaining variation within natural populations to bolster species' adaptive capacity as global change progresses.

A successful integrated ocean acidification (OA) observing network must include (1) scientists and technicians from a range of disciplines from physics to chemistry to biology to technology development; (2) government, private, and intergovernmental support; (3) regional cohorts working together on regionally specific issues; (4) publicly accessible data from the open ocean to coastal to estuarine systems; (5) close integration with other networks focusing on related measurements or issues including the social and economic consequences of OA; and (6) observation-based informational products useful for decision making such as management of fisheries and aquaculture. The Global Ocean Acidification Observing Network (GOA-ON), a key player in this vision, seeks to expand and enhance geographic extent and availability of coastal and open ocean observing data to ultimately inform adaptive measures and policy action, especially in support of the United Nations 2030 Agenda for Sustainable Development. GOA-ON works to empower and support regional collaborative networks such as the Latin American Ocean Acidification Network, supports new scientists entering the field with training, mentorship, and equipment, refines approaches for tracking biological impacts, and stimulates development of lower-cost methodology and technologies allowing for wider participation of scientists. GOA-ON seeks to collaborate with and complement work done by other observing networks such as those focused on carbon flux into the ocean, tracking of carbon and oxygen in the ocean, observing biological diversity, and determining short-and long-term variability in these and other ocean parameters through space and time.

Coastal marine ecosystems experience dynamic fluctuations in seawater carbonate chemistry. The importance of this variation in the context of ocean acidification requires knowing what aspect of variability biological processes respond to. We conducted four experiments (ranging from 3 to 22 days) with different variability regimes (pH T 7.4–8.1) assessing the impact of diel fluctuations in carbonate chemistry on the early development of the mussel Mytilus galloprovincialis . Larval shell growth was consistently correlated to mean exposures, regardless of variability regimes, indicating that calcification responds instantaneously to seawater chemistry. Larval development was impacted by timing of exposure, revealing sensitivity of two developmental processes: development of the shell field, and transition from the first to the second larval shell. Fluorescent staining revealed developmental delay of the shell field at low pH, and abnormal development thereof was correlated with hinge defects in D-veligers. This study shows, for the first time, that ocean acidification affects larval soft-tissue development, independent from calcification. Multiple developmental processes additively underpin the teratogenic effect of ocean acidification on bivalve larvae. These results explain why trochophores are the most sensitive life-history stage in marine bivalves and suggest that short-term variability in carbonate chemistry can impact early larval development.

4. L178-182 “Precision of CT and AT was less than 3 μmol kg-1, and the average accuracy was 2.6 and 3 μmol kg-1, as compared with seawater certified reference material (CRM) provided by A. Dickson” To my understanding, the accuracy must always be less good (i.e., higher absolute number) than the precision, because it is superimposed on the precision. Accuracy was determined as the mean offset between the measured and reference value of the CRMs. Precision refers to the repeatability of attaining the same value across replicate bottle samples (which includes errors associated with sample collection, preservation, etc.). We have updated the text to use the phrase repeatability instead of precision: “Average accuracy of CT and AT measurements was 2.6 and 3 μmol kg, respectively, as compared with seawater certified reference material (CRM) provided by A. Dickson (Scripps Institution of Oceanography). Repeatability of replicate samples was better than 3 μmol kg.”

Abstract. Coastal time series of ocean carbonate chemistry are critical for understanding how global anthropogenic change manifests in near-shore ecosystems. Yet, they are few and have low temporal resolution. At the time series station Point B in the northwestern Mediterranean Sea, seawater was sampled weekly from 2007 through 2015, at 1 and 50 m, and analyzed for total dissolved inorganic carbon (CT) and total alkalinity (AT). Parameters of the carbonate system such as pH (pHT, total hydrogen ion scale) were calculated and a deconvolution analysis was performed to identify drivers of change. The rate of surface ocean acidification was −0.0028 ± 0.0003 units pHT yr−1. This rate is larger than previously identified open-ocean trends due to rapid warming that occurred over the study period (0.072 ± 0.022 °C yr−1). The total pHT change over the study period was of similar magnitude as the diel pHT variability at this site. The acidification trend can be attributed to atmospheric carbon dioxide (CO2) forcing (59 %, 2.08 ± 0.01 ppm CO2 yr−1) and warming (41 %). Similar trends were observed at 50 m but rates were generally slower. At 1 m depth, the increase in atmospheric CO2 accounted for approximately 40 % of the observed increase in CT (2.97 ± 0.20 µmol kg−1 yr−1). The remaining increase in CT may have been driven by the same unidentified process that caused an increase in AT (2.08 ± 0.19 µmol kg−1 yr−1). Based on the analysis of monthly trends, synchronous increases in CT and AT were fastest in the spring–summer transition. The driving process of the interannual increase in AT has a seasonal and shallow component, which may indicate riverine or groundwater influence. This study exemplifies the importance of understanding changes in coastal carbonate chemistry through the lens of biogeochemical cycling at the land–sea interface. This is the first coastal acidification time series providing multiyear data at high temporal resolution. The data confirm rapid warming in the Mediterranean Sea and demonstrate coastal acidification with a synchronous increase in total alkalinity.

Research assessing the biological impacts of global ocean change often requires a burdensome characterization of seawater carbonate chemistry. For laboratory-based ocean acidification research, this impedes the scope of experimental design. Honeywell Durafet (R) Ill pH electrodes provide precise and continuous seawater pH measurements. In addition to use in oceanographic sensor packages, Durafets can also be used in the laboratory to track and control seawater treatments via Honeywell Universal Dual Analyzers (UDAs). Here we provide performance data, instructions, and step-by-step recommendations for use of multiple UDA-Durafets. Durafet pH measurements were within +/-0.005 units pH(T) of spectrophotometric measurements and agreement among eight Durafets was better than +/-0.005 units pH(T). These results indicate equal performance to Durafets in oceanographic sensor packages, but methods for calibration and quality control differ. Use of UDA-Durafets vastly improves time-course documentation of experimental conditions and reduces person-hours dedicated to this activity. Due to the versatility of integrating Durafets in laboratory seawater systems, this technology opens the door to advance the scale of questions that the ocean acidification research community aims to address.

AbstractIn the coastal ocean, temporal fluctuations in pH vary dramatically across biogeographic ranges. How such spatial differences in pH variability regimes might shape ocean acidification resistance in marine species remains unknown. We assessed the pH sensitivity of the sea urchin Strongylocentrotus purpuratus in the context of ocean pH variability. Using unique male–female pairs, originating from three sites with similar mean pH but different variability and frequency of low pH (pHT ≤ 7.8) exposures, fertilization was tested across a range of pH (pHT 7.61–8.03) and sperm concentrations. High fertilization success was maintained at low pH via a slight right shift in the fertilization function across sperm concentration. This pH effect differed by site. Urchins from the site with the narrowest pH variability regime exhibited the greatest pH sensitivity. At this site, mechanistic fertilization dynamics models support a decrease in sperm–egg interaction rate with decreasing pH. The site differences in pH sensitivity build upon recent evidence of local pH adaptation in S. purpuratus and highlight the need to incorporate environmental variability in the study of global change biology.

Eastern boundary current systems (EBCSs) experience dynamic fluctuations in seawater pH due to coastal upwelling and primary production. The lack of high‐resolution pH observations in EBCSs limits the ability to relate field pH exposures to performance of coastal marine species under future ocean change (acidification, warming). This 3‐yr study describes spatio‐temporal pH variability across the northern Channel Islands, along a persistent temperature gradient (1–4 ° C) within the eastern boundary California Current System. pH and Conductivity, Temperature, Depth, and Oxygen sensors were deployed on island piers in eelgrass and kelp habitat and on a subtidal mooring. Due to event‐scale primary production, the temperature gradient across the islands did not manifest in a pH gradient. We resolved spatial pH variability on diel (ΔpH T 0.05–0.2: photosynthesis), event‐scale (ΔpH T <0.1–0.2: upwelling, phytoplankton blooms, wind relaxation), and seasonal (ΔpH T 0.06: warming) time frames. In the kelp forest, summer mean pH T (8.01–8.02) and magnitude of diel pH T cycles (ΔpH T 0.12–0.10) were comparable year‐to‐year, despite 2.1 ° C warming from 2012 to 2014. Compared to nearby mainland sites, the northern Channel Islands experienced few low pH events. The majority of pH T observations were >7.9. The lowest pH observations (>1 SD below mean pH T ) occurred under either warm (respiration during warm nights) or cold (advection of upwelled water) temperatures. We emphasize the importance of incorporating site‐specific environmental variability in studies of ocean change biology, particularly in the design of multistressor experiments.

To better understand the impact of ocean acidification on marine ecosystems, an important ongoing research priority for marine scientists is to characterize present-day pH variability. Following recent technological advances, autonomous pH sensor deployments in shallow coastal marine environments have revealed that pH dynamics in coastal oceans are more variable in space and time than the discrete, open-ocean measurements that are used for ocean acidification projections. Data from these types of deployments will benefit the research community by facilitating the improved design of ocean acidification studies as well as the identification or evaluation of natural and human-influenced pH variability. Importantly, the collection of ecologically relevant pH data and a cohesive, user-friendly integration of results across sites and regions requires (1) effective sensor operation to ensure high-quality pH data collection and (2) efficient data management for accessibility and broad reuse by the marine science community. Here, we review the best practices for deployment, calibration, and data processing and quality control, using our experience with Durafet®-based pH sensors as a model. Next, we describe information management practices for streamlining preservation and distribution of data and for cataloging different types of pH sensor data, developed in collaboration with two U.S. Long Term Ecological Research (LTER) sites. Finally, we assess sensor performance and data recovery from 73 SeaFET deployments in the Santa Barbara Channel using our quality control guidelines and data management tools, and offer recommendations for improved data yields. Our experience provides a template for other groups contemplating using SeaFET technology as well as general steps that may be helpful for the design of data management for other complex sensors.

Abstract. Monitoring of global ocean change is necessary in coastal zones due to their physical and biological complexity. Here, we document changes in coastal carbonate chemistry at the coastal time-series station, Point B, in the NW Mediterranean Sea from 2007 through 2015 at 1 and 50 m. The rate of surface ocean acidification (−0.0028 ± 0.0003 units pHT yr−1) was faster-than-expected based on atmospheric carbon dioxide forcing alone. Changes in carbonate chemistry were predominantly driven by an increase in total dissolved inorganic carbon (CT, +2.97 ± 0.20 μmol kg−1 yr−1), > 50 % of which was buffered by a synchronous increase in total alkalinity (AT, +2.08 ± 0.19 μmol kg−1 yr−1). The increase in AT was unrelated to salinity and its cause remains to be identified. Interestingly, concurrent increases in AT and CT were most rapid from May to July. Changes at 50 m were slower compared to 1 m. It seems therefore likely that changes in coastal AT cycling via a shallow coastal process gave rise to these observations. This study exemplifies the importance of understanding coastal ocean acidification through localized biogeochemical cycling that extends beyond simple air-sea gas exchange dynamics, in order to make relevant predictions about future coastal ocean change and ecosystem function.