Effects of pH-equilibrated ocean alkalinization on macro-benthic calcifiers: A mesocosm study from the port of La Spezia (Italy).

Ocean alkalinity enhancement (OAE) is emerging as a promising carbon dioxide removal (CDR) strategy, yet its ecological implications for benthic communities remain poorly understood. This study presents the first experimental evaluation of pH-equilibrated ocean alkalinization (pHeqOA), a technique designed to produce bicarbonate-enriched seawater without increasing pH, and investigates whether increasing alkalinity levels affect the recruitment, growth and biomass of macro-benthic calcifiers (MBCs), as well as whether excessive alkalinization can trigger abiotic carbonate precipitation with negative ecological consequences. A mesocosm experiment was conducted in the port area of La Spezia (Liguria, Italy) using five alkalinization levels (Control, Low, Medium, High, and Oversaturated), and limestone settlement plates were deployed to quantify recruitment, growth, surface cover, and biomass of MBCs. Colonization was dominated by the polychaete Serpula sp. Abundance and growth-related metrics did not differ significantly among the Control, Low, Medium, and High treatments, with maximum values generally observed under Medium and High alkalinization level. In contrast, the Oversaturated treatment exhibited negligible recruitment and growth, driven by extensive abiotic calcium carbonate precipitation on settlement surfaces. These results indicate that alkalinity enhancement doesn't affect the performance of MBCs when maintained below a system-specific saturation threshold, whereas excessive alkalinization suppresses settlement and growth. Overall, pHeqOA appears ecologically compatible with local macro-benthic communities within an optimal alkalinization range, underscoring the importance of defining site-specific saturation thresholds that balance carbon removal efficiency with ecological integrity when implementing OAE technologies.