Ocean alkalinity enhancement (OAE) has been proposed as a carbon dioxide (CO₂) removal strategy via the addition of alkaline substances to seawater. However, comprehensive assessments of its ecological impacts remain incomplete, particularly regarding effects on natural phytoplankton communities, hindering large-scale OAE evaluation.

To address these gaps, we conducted a microcosm experiment with a natural plankton community from Qingdao, China, under two OAE scenarios targeting an alkalinity increase of total alkalinity (ΔTA ≈ 500 µmol kg^-1) using NaOH and NaHCO₃. The experimental design was deliberately chosen to balance ecological realism and perturbation relevance over weekly timescales while ensuring the detectability of biological responses. This study is part of the Ocean Alkalinity Enhancement Pelagic Impact Intercomparison Project (OAEPIIP), which aims to standardise microcosm experiments and systematically assess the environmental impacts of OAE across diverse marine ecosystems globally.

Here, we report the effects of OAE on plankton community structure and biogeochemistry in the eutrophic North China Sea (NCS). Our microcosm compared CO₂-equilibrated and non-equilibrated OAE treatments, revealing that neither approach significantly affected the abundance of phytoplankton or microzooplankton. Moreover, OAE-exposed communities maintained natural levels of chlorophyll a, taxonomic composition, biodiversity, and particulate organic carbon throughout the experiment. These results indicate a high tolerance of the NCS plankton community to both OAE scenarios.

Nevertheless, we observed a notable loss of TA in the non-equilibrated treatment, which was likely driven by elevated temperatures during the experiment. This unintended TA drawdown underscores the importance of optimising OAE deployment methodologies to minimise alkalinity loss before it contributes to CO₂ uptake.

Taken together, our findings represent a critical step toward understanding the ecological responses of phytoplankton communities to OAE and help define the safe operating space for future implementations. While the results support the biological feasibility of OAE in coastal eutrophic waters, they also highlight the urgent need for careful monitoring and mitigation of TA loss during real-world applications.