Optimising reef-scale CO ₂ removal by seaweed to buffer ocean acidification

Abstract

The equilibration of rising atmospheric CO 2 with the ocean is lowering pH in 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 a reef-scale (a few kilometers) carbon removal strategy, through the addition of seaweed (fleshy multicellular algae) farms within the Great Barrier Reef at the Heron Island reef. First, using diagnostic time-dependent age tracers in a hydrodynamic model, we determine the optimal location and size of the seaweed farm. Secondly, we analytically calculate the optimal density of the seaweed and harvesting strategy, finding, for the seaweed growth parameters used, a 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 an optimally located 1.9 km 2 farm and optimally harvested seaweed (removing biomass above 42 g N m −2 every 7 d) increased aragonite saturation by 0.1 over 24 km 2 of the Heron Island reef. Thus, 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 only a kilometer-scale farm can partially mitigate global ocean acidification for a particular reef.

Flow and Drag in a Seagrass Bed

“…One particular case of interest is that of flows through seagrass beds. Among the reasons is the possibility that primary production by seagrass beds may enhance local buffering against ocean acidification and thereby help colocated calcifying organisms survive in conditions that would otherwise be detrimental (Cyronak et al, ; Mongin et al, ). Thus, the dynamics of the carbon system in seagrass beds as influenced by flows plays an important role in determining their ability to provide this potential ecosystem service.…”

CSIRO Environmental Modelling Suite (EMS): scientific description of the optical and biogeochemical models (vB3p0)

“…With this change, the scheme converges over a broad range of DIC and A T values (Munhoven, 2013). For more details, see Mongin and Baird (2014) and Mongin et al (2016b).…”

“…The assessment of version B1p0 of the eReefs marine model configuration of the EMS included a range of model configurations (4 km, 1 km and relocatable fine-resolution versions) (Herzfeld et al, 2016). The optical and carbon chemistry outputs were assessed in Baird et al (2016b) and Mongin et al (2016b), respectively. A more recent assessment of the BGC model (vB2p0) in the GBR compared simulations against a range of in situ observations that included 24 water quality moorings, two nutrient sampling programmes (with a total of 18 stations) and time series of taxon-specific plankton abundance.…”

Management strategies for coral reefs and people under global environmental change: 25 years of scientific research

“…Alternatively, waste carbon dioxide can be converted to ocean alkalinity . Novel engineering techniques to locally increase pH to delay the negative impacts of ocean acidification are starting to emerge, such as bubble stripping (Koweek et al, 2016) or installing seaweed farms in close proximity to coral reefs to remove carbon from the ocean (Mongin et al, 2016). Engineering solutions proposed to buffer the increase in ocean temperature include artificial shading and artificial upwelling to cool the sea temperature around corals , and low-voltage direct current to improve coral resistance to environmental change .…”