Optimising island-scale biological buffering of ocean acidification

Chair: Jessica Ericson

Mathieu Mongin(1*), Mark E. Baird(1), Scott Hadley, (1), , Andrew Lenton(1),

1CSIRO Oceans and Atmosphere Flagship, Hobart, Australia

The equilibration of rising CO2 between the atmosphere and the ocean is lowering pH in the tropical waters by about 0.01 every decade. Coral reefs and the ecosystems they support are regarded as one of the most vulnerable ecosystems to ocean acidification, threatening their long-term viability. In response to this threat, different strategies for buffering the impact of ocean acidification have been proposed. As the pH experienced by individual corals on a natural reef system depends on many processes over different time scales, the efficacy of these buffering strategies remains largely unknown. Here we assess the feasibility and potential efficacy of reef-scale biological buffering, through the addition of seaweed farms within the Great Barrier Reef (GBR) at the Heron Island. First, using diagnostic time-dependent age tracers in a hydrodynamic model, we determine the optimal location and size of the seaweed farms. Secondly, we analytically calculate the optimal density of the seaweed and harvesting strategy, finding, for the seaweed growth parameters used, a mean biomass of 42 g N m−2 with a harvesting rate of up 3.2 g N m−2 d−1 maximises the carbon sequestration and removal. Numerical experiments show that the optimally-sized (1.9 km2) and optimally-harvested (removing biomass above 42 g N m−2 every 7 days) seaweed farm increased aragonite saturation by 0.1 over 24 km2 of the Heron Island reef. The most effective seaweed farm can only delay the impacts of global ocean acidification at the reef scale by 7-21 years, depending on future global carbon emissions. Our results highlight that large seaweed farms will be required to locally mitigate ocean acidification in natural reef environments.