Ocean alkalinity enhancement (OAE) includes a branch of marine carbon dioxide removal (mCDR) methods that add alkalinity to the surface ocean, leveraging the ocean’s vast natural ability to capture and store atmospheric CO2. The impact of OAE on marine ecosystems will depend on the type and delivery of alkaline feedstock to the ocean, which typically results in elevated pH and total alkalinity and decreased pCO2 in the near-field of an OAE application. These signals will decrease in space and time away from the point of alkaline addition until are no longer measurable against the background of natural variability in the marine environment. It is important to evaluate potential impacts of OAE on marine ecosystems within the context of realistic OAE deployments. This study highlights the use of an effluent dilution model to describe the measurable extent of the release of electrochemically-generated aqueous alkalinity from Ebb Carbon’s research pilot in Port Angeles, WA. We describe a novel laboratory method to simulate the potential exposure of juvenile coho salmon to the pilot’s alkaline discharge, representing exposure to OAE field conditions as salmon swim through the pilot’s mixing zone. Salmon were exposed to an electrochemically generated alkalinity-enhanced seawater solution pulsed into a test chamber at a dilution factor predicted approximately 3 m from the alkaline outfall. The alkalinity-enhanced seawater was held for 30 s, 1 min, and 5 mins, then was slowly flushed with ambient seawater. The alkaline solution, initially at pHNBS 10.0, was released into seawater at pHNBS 7.6, resulting in peak pH of the mixed solution of 8.04-8.09, with an increase in total alkalinity of ~60 μmol/kg. The results of the study indicated no impact on juvenile coho salmon behavior, survival, or physical effects on gills, eyes, or external body tissues, relative to control tests. The experimental design, developed for performance by a commercial toxicology laboratory and supported by standard mixing analyses, allows for rapid repetition with species of interest near OAE deployments.

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