Biogeochemical responses to ocean alkalinity enhancement in the North China Sea

Abstract. Ocean Alkalinity Enhancement (OAE) has been proposed as a promising marine carbon dioxide removal strategy. Yet its ecological consequences remain poorly understood, particularly in dynamic coastal systems characterised by high environmental variability. Here, we investigated the effects of OAE on plankton community structure and biogeochemical processes in the eutrophic North China Sea (NCS) using a microcosm experiment with natural seawater collected offshore. Two OAE scenarios were evaluated: CO₂-equilibrated and CO₂-non-equilibrated treatments, both targeting an increase in total alkalinity (ΔTA) of approximately 500 µmol kg⁻¹. Neither OAE treatment significantly affected the abundance of phytoplankton and microzooplankton. Similarly, plankton communities exposed to OAE maintained their natural levels of chlorophyll a, taxonomic composition, biodiversity, and particulate organic carbon throughout the experiment. In contrast, substantial TA loss occurred in the CO₂-non-equilibrated OAE treatment, indicating reduced alkalinity retention under non-equilibrated conditions. This study was conducted within the framework of the Ocean Alkalinity Enhancement Pelagic Impact Intercomparison Project (OAEPIIP), which was established to standardise OAE microcosm experiments and systematically assess the environmental impacts of OAE across diverse marine ecosystems. Our findings indicate that NCS phytoplankton and microzooplankton exhibited high tolerance to both CO₂-equilibrated and non-equilibrated OAE scenarios under the experimental conditions tested, while highlighting the need to monitor and mitigate TA loss alkalinity loss during OAE implementation in warm coastal environments.