Optimising reef-scale CO2 removal by seaweed to buffer ocean acidification

Abstract

The equilibration of rising atmospheric CO2 with the ocean is lowering pHin tropical waters by about 0.01 every decade. oral reefs and the ecosystems they support are regarded as one of the most vulnerable ecosystems to ocean acidification, hreatening their long-term viability. In response to this threat, different strategies for buffering the impact of ocean cidification have been proposed. As the pHexperienced by individual corals on a natural reef system depends on many processes ver different time scales, the efficacy of these buffering strategies remains largely unknown. Here we assess the feasibility nd potential efficacy of a reef-scale (a few kilometers) carbon removal strategy, through the addition of seaweed (fleshy ulticellular algae) farms within the Great Barrier Reef at the Heron Island reef. First, using diagnostic time-dependent age racers in a hydrodynamic model, we determine the optimal location and size of the seaweed farm. Secondly, we analytically alculate the optimal densityof the seaweed and harvesting strategy, finding, for the seaweed growth parameters used, a iomass of 42 gNm-2 with a harvesting rate of up 3.2 gNm-2 d-1 maximises the carbon sequestration and removal. Numerical xperiments show that an optimally located 1.9 km2 farm and optimally harvested seaweed (removing biomass above 42 gNm-2 every ) increased aragonite saturation by 0.1 over 24 km2 of the Heron Island reef. Thus, the most effective seaweed farm can only elay the impacts of global ocean acidification at the reef scale by 7-21 years, depending on future global carbonemissions. ur results highlight that only a kilometer-scale farm can partially mitigate global ocean acidification for a particular eef.