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_eqOA) has emerged as a promising method to enhance CO₂ sequestration while minimizing potential environmental risks. This study investigates the ecological impact of pH_eqOA 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_eqOA 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_eqOA 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_eqOA.