Ocean Alkalinity Enhancement (OAE) is a marine carbon dioxide removal (mCDR) strategy that involves adding alkaline substances to surface waters to enhance CO2 uptake and storage. The dispersal of alkaline materials such as sodium hydroxide (NaOH) into seawater can cause rapid increases in pH and total alkalinity (TA) that substantially exceeds natural variability in marine environments. Such fluctuations may negatively affect marine life, especially small animals like copepods who cannot avoid OAE plumes and whose physiological processes could be disrupted by large and rapid shifts in seawater pH. To address knowledge gaps regarding potential biological impacts of OAE, we studied these effects in Calanus finmarchicus, an ecologically critical copepod species in the Northwest Atlantic Ocean. We exposed C. finmarchicus from the late juvenile copepodite stages and adult females to NaOH-dosed seawater at pH 10.5 (similar to 5000 & micro;mol kg-1 TA) and pH 9.0 (similar to 3150 & micro;mol kg-1 TA) for durations that reflect expected short-term exposure times during field OAE deployments (pH 10.5: 1, 5, 10 minutes; pH 9.0: 1, 15, 30 minutes). None of the treatment combinations resulted in mortality immediately after the initial exposure. Individuals were monitored for survival for 72 hours post-exposure (hpe), and only one treatment group (juveniles exposed to pH 10.5 for 10 minutes) showed a significant reduction in final survival; no other pH-duration combination showed increased mortality. Effects on the ability to initiate an escape response were more substantial. Adult females treated with pH 10.5 for 5 or 10 minutes showed a significant reduction in escape response immediately after exposure. In contrast, juveniles showed no immediate change in escape response following exposure to pH 10.5 or pH 9.0, although juveniles exposed to pH 10.5 for 10 minutes exhibited reduced escape response at 24 hpe. Using microrespirometry, we measured oxygen consumption following a 10-minute exposure to pH 10.5 and detected no effect on routine metabolic rate immediately post-exposure or at 12 hpe. Overall, our results suggest that C. finmarchicus is relatively tolerant to short-term exposures to very high pH and alkalinity. Future work should prioritize longer-term exposure under more moderate ocean OAE conditions.

Marine carbon dioxide removal (mCDR) is an emerging field where independent, responsibly conducted research is necessary to address uncertainties about effectiveness, safety, feasibility, and desirability. Codes of conduct and calls for community engagement related to mCDR research are growing as public and private entities move forward with field trials. However, these guidelines may not be readily applicable to all projects amidst a rapidly shifting landscape of societal perceptions, media coverage, funding constraints, and local and regional political climates. The Woods Hole Oceanographic Institution's Locking Ocean Carbon in the Northeast Shelf and Slope project, a noncommercial mCDR research program based in the United States, conducted 55 community engagement efforts with local community, industry, and tribal members in 2023-2025, centered around a proposed field trial in the Northeast region. In addition to providing valuable input on project design and implementation, our iterative engagement process honed a framing for the project, explanations of scientific concepts relevant to the study, and answers to common questions raised by community members. We found that open and straightforward dialogue with the strongest opponents to our project led to a set of core messages that spoke to diverse audiences with only slight modifications for different communities. In this narrative account, we reflect on lessons learned through this iterative process, and discuss unresolved questions for community engagement during ongoing and future mCDR field research.

Ocean alkalinity enhancement is being explored to remove carbon dioxide from the atmosphere, but its interactions with marine ecosystems remain uncertain. Robust data-based baseline frameworks are needed to assess where and when ecosystems may be increasingly exposed to ocean alkalinity enhancement. Here we present the Ecological Activity Index (EAI) that integrates publicly available datasets on chlorophyll a, zooplankton, and fish larvae abundance, endangered species distributions and commercial fishing activity across the United States Northeast Shelf and Slope. This framework provides a spatially and temporally resolved measure of ecosystem activity and human use. Ecological activity varies strongly across regions and seasons, with patterns that differ between parts of the food web and fisheries. Higher EAI values indicate greater overlap between ecological activity and human use, and thus potentially higher exposure. This framework establishes a quantitative approach to inform site selection, monitoring design, and environmental assessment of ocean alkalinity enhancement. Ecological activity varies across space and time and shapes potential exposure to ocean alkalinity enhancement, as revealed by an index integrating ecological and human use datasets for a quantitative baseline.

The impacts of El Niño‐Southern Oscillation (ENSO) on salinity and alkalinity in an equatorial coral reef lagoon (Kanton) are investigated using water samples collected in three non‐El Niño years (1973, 2012, and 2018) and one El Niño year (2015). A one‐dimensional, advective‐diffusive model is developed to aid in the interpretation of the sparse observations and make estimates of net ecosystem calcification (NEC) rates. The Kanton lagoon experiences extreme salinity and alkalinity variations driven by ENSO variations in precipitation. During the non‐El Niño years, salinity increases from the ocean (35.5 psu) to the back of the lagoon (38 psu) because evaporation exceeds precipitation, and water resides in the back of the lagoon for ∼180 days. Early in the 2015–2016 El Niño, the back of the lagoon is only ∼1 psu saltier than the ocean because precipitation had begun to exceed evaporation. The model suggests that during El Niño events, when precipitation substantially exceeds evaporation, the back of the lagoon is less salty than the ocean (30–32 psu). Alkalinity variations in the lagoon are primarily due to dilution or concentration driven by the ENSO variations in precipitation and NEC that causes an alkalinity deficit of ∼250 μmol/kg in the back of the lagoon. The estimated NEC rate in 2015 is ∼25% lower (4.1 mmol/day) than in the non‐El Niño years (5.3–5. 7 mmol/day). The NEC rates and coral cover measurements indicate that the Kanton lagoon has recovered from the complete loss of coral cover during the 2002–2003 El Niño.

Ocean alkalinity enhancement (OAE) is a marine carbon dioxide (CO 2 ) removal strategy that relies on lowering the ocean's p CO 2 via the addition of alkaline materials to facilitate enhanced CO 2 uptake with the potential for durable, long‐term, storage. This strategy has gained recent scientific and private sector attention as a possible component of climate mitigation portfolios, yet many research questions remain. This work describes an analysis of historical reconstructions of regional carbonate chemistry developed via application of machine learning algorithms to an ocean reanalysis product. Model skill assessment demonstrated excellent performance when compared to regional observations, and this work focuses on four carbonate system variables that may influence OAE applications: total scale pH, calcite saturation state, the theoretical molar change in dissolved inorganic carbon associated with a molar change in total alkalinity (ΔDIC/ΔTA), and the timescale of CO 2 equilibrium of the surface mixed layer (). These metrics were combined into a suitability index to quantify locations and times of year more favorable for OAE. Much of the US Northeast Shelf and Slope region has seasonally similar suitability for small‐scale OAE applications, with nearshore environments exhibiting high suitability year‐round. Lagrangian particle tracking experiments show strong reductions in ΔDIC/ΔTA and increases in due to horizontal and vertical transport, suggesting that when water motion is accounted for, reduced efficiency and longer equilibration times may impact successful observations of carbon uptake and storage. This analysis and framework were developed with publicly available tools, data sets, and global data products allowing for global scalability and application.

Ocean alkalinity enhancement (OAE) is a carbon dioxide (CO2) removal approach that involves the addition of alkaline substances to the marine environment to increase seawater buffering capacity and allow it to absorb more atmospheric CO2. Increasing seawater alkalinity leads to an increase in the saturation state (Omega) with respect to several minerals, which may trigger mineral precipitation, consuming the added alkalinity and thus decreasing the overall efficiency of OAE. To explore mineral formation due to alkalinity addition, we present results from shipboard experiments in which an aqueous solution of NaOH was added to unfiltered seawater collected from the surface ocean in the Sargasso Sea. Alkalinity addition ranged from 500 to 2000 mu mol kg-1, and the carbonate chemistry was monitored through time by measuring total alkalinity (TA) and dissolved inorganic carbon (DIC), which were used to calculate Omega. The amount of precipitate and its mineralogy were determined throughout the experiments. Mineral precipitation took place in all experiments over a timescale of hours to days. The dominant precipitate phase is aragonite with trace amounts of calcite and magnesium hydroxide (MgOH2, i.e., brucite). Aragonite crystallite size increases and its micro-strain decreases with time, consistent with Ostwald ripening. The precipitation rate (r) in our experiments and those of other OAE-related calcium carbonate precipitation studies correlate with the aragonite saturation state (Omega A), and the resulting fit of log10(r) = n x log10 (Omega A-1) + log10 (k) yields a reaction order n=2.15 +/- 0.50 and a rate constant k=0.20 +/- 0.10 mu mol h-1. The reaction order is comparable to that derived from previous studies, but the rate constant is 1 order of magnitude lower, which we attribute to the fact that our experiments are unseeded compared with previous studies that used aragonite seeds which act as nuclei for precipitation. Observable precipitation was delayed by an induction period, the length of which is inversely correlated with the initial Omega. Mineral precipitation occurred in a runaway manner, decreasing TA to values below those of seawater prior to alkalinity addition.This study demonstrates that the highest risk of mineral precipitation is immediately following alkalinity addition and before dilution and CO2 uptake by seawater, both of which lower Omega. Aragonite precipitation will decrease OAE efficiency because aragonite is typically supersaturated in surface ocean waters. Thus, once formed, aragonite essentially permanently removes the precipitated alkalinity from the CO2 uptake process. Runaway mineral precipitation also means that mineral precipitation following OAE may not only decrease OAE efficiency at sequestering CO2 but could also render this approach counterproductive. As such, mineral precipitation should be avoided by keeping Omega below the threshold of precipitation and quantifying its consequences for OAE efficiency if it occurs. Lastly, in order to be able to quantitatively determine the impact of mineral precipitation during OAE, a mechanistic understanding of precipitation in the context of OAE must be developed.

Ocean alkalinity enhancement (OAE) is a marine carbon dioxide removal (mCDR) approach that relies on the addition of liquid or solid alkalinity into seawater to take up and neutralize carbon dioxide (CO2) from the atmosphere. Documenting the effectiveness of OAE for carbon removal requires research and development of measurement, reporting, and verification (MRV) frameworks. Specifically, direct observations of carbon uptake via OAE will be critical to constrain the total carbon dioxide removal (CDR) and to validate the model-based MRV approaches currently in use. In September 2023, we conducted a ship-based rhodamine water tracer (RT) release in United States federal waters south of Martha's Vineyard, MA, followed by a 36 h tracking and monitoring campaign. We collected RT fluorescence data and a suite of physical and chemical parameters at the sea surface and through the upper water column using the ship's underway system, a conductivity-temperature-depth (CTD) rosette, and Lagrangian drifters. We developed an OAE analytical framework that explicitly references the OAE intervention and the resulting CDR to the baseline ocean state using these in situ observations. We evaluated the effectiveness of defining a "dynamic" baseline, in which the carbonate chemistry was continuously constrained spatially and temporally using the shipboard data outside of the tracer patch. This approach reduced the influence of baseline variability by 25 % for CO2 fugacity (fCO(2)) and 60 % for TA. We then constructed a hypothetical alkalinity release experiment using RT as a proxy for OAE. With appropriate sampling, and with suitable ocean conditions, OAE signals were predicted to be detectable in total alkalinity (TA > 10 mu mol kg(-1)), pH (> 0.01), and CO2 fugacity (fCO(2) > 10 mu atm). Over 36 h, an ensuing additional CO2 uptake was driven by this persistent gradient in surface fCO(2). The calculated CDR signal was detectable as a 4 mu atm surface fCO(2) increase, a pH decrease of 0.004 units, and a dissolved inorganic carbon (DIC) increase of 1.8 mu mol kg(-1), translating to 10 % of the total potential CDR. This signal, and the CDR itself, would continue to grow as long as an fCO(2) gradient persisted at the sea surface. Climatological results from a regional physical circulation model supported these findings and indicated that models and in-water measurements can be used in concert to develop a comprehensive MRV framework for OAE-based mCDR.

Ocean alkalinity enhancement is a marine-based carbon dioxide removal strategy that involves adding alkaline material to the surface ocean to boost carbon uptake and storage. The physical circulation of ocean water exerts fundamental control on the dilution, spreading, and retention of alkaline materials, influencing carbon removal effectiveness, environmental impacts, and monitoring feasibility. Here we evaluate potential sites and timing for ocean alkalinity enhancement on the U.S. Northeast Shelf by conducting passive tracer simulations from 2009 to 2017. Monthly dye release experiments across ten locations were analyzed by quantifying dye evolution metrics such as surface spread, lateral movement, upper-ocean concentration, and gas transfer velocity. A site selection index was developed to assess site and time suitability for tracer dispersal for ocean alkalinity enhancement. Results showed strong seasonality, with optimal conditions in summer and less favorable conditions in winter. Among the tested locations, Wilkinson Basin emerged as the most favorable tracer release site due to its larger spreading area, higher tracer concentrations, and longer decay time. These findings inform a future field experiment in the region and offer a scalable framework for guiding future research on ocean alkalinity enhancement in other regions based on physical characteristics of tracer evolution.

In this study, we used stable isotopes of oxygen (delta O-18), deuterium (delta D), and dissolved inorganic carbon (delta C-13(DIC)) in combination with temperature, salinity, oxygen, and nutrient concentrations to characterize the coastal (71 degrees-78 degrees W) and an oceanic (82 degrees-98 degrees W) water masses (SAAW-Subantarctic Surface Water; STW-Subtropical Water; ESSW-Equatorial Subsurface water; AAIW-Antarctic Intermediate Water; PDW-Pacific Deep Water) of the Southeast Pacific (SEP). The results show that delta O-18 and delta D can be used to differentiate between SAAW-STW, SAAW-ESSW, and ESSW-AAIW. delta C-13(DIC) signatures can be used to differentiate between STW-ESSW (oceanic section), SAAW-ESSW, ESSW-AAIW, and AAIW-PDW. Compared with the oceanic section, our new coastal section highlights differences in both the chemistry and geometry of water masses above 1,000 m. Previous paleoceanographic studies using marine sediments from the SEP continental margin used the present-day hydrological oceanic transect to compare against, as the coastal section was not sufficiently characterized. We suggest that our new results of the coastal section should be used for past characterizations of the SEP water masses that are usually based on continental margin sediment samples.

Ocean chemistry is changing as a result of human activities. Atmospheric carbon dioxide (CO 2 ) concentrations are increasing, causing an increase in oceanic p CO 2 that drives a decrease in oceanic pH, a process called ocean acidification (OA). Higher CO 2 concentrations are also linked to rising global temperatures that can result in more stratified surface waters, reducing the exchange between surface and deep waters; this stronger stratification, along with nutrient pollution, contributes to an expansion of oxygen-depleted zones (so called hypoxia or deoxygenation). Determining the response of marine organisms to environmental changes is important for assessments of future ecosystem functioning. While many studies have assessed the impact of individual or paired stressors, fewer studies have assessed the combined impact of p CO 2 , O 2 , and temperature. A long-term experiment (∼10 months) with different treatments of these three stressors was conducted to determine their sole or combined impact on the abundance and survival of a benthic foraminiferal community collected from a continental-shelf site. Foraminifera are well suited to such study because of their small size, relatively rapid growth, varied mineralogies and physiologies. Inoculation materials were collected from a ∼77-m deep site south of Woods Hole, MA. Very fine sediments (<53 μm) were used as inoculum, to allow the entire community to respond. Thirty-eight morphologically identified taxa grew during the experiment. Multivariate statistical analysis indicates that hypoxia was the major driving factor distinguishing the yields, while warming was secondary. Species responses were not consistent, with different species being most abundant in different treatments. Some taxa grew in all of the triple-stressor samples. Results from the experiment suggest that foraminiferal species’ responses will vary considerably, with some being negatively impacted by predicted environmental changes, while other taxa will tolerate, and perhaps even benefit, from deoxygenation, warming and OA.

The naturally-occurring radionuclides protactinium-231 (Pa-231) and thorium-230 (Th-230) are produced at approximately uniform rates in the ocean and thought to be removed from the water column through a reversible exchange with settling particles. Recent measurements along the U.S. GEOTRACES North Atlantic transect (GA03) revealed two features which are at odds with current understanding about Pa-231 and Th-230 cycling in the ocean: (i) a sharp decrease in dissolved Pa-231 (Pa-231(d)) and Th-230 (Th-230(d)) activities with depth below 2000-4000 m and (ii) very high particulate Pa-231 (Pa-231(p)) and Th-230 (Th-230(p)) activities near the bottom, at a number of stations between the New England continental shelf and Bermuda. Concomitant measurements of light attenuation from beam transmissometry showed that both features occur in benthic nepheloid layers (BNLs), which suggests that these features may stem, at least partly, from the presence of resuspended sediment in the deep water column. Here we explore the behaviour of Pa-231 and Th-230 in BNLs by using (i) radionuclide, optical, and hydrographic data from the western segment of GA03 (west of Bermuda) and (ii) a simplified model of particle and radionuclide cycling that includes a lateral particle source. First, the BNLs observed at GA03 stations are characterized from measurements of the beam attenuation coefficient converted to particle concentrations. At all stations, particle concentrations below the clear water minimum were the highest in the bottom mixed layer, whose thickness ranged from 95 to 320 m, and decreased generally with height above the bottom. The thickness of strong BNLs varied from 482 to 1358 m and the vertical integral of particle concentration in excess to that at the clear water minimum varied from 1x10(4) to 2x10(6) mg m(-2), among different stations. Second, the particle-radionuclide model is fitted to data from stations GT11-04 (New England continental rise) and GT11-08 (Hatteras abyssal plain), where samples for radionuclide analyses were collected in the BNL. The model can reproduce simultaneously the increase of particle concentration with depth, the low Pa-231(d) and Th-230(d) in the BNLs, and the high Pa-231(p) and Th-230(p) near the bottom. According to the model, at heights less than about 300 m above the seafloor, the dissolved phase was set primarily by a balance between adsorption and desorption, with vertical turbulent mixing playing a secondary role, whilst the particulate phase behaved largely as a non-reactive constituent supplied laterally and transported vertically by particle settling and turbulent mixing. Sensitivity tests with the model suggest that lateral particle sources near continental slopes and similar reliefs can produce significant biases both in the Th-230 normalization method and in the interpretation of sediment Pa-231/Th-230 records. Our findings yield insights into the influence of sediment resuspension and transport on Pa-231 and Th-230 in the deep ocean and highlight the need for considering these processes in paleoceanographic applications.

Benthic nepheloid layers (BNLs) are particle-rich layers that can extend over a thousand meter or more above the seafloor and are thought to be produced by the resuspension of fine sediments from strong bottom currents. They can often be subdivided into two sublayers: (i) a lower sublayer in contact with the seabed, where particle concentrations are the largest and which roughly coincides with the bottom mixed layer (BML); and (ii) an upper sublayer in which particle concentration decreases up to a clear water minimum (CWM). Although BNLs have long been recognised in vertical traces of optical instruments lowered to abyssal depths, their influence on ocean biogeochemical cycles – on the cycling of particle-reactive metals in particular – remains poorly understood. In this study, we characterize the BNLs observed between the New England continental shelf and Bermuda and explore their influence on the cycling of 230Th and 231Pa – two naturally-occurring particle-reactive radionuclides that have found different applications in chemical oceanography and paleoceanography. To this end, we use concomitant measurements of temperature, salinity, particle concentration derived from light beam transmissometry, and 230Th and 231Pa activities in the dissolved and particulate fractions, which have been collected along the western segment of the U.S. GEOTRACES GA03 transect. We estimate that the thickness of strong BNLs (particle concentration > 20 µg l-1) varied from about 72 to 1358 m between different deep stations. At all stations, particle concentrations below the CWM were the highest in the BML, whose thickness ranged from 95 to 320 m, and decreased generally with height above the seafloor. A simplified model of particle-radionuclide cycling in the deep water column, which includes a particle source representing sediment resuspension at topographic reliefs and their subsequent lateral transport, is fitted to observed profiles of particle concentration and radionuclide activities at two selected stations. The model can reproduce simultaneously the increase of particle concentration with depth, the low dissolved activities in the BNLs, and the extremely large particulate activities near the bottom. Analysis of 230Th and 231Pa budgets reveals that the behaviour of both radionuclides in the BNL is fundamentally different from that envisioned in reversible exchange theory. Sensitivity tests with the model suggest that lateral particle sources near continental slopes and similar reliefs can produce significant biases in the paleoceanographic applications of both radionuclides, including the 230Th-normalization method and the interpretation of sediment 231Pa/230Th records.

Ocean and coastal acidification (OCA) present a unique set of sustainability challenges at the human-ecological interface. Extensive biogeochemical monitoring that can assess local acidification conditions, distinguish multiple drivers of changing carbonate chemistry, and ultimately inform local and regional response strategies is necessary for successful adaptation to OCA. However, the sampling frequency and cost-prohibitive scientific equipment needed to monitor OCA are barriers to implementing the widespread monitoring of dynamic coastal conditions. Here, we demonstrate through a case study that existing community-based water monitoring initiatives can help address these challenges and contribute to OCA science. We document how iterative, sequential outreach, workshop-based training, and coordinated monitoring activities through the Northeast Coastal Acidification Network (a) assessed the capacity of northeastern United States community science programs and (b) engaged community science programs productively with OCA monitoring efforts. Our results (along with the companion manuscript) indicate that community science programs are capable of collecting robust scientific information pertinent to OCA and are positioned to monitor in locations that would critically expand the coverage of current OCA research. Furthermore, engaging community stakeholders in OCA science and outreach enabled a platform for dialogue about OCA among other interrelated environmental concerns and fostered a series of co-benefits relating to public participation in resource and risk management. Activities in support of community science monitoring have an impact not only by increasing local understanding of OCA but also by promoting public education and community participation in potential adaptation measures.

Comprehensive sampling of the carbonate system in estuaries and coastal waters can be difficult and expensive because of the complex and heterogeneous nature of near-shore environments. We show that sample collection by community science programs is a viable strategy for expanding estuarine carbonate system monitoring and prioritizing regions for more targeted assessment. ‘Shell Day’ was a single-day regional water monitoring event coordinating coastal carbonate chemistry observations by 59 community science programs and seven research institutions in the northeastern United States, in which 410 total alkalinity (TA) samples from 86 stations were collected. Field replicates collected at both low and high tides had a mean standard deviation between replicates of 3.6 ± 0.3 µ mol kg −1 ( σ mean ± SE, n = 145) or 0.20 ± 0.02%. This level of precision demonstrates that with adequate protocols for sample collection, handling, storage, and analysis, community science programs are able to collect TA samples leading to high-quality analyses and data. Despite correlations between salinity, temperature, and TA observed at multiple spatial scales, empirical predictions of TA had relatively high root mean square error >48 µ mol kg −1 . Additionally, ten stations displayed tidal variability in TA that was not likely driven by low TA freshwater inputs. As such, TA cannot be predicted accurately from salinity using a single relationship across the northeastern US region, though predictions may be viable at more localized scales where consistent freshwater and seawater endmembers can be defined. There was a high degree of geographic heterogeneity in both mean and tidal variability in TA, and this single-day snapshot sampling identified three patterns driving variation in TA, with certain locations exhibiting increased risk of acidification. The success of Shell Day implies that similar community science based events could be conducted in other regions to not only expand understanding of the coastal carbonate system, but also provide a way to inventory monitoring assets, build partnerships with stakeholders, and expand education and outreach to a broader constituency.

We use observations of dissolved inorganic carbon (DIC) and total alkalinity (TA) to assess the impact of ecosystem metabolic processes on coastal waters of the eastern Red Sea. A simple, single-end-member mixing model is used to account for the influence of mixing with offshore waters and evaporation–precipitation and to model ecosystem-driven perturbations on the carbonate system chemistry of coral reefs, seagrass meadows and mangrove forests. We find that (1) along-shelf changes in TA and DIC exhibit strong linear relationships that are consistent with basin-scale net calcium carbonate precipitation; (2) ecosystem-driven changes in TA and DIC are larger than offshore variations in >70 % of sampled seagrass meadows and mangrove forests, changes which are influenced by a combination of longer water residence times and community metabolic rates; and (3) the sampled mangrove forests show strong and consistent contributions from both organic respiration and other sedimentary processes (carbonate dissolution and secondary redox processes), while seagrass meadows display more variability in the relative contributions of photosynthesis and other sedimentary processes (carbonate precipitation and oxidative processes). The results of this study highlight the importance of resolving the influences of water residence times, mixing and upstream habitats on mediating the carbonate system and coastal air–sea carbon dioxide fluxes over coastal habitats in the Red Sea.

The U.S. Northeast is vulnerable to ocean and coastal acidification because of low alkalinity freshwater discharge that naturally acidifies the region, and high anthropogenic nutrient loads that lead to eutrophication in many estuaries. This study describes a combined nutrient and carbonate chemistry monitoring program in five embayments of Buzzards Bay, Massachusetts to quantify the effects of nutrient loading and freshwater discharge on aragonite saturation state (omega). Monitoring occurred monthly from June 2015 to September 2017 with higher frequency at two embayments (Quissett and West Falmouth Harbors) and across nitrogen loading and freshwater discharge gradients. The more eutrophic stations experienced seasonal aragonite undersaturation, and at one site, nearly every measurement collected was undersaturated. We present an analytical framework to decompose variability in aragonite omega into components driven by temperature, salinity, freshwater endmember mixing, and biogeochemical processes. We observed strong correlations between apparent oxygen utilization and the portion of aragonite omega variation that we attribute to biogeochemistry. The regression slopes were consistent with Redfield ratios of dissolved inorganic carbon and total alkalinity to dissolved oxygen. Total nitrogen and the contribution of biogeochemical processes to aragonite omega were highly correlated, and this relationship was used to estimate the likely effects of nitrogen loading improvements on aragonite omega. Under nitrogen loading reduction scenarios, aragonite omega in the most eutrophic estuaries could be raised by nearly 0.6 units, potentially increasing several stations above the critical threshold of 1. This analysis provides a quantitative framework for incorporating ocean and coastal acidification impacts into regulatory and management discussions.

Ocean acidification poses a major threat to marine organisms, but little is known about how pH affects larval behavior. The transition from a pelagic larval stage to adult life in the benthos, known as settlement, is a critical period in invertebrate life-histories and is influenced by larval responses to multiple chemical and physical settlement cues. We investigated the effects of low pH on the swimming behavior during settlement of the important reef-forming eastern oyster Crassostrea virginica, by tracking larval behaviors in ambient (8.1) and low (7.4) pH conditions, in the presence and absence of a chemical settlement cue. We hypothesized that low pH would impede the ability of C. virginica larvae to interpret or respond to the cue. In a preliminary experiment, observations of swimming behavior in larvae of 3 different stages (pre-competent, competent-to-settle, and post-competent) in the presence of the settlement cue showed no significant effect of pH on downward velocity or proportion of larvae remaining on bottom. This result indicates the response to the cue was not impaired by low pH. There were, however, trends in the proportion of larvae remaining on the bottom or swimming in helices, both in the presence and absence of the cue, that suggested larvae in one or more stages may have become more active or exploratory in low pH. Results from a follow-up experiment confirmed the lack of a significant effect of pH on larval response to the cue, but did not demonstrate a consistent or significant effect of pH on larval activity or exploration.

The oxygen concentration in marine ecosystems is influenced by production and consumption in the water column and fluxes across both the atmosphere-water and benthic-water boundaries. Each of these fluxes has the potential to be significant in shallow ecosystems due to high fluxes and low water volumes. This study evaluated the contributions of these three fluxes to the oxygen budget in two contrasting ecosystems, a Zostera marina (eelgrass) meadow in Virginia, U.S.A., and a coral reef in Bermuda. Benthic oxygen fluxes were evaluated by eddy covariance. Water column oxygen production and consumption were measured using an automated water incubation system. Atmosphere-water oxygen fluxes were estimated by parameterizations based on wind speed or turbulent kinetic energy dissipation rates. We observed significant contributions of both benthic fluxes and water column processes to the oxygen mass balance, despite the often-assumed dominance of the benthic communities. Water column rates accounted for 45% and 58% of the total oxygen rate, and benthic fluxes accounted for 23% and 39% of the total oxygen rate in the shallow (~ 1.5 m) eelgrass meadow and deeper (~ 7.5 m) reef site, respectively. Atmosphere-water fluxes were a minor component at the deeper reef site (3%) but a major component at the shallow eelgrass meadow (32%), driven by diel changes in the sign and strength of atmosphere-water gradient. When summed, the measured benthic, atmosphere-water, and water column rates predicted, with 85-90% confidence, the observed time rate of change of oxygen in the water column and provided an accurate, high temporal resolution closure of the oxygen mass balance.