The response of phytoplankton to pH-Equilibrated Ocean Alkalinization: a mesocosm experiment in the Gulf of La Spezia

To combat global warming and achieve carbon neutrality, carbon storage solutions for Carbon Dioxide Removal (CDR) from the atmosphere are being explored for their potential to sequester atmospheric CO₂. This study evaluates the ecological impacts of pH-equilibrated alkalinization, using the Limenet® process, a method that combines CO₂, calcium hydroxide (Ca(OH)₂), and seawater to generate bicarbonate-enriched solutions. Similar to other marine CDR based on Ocean Alkalinity Enhancement (OAE), this approach addresses not only climate change but also ocean acidification. A mesocosm experiment conducted in La Spezia, Italy, assessed how natural plankton communities respond to varying levels of alkalinization treatments. Fifteen land-based mesocosms were filled with seawater treated with five different levels of alkalinization: untreated, Low, Medium, High and Oversaturated bicarbonate conditions. Over a 15-day period, the physicochemical properties, nutrient dynamics, and plankton community responses were systematically monitored. Results showed effective bicarbonate stabilization in the Low, Medium, and High treatments, while oversaturation and runaway carbonate precipitation were recorded, as expected, in the oversaturated condition. The monitored pH and alkalinity demonstrated that the treated seawater maintained near-natural conditions while enhancing bicarbonate availability. Nutrient analysis revealed a rapid depletion of silica across all treatments, limiting diatom growth and triggering shifts in plankton community composition. Diatoms initially dominated the community in all mesocosms but declined over time as silica became depleted, giving way to a relative increase in dinoflagellates. The diatom-to-dinoflagellate ratio served as a key indicator of community response. In control and oversaturated treatments, the ratio exhibited steep declines, reflecting significant ecological shifts and reduced stability in plankton dynamics. By contrast, Low, Medium, and High treatments showed more gradual changes in the ratio, suggesting that pH-equilibrated alkalinization mitigates sharp shifts in planktonic communities. When compared with diatoms and dinoflagellates, the calcareous plankton component is negligible in mass in the La Spezia harbour, however, a dedicated effort is ongoing to assess the effect of pH-equilibrated alkalinization on nannoplankton dynamics. These findings highlight the dual potential of pH equilibrated alkalinization as a CDR strategy: effectively enhancing marine carbon sequestration while buffering ecosystems against extreme shifts in plankton structure. The experimental results suggest that this approach promotes ecological resilience by supporting balanced carbon uptake processes and mitigating the risks of bicarbonates (HCO₃^-) precipitation in Calcium Carbonate (CaCO₃) and pH destabilization, which are critical challenges in OAE implementation. This study provides valuable insights into the feasibility of pH equilibrated alkalinization as a CDR technology, underscoring the importance of sustainable practices and renewable energy integration to maximize its environmental benefits. While the results are promising, further research is needed to evaluate long-term ecological impacts, trophic-level interactions, and the scalability of this approach in diverse marine environments.