Plankton do not care: Minimal effects of ocean liming on plankton growth and grazing in the Eastern Mediterranean

Increasing CO2 emissions have led to the development of CO2 removal strategies to counteract ocean acidification. Among these, ocean alkalinity enhancement techniques, particularly ocean liming, may represent a promising approach to restore seawater pH and boost CO2 sequestration. Yet, the impact of liming on plankton communities remains underexplored. In the framework of a mesocosm experiment, we conducted three dilution incubations to assess natural plankton response (abundance, composition, growth, grazing, diet, and food selectivity) to liming, achieved with calcium hydroxide (Ca(OH)2) additions. Experiments included two liming treatments (low concentration, and high concentration) and a control treatment without Ca(OH)2. The community was dominated by small-sized plankton (bacteria, Synechococcus and pigmented picoflagellates), outnumbering larger diatoms, dinoflagellates, and ciliates. While chlorophyll a, heterotrophic bacteria, and pigmented picoflagellates remained stable across treatments, the abundance of Synechococcus and dinoflagellates increased, whereas diatoms and ciliates declined particularly under high Ca(OH)2. Growth and grazing rates were largely unaffected by alkalinity, except for increased growth in pigmented picoflagellates upon liming. Microzooplankton showed low ingestion of Synechococcus and pigmented picoflagellates and higher intake of diatoms, dinoflagellates, and ciliates. Food selectivity was unresponsive to liming, as the grazers selected prey based on size, regardless of Ca(OH)2 concentrations. Increased alkalinity and pH, and the parallel effect of trophic cascades might have driven nutrient fluctuations and shaped downstream trophic interactions. Despite positive responses highlighted in this study, further research is needed to explore liming potentiality on a wider range of food-web components and larger scales in the framework of ocean alkalinity enhancement research.