Biological response of eelgrass epifauna, Taylor’s sea hare (Phyllaplysia taylori) and eelgrass isopod (Idotea resecata), to elevated ocean alkalinity

Abstract. Marine carbon dioxide removal (mCDR) approaches are under development to mitigate the effects of climate change with potential co-benefits of local reduction of ocean acidification impacts. One such method is ocean alkalinity enhancement (OAE). A specific OAE method that avoids issues of solid dissolution kinetics and the release of impurities into the ocean is the generation of aqueous alkalinity via electrochemistry to enhance the alkalinity of the surrounding water and extract acid from seawater. While electrochemical acid extraction is a promising method for increasing the carbon dioxide sequestration potential of the ocean, the biological effects of this method 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 base, on two ecologically important eelgrass epifauna in the U.S. Pacific Northwest, Taylor’s sea hare (Phyllaplysia taylori) and eelgrass isopod (Idotea resecata), across pH treatments ranging from 7.8 to 9.3. 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. Sea hares experienced mortality in all pH treatments, ranging from 40 % mortality at pH 7.8 to 100 % mortality at pH 9.3. Isopods experienced lower mortality rates in all treatment groups, which did not significantly increase with higher pH treatments. 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.