Ocean alkalinity enhancement in an open ocean ecosystem: Biogeochemical responses and carbon storage durability

Abstract. Ocean alkalinity enhancement (OAE) is considered for the long-term removal of gigatons of carbon dioxide (CO₂) from the atmosphere to achieve our climate goals. Little is known, however, about the ecosystem-level changes in biogeochemical functioning that may result from the chemical sequestration of CO₂ in seawater, and how stable the sequestration is. We studied these two aspects in natural plankton communities under carbonate-based, CO₂-equilibrated OAE in the nutrient-poor North Atlantic. During a month-long mesocosm experiment, the majority of biogeochemical pools, including inorganic nutrients, particulate organic carbon and phosphorus as well as biogenic silica, remained unaltered across all OAE levels of up to a doubling of ambient alkalinity (+2400 µeq kg^-1). Noticeable exceptions were a minor decrease in particulate organic nitrogen and an increase in the carbon to nitrogen ratio (C:N) of particulate organic matter in response to OAE. Thus, in our nitrogen limited system, nitrogen turnover processes appear more susceptible than those of other elements leading to decreased food quality and increased organic carbon storage. However, alkalinity and chemical CO₂ sequestration were not stable at all levels of OAE. Two weeks after alkalinity addition, we measured a loss of added alkalinity and of the initially stored CO₂ in the mesocosm where alkalinity was highest (+2400 µeq kg^-1, Ω_aragonite ~10). The loss rate accelerated over time. Additional tests showed that such secondary precipitation can be initiated by particles acting as precipitation nuclei and that this process can occur even at lower levels of OAE. In conclusion, on the one hand, our study under carbonate-based OAE where the carbon is already sequestered, the risk of major and sustained impacts on biogeochemical functioning may be low in the nutrient-poor ocean. On the other hand, the durability of carbon sequestration using OAE could be constrained by alkalinity loss in supersaturated waters with precipitation nuclei present. Our study provides evaluation of ecosystem impacts of an idealised OAE deployment for monitoring, reporting and verification (MRV) in an oligotrophic system. Whether biogeochemical functioning is resilient to more technically simple and economically more viable approaches that induce stronger water chemistry perturbations remains to be seen.