Ocean Alkalinity Enhancement (OAE) is proposed as a Negative Emission Technology method to remove CO₂ from the atmosphere and sequester it in the ocean as dissolved inorganic carbon. However, little is known about the potential ecological and biogeochemical changes that OAE might induce. To investigate this, we conducted two mesocosm experiments (3 × 50,000 L lined ponds) and one incubation experiment (28 × 55 L ventilated incubators) in Daya Bay, Shenzhen. We used magnesium hydroxide as an alkalinity enhancement agent. In the two mesocosm experiments, the addition of different concentrations of magnesium hydroxide did not significantly alter most biogeochemical timeseries, including inorganic nutrients (N, P, Si) and chlorophyll-a. A notable exception occurred in the 55 L incubation experiment during the dry season, where the NO_x^- concentrations in the three treatments with varying doses of magnesium hydroxide were significantly lower than the control after 22 days. This might have resulted from the incorporation of magnesium hydroxide into organic flocs and associated changes in pH-sensitive remineralization rates. Ecological changes, as assessed by DNA, were minor across both mesocosms with no significant changes relative to the control treatment in either the wet or dry season. In summary, our study, based on magnesium hydroxide-induced OAE, suggests that the risk of substantial biogeochemical or ecological side-effects is low.



In 2025 we will participate in a 55 L scale intercomparison project (OAEPIIP) which will assess whether there are critical differences in the responses of marine ecosystems to OAE worldwide. We will also expand our mesocosms to 1 million litre scale with a flow-through design which will better mimic how a trial OAE scheme would operate.