Using Variable Chlorophyll-a Fluorescence to Assess the Impact of Ocean Alkalinity Enhancement on the Photochemical Efficiency of Phytoplankton

Abstract

Ocean Alkalinity Enhancement (OAE) is a potential carbon dioxide removal strategy to mitigate climate change by enhancing oceanic carbon uptake. This thesis contributes to the biological risk assessment of OAE by examining its effects on phytoplankton photophysiology through laboratory and field experiments. In vitro studies (Chapter 3) revealed species-specific responses to elevated pH (~8.7), with varying resilience among phytoplankton. Mesocosm experiments (Chapter 4) showed changes in beam attenuation, particle size distribution, and pH following magnesium hydroxide additions, but no clear biological responses. Similarly, field trials with brucite altered optical properties but did not impair photosynthetic function. However, scattering artifacts in bio-optical sensors affected chlorophyll-a fluorescence readings, emphasizing the need for careful methodological consideration. Instrument design influenced sensitivity to scattering, affecting data reliability. Overall, findings support existing evidence that OAE, when applied at appropriate scales, is unlikely to pose significant risks to marine phytoplankton but highlight challenges in measurement accuracy.