The response of phytoplankton to pH-equilibrated ocean alkalinization: A mesocosm experiment with harbour waters

Mitigating global warming requires urgent reductions in carbon dioxide (CO₂) emissions and the development of Carbon Dioxide Removal (CDR) strategies. pH-equilibrated Ocean Alkalinization (pH eq OA) has emerged as a promising method to enhance CO₂ sequestration while minimizing potential environmental risks. This study investigates the ecological impact of pH eq OA on phytoplankton communities through a mesocosm experiment in the Gulf of La Spezia (Italy) using local seawater treated with Limenet® technology. We assessed planktonic responses to different levels of bicarbonate-enriched seawater (Control, Low, Medium, High, Oversaturated) by monitoring changes in carbonate chemistry, nutrient availability, and phytoplankton community composition over 15 days. Results revealed that pH eq OA treatments (excluding the oversaturated condition) helped maintain greater ecological stability, reducing the rate of diatom/dinoflagellate community shifts and supporting higher silicon uptake, particularly by diatoms. In contrast, oversaturation led to carbonate precipitation and a significant loss of added alkalinity. The findings suggest that moderate pH eq OA may enhance phytoplankton resilience and promote diatom activity under altered carbonate chemistry. This study underscores the need for further research to evaluate the broader ecological implications of pH eq OA. • pH eq OA enhances CO₂ sequestration while minimizing ecological risks • Diatoms declined due to silicon depletion, favoring dinoflagellates • Bicarbonate-enriched seawater stabilized the diatom/dinoflagellate ratio • Diatom Si uptake increased with alkalinity but dropped for strong carbonate oversaturation