Influence of Ocean Alkalinity Enhancement on the Stability of Phytoplankton and Microzooplankton Communities
The rapid increase in anthropogenic CO₂ emissions since the Industrial Revolution has led to climate change, impacting ecosystems, seasonal patterns, and economies. To meet the 1.5°C Paris Agreement target, CO₂ emissions must be reduced, with Negative Emission Technologies (NETs) including Ocean Alkalinity Enhancement (OAE) gaining attention. OAE involves adding alkaline materials (e.g., minerals, industrial by-products) to seawater, increasing CO₂ absorption and altering ocean chemistry, potentially affecting plankton communities fundamental to marine food webs. However, its ecological impacts remain poorly understood. This doctoral thesis examines how OAE affects phytoplankton and microzooplankton communities through two mesocosm studies and a laboratory experiment. The first study tested CO₂-equilibrated OAE in oligotrophic waters, showing no significant effects on plankton biomass, diversity, or composition, suggesting resilience and environmental safety in such conditions. The second study explored CO₂-non-equilibrated OAE with quicklime and olivine additions, revealing that, under oligotrophic conditions, phytoplankton remained unaffected. However, after nutrient enrichment, diatoms thrived, particularly in silicate-based OAE, while coccolithophores declined, highlighting potential ecosystem shifts in eutrophic waters. The third study assessed nickel (Ni) toxicity, a trace metal found in certain alkaline materials. While some phytoplankton species showed resilience, others, like diatoms, were highly sensitive, indicating that trace metals could influence community structure. Overall, OAE appears environmentally benign under tested CO₂-equilibrated conditions, though CO2-non-equilibrated conditions and trace metal exposure may introduce ecological risks. The findings highlight the need for further research to refine OAE applications and minimize unintended consequences.
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