The effect of ocean alkalinity enhancement on zooplankton standing stock and community composition in the Eastern Mediterranean Sea: a mesocosm study

Ocean acidification (OA) due to anthropogenic CO2 emissions has significantly altered ocean chemistry since the industrial era. Ocean alkalinity enhancement (OAE) is an innovative strategy to mitigate excess CO2, with ocean liming (OL) serving as a potential carbon dioxide removal (CDR) method, through the spreading of Ca(OH)2 (slaked lime) at the ocean surface. This study examined the ecological effects of OL on a natural zooplankton community from the ultraoligotrophic Eastern Mediterranean Sea during a 14-day mesocosm experiment conducted in spring-summer. We investigated how varying concentrations of slaked lime (low: 0.00074 g L−1, high: 0.0067 g L−1) affected zooplankton community structure, abundance, composition and species diversity The experiment revealed complex temporal dynamics in the zooplankton community across all treatments. Tintinnina and Copepoda dominated the community (80–100 %), with nauplii significantly outnumbering copepodites and adults by an order of magnitude. Redundancy Analysis and correlations showed that pH, temperature and nutrient availability were crucial factors shaping the zooplankton community structure. These environmental variables explained more of the community variation than the lime treatments themselves, highlighting the importance of considering multiple ecological factors when assessing OAE impacts. The overall community response to OL treatments was subtle, suggesting potential resilience of the zooplankton community to short-term alkalinity enhancement. This research provides insights into the ecological implications of OL as a potential OA mitigation strategy. It emphasizes the need for longer-term studies to fully understand the cascading effects on marine food webs and ecosystem functioning. Future research should focus on the interplay between OL, nutrient dynamics, and trophic interactions to better predict the ecological consequences of large-scale OAE implementation.