Ocean alkalinity enhancement (OAE) is an emerging carbon dioxide removal (CDR) strategy that leverages the natural processes of weathering and acid neutralisation to durably store atmospheric CO2 in seawater. OAE can be achieved with a variety of methods, all of which have different environmental implications. One widely considered method utilizes electrochemistry to remove strong acid from seawater, leaving sodium hydroxide (NaOH) behind. This study evaluates the impacts of OAE via NaOH (NaOH-OAE) on a coastal plankton bloom, with particular focus on how macronutrient regeneration in the aftermath of the bloom responds to the perturbation. To investigate this, we enclosed a natural coastal phytoplankton community, including coccolithophores, in nine microcosms. The microcosms were divided into three groups: control, unequilibrated (512.1 ± 2.5 µmol kg-1 alkalinity increase) and equilibrated (499.3 ±5.65 µmol kg-1 alkalinity increase). Light was provided for 11 days to stimulate a bloom (light phase) and lights were turned off thereafter to investigate alkalinity and nutrient changes for 21 days (dark phase). We found no detectable effect of equilibrated NaOH-OAE on phytoplankton community and bacteria abundances determined with flow cytometry but observed a small yet detectable restructuring of phytoplankton communities under unequilibrated conditions. NaOH-OAE had no significant effect on alkalinity, NOx- and phosphate regeneration, but increased silicate regeneration by 64% over 21 days under darkness in the unequilibrated treatments where seawater pH was highest (8.65 relative to 7.92 in the control). Additional dissolution experiments with two diatom species supported this outcome on silicate regeneration for one of the two species, thereby suggesting that the effect is species specific. Our results point towards the potential of NaOH-OAE to influence regeneration of silicate in the surface ocean and thus the growth of diatoms, at least under the very extreme NaOH-OAE conditions simulated here.

To investigate how the unavoidable physical and chemical perturbations associated with Ocean Alkalinity Enhancement (OAE) could influence marine plankton communities and how potential side-effects compare to impacts of climate change, we conducted 19 ship-based experiments in the Equatorial Pacific, examining three prevalent OAE source (NaOH, olivine, and steel slag) and their impacts on natural phytoplankton populations. Our experiments simulated realistic and moderate alkalinity enhancements between 29-16 μmol kg-1. The monitored parameters included total chlorophyll-a concentrations, macro nutrients, trace elements, total alkalinity, Fv/Fm, pH,and flowcytometry.

The effect of ocean alkalinity enhancement on a coastal phytoplankton community was assessed via a microcosm experiment. The effect of alkalinity enhancement in two scenarios (i) when enclosed seawater was in equilibrium with atmospheric CO2 and (ii) when enclosed seawater was not in equilibrium with atmospheric CO2 were explored. Alkalinity was increased by ~497 umol/kg in these two treatments and plankton communities, carbonate chemistry, dissolved inorganic nutrients, particulate matter and chlorophyll a dynamics monitored over a 22 day period where a spring bloom occurred.

Ocean alkalinity enhancement is a proposed method of marine carbon dioxide removal that enhances the ocean’s uptake of atmospheric carbon dioxide (CO2) and converts it to dissolved bicarbonate for long-term ocean storage. This method of marine carbon dioxide removal has been gaining attention for its potential to durably (10,000+ years) store large amounts of CO2 (Gt+ where 1 Gt = 1 x 10^9 tons), while potentially ameliorating acidification in the vicinity of the alkalinity release. This study focuses on a novel release of electrochemically derived aqueous alkalinity into Sequim Bay, WA, through a previously established wastewater treatment plant (WWTP). This research was made possible through the collaboration of industry, academic, and federal partners, which enabled the establishment of an Ebb Carbon electrochemical mCDR system at the Pacific Northwest National Laboratory in Sequim, WA, for ocean alkalinity enhancement field trials. During these field trials, pH was measured across the WWTP system from the initial alkalinity dosing, throughout the WWTP, and at the outfall. We use the NBS scale for pH throughout this study as it is the scale used in discharge permit limits specified for WWTP and NPDES regulation and compliance monitoring. The background pHNBS of Sequim Bay seawater was between 7.5 to 7.7 for the November and February field tests. The mixing tank's pHNBS was raised to the maximum value permitted for the WWTP (9.0) and maintained across the system (±0.2) during the outfall releases. At the outfall, the elevated pH and alkalinity was quickly diluted, such that the region with a measurable signal was limited to within ~2.5 m of the discharge pipe. We were able to successfully monitor an increase in pHNBS across all four pulses of alkalinity-enhanced seawater discharge during the February 2025 field trial, with peak pHNBS values of 8.3 or 8.1, as recorded by outfall-adjacent YSI Exo 2 sonde and SAMI-pH sensors, respectively. The alkalinity-enhanced seawater did not measurably alter the surrounding waters' temperature, salinity, turbidity, or oxygen. This study provides proof-of-concept for a conservative small-scale release of electrochemically generated alkalinity-enhanced seawater from a coastal outfall.

Marine carbon dioxide removal (mCDR) approaches are under development to mitigate the effects of climate change by sequestering carbon in stable reservoirs, with potential co-benefits of local reduction of coastal acidification impacts. One such method is ocean alkalinity enhancement (OAE). A specific OAE method is the generation of aqueous alkalinity via electrochemistry to enhance the alkalinity of the receiving water by the extraction of acid from seawater, thereby avoiding issues of solid dissolution kinetics and the release of impurities into the ocean from alkaline minerals. While electrochemical acid extraction is a promising method for increasing the carbon dioxide sequestration potential of the ocean, the biological effects of increasing seawater alkalinity and pH within an OAE project site are relatively unknown. This study aims to address this knowledge gap by testing the effects of increased pH and alkalinity, delivered in the form of aqueous NaOH, on two eelgrass epifauna in the U.S. Pacific Northwest, Taylor’s sea hare (Phyllaplysia taylori) and eelgrass isopod (Idotea resecata), chosen for their ecological importance as salmon prey and for their roles in eelgrass ecosystems. Four-day experiments were conducted in closed bottles to allow measurements of the evolution of carbonate species throughout the experiment with water refreshed twice daily to maintain elevated pH, across pHNBS treatments ranging from 7.8 to 9.3. Sea hares experienced mortality in all pH treatments, ranging from 37% mortality at pHNBS 7.8 to 100% mortality at pHNBS 9.3. Isopods experienced lower mortality rates in all treatment groups, ranging from 13% at pHNBS 7.8 to 21% at pHNBS 9.3, which did not significantly increase with higher pH treatments. These experiments represent an extreme of constant exposure to elevated pH and alkalinity, which should be considered in the context of both the natural variation and the dilution of alkalinity experienced by marine communities across an OAE project site. Different invertebrate species will likely have different responses to increased pH and alkalinity, depending on their physiological vulnerabilities. Investigation of the potential vulnerabilities of local marine species will help inform the decision-making process regarding mCDR planning and permitting.

This dataset contains full-depth carbonate chemistry profile data at the seasonal frequency of ship-based hydrographic surveys for the Florida Straits at 27°N from 2002 to 2018. We firstly developed an algorithm to estimate subsurface (~50 m) dissolved inorganic carbon (DIC) using measured data from carbonate chemistry-focused research cruise surveys across the Florida Straits at 27°N. This algorithm was then applied to the long-term non-carbonate chemistry time-series dataset to generate a DIC record. Total alkalinity of the full water column was estimated from a linear relationship with salinity based on measured data from carbonate chemistry focused research cruise surveys across the Florida Straits at 27°N. Furthermore, subsurface pH and aragonite saturation state (Ωarag) were calculated from DIC and TA using CO2SYS (Van Heuven et al., 2009) with the first and second carbonic acid dissociation constants K1 and K2 from Lueker et al. (2000), the acidity constant of the ion HSO4- from Dickson (1990), and borate-to-salinity ratio from Lee et al. (2010). For surface mixed layer (<50 m), DIC, pH, and (Ωarag) were calculated from TA and fCO2, where the latter was calculated using algorithms provided by Wanninkhof et al. (2020) based on the observational data from the Atlantic Oceanographic and Meteorological Laboratory (AOML) Ship of Opportunity-CO2 (SOOP-CO2) program from Royal Caribbean international cruise ships.

To understand the environmental impacts of added alkaline minerals on plankton communities, we enclosed natural coastal plankton communities using 53L microcosms and exposed these communities to ground factory slag (2g/53L) and olivine (100g/53L). The microcosms of seawater were kept at 13.5 °C with circulations. The biochemical changes and responses in microcosms were monitored and measured for 21 days. The measured parameters are pH, total alkalinity, temperature, macro-nutrients concentrations, total chlorophyll-a, flow cytometry data, POC/PON, BSi, Rapid Light Curves, zooplankton abundance, the dissolved trace metal concentrations, and the particulate trace metal concentrations.

Time-series of seawater carbonate chemistry variables, including salinity, dissolved inorganic nutrients, pH, total alkalinity, and dissolved inorganic carbon from sites along Kahekili Beach Park, west Maui near submarine groundwater seeps and living coral reefs. Samples for seawater were collected by pumping bottom water from the seafloor using a peristaltic pump and collecting discrete water samples every 4-hrs over a 6-day period.

The NOAA Atlantic Oceanographic and Meteorological Laboratory (AOML) through a collaboration with the National Park Service collected ocean acidification discreet samples at select National Parks sites along the East Coast of the United States and in the Gulf of Mexico to increase near-shore data collection on a bi-annual frequency. Whenever possible, sample collection is coordinated with the OAP-funded GOMECC and ECOA cruises.