Assessing ocean alkalinity for carbon storage

Proposals to remove CO2 from the atmosphere are becoming increasingly important in climate change policy. However, considerable uncertainty remains regarding the cost, scalability, and socioenvironmental consequences. One proposal for removing CO2 from the atmosphere involves increasing total alkalinity (TA) in the ocean. This is known as ocean alkalinity enhancement (OAE) and involves the oceanic uptake of CO2 from the atmosphere and its’ conversion to carbonate (CO3 2- ) or bicarbonate (HCO3 - ) ions. Increasing ocean TA, pH, and calcium carbonate saturation state (ΩCaCO3) could potentially alleviate sensitive ecosystems from ocean acidification. However, OAE could raise pH and ΩCaCO3 well above modern day levels and there is little data on the environmental impact of this. These potential perturbations could substantially influence marine biology, particularly those taxa found in coastal environments as these regions are a favourable site for TA addition. This thesis sets out to determine how elevated TA influences the physiology of two important coastal taxa, a benthic calcifying macroalgae found on rocky shores of the continental shelf (Corallina spp.) and Synechococcus 8806, a calcifying pico-sized phytoplankton found in the surface of deeper continental shelf waters. How elevated TA affects the growth rates, calcification rates, productivity rates and photophysiology of these two physiologically different species was investigated as part of four separate ex-situ experiments. The results from this thesis provide the first insights to the practical use of OAE as a carbon dioxide removal approach in terms of the response of the marine environment. Results show that increasing seawater TA significantly increases Corallina spp. calcification rates and productivity rates and so could potentially lessen the impacts of ocean acidification. Furthermore, these results suggest that elevated TA increases Synechococcus 8806 growth and under elevated TA higher rates of CaCO3 precipitation rates occurred. However, the results from this thesis do not show whether this was due to Synechococcus 8806 growth. The results show that OAE would not be intrinsically detrimental for Corallina spp. and Synechococcus 8806 in terms of direct effects to physiology. However, further research is needed to determine what the indirect effects of elevated TA are and how this could impact Corallina spp. and Synechococcus 8806