The winners and losers of ocean acidification

Populations of certain types of marine organisms known collectively as the ‘biofouling community’ – tiny creatures that attach themselves to ships’ hulls and rocks – may quadruple within decades, while others may see their numbers reduced by as much as 80%, if the world’s oceans continue to become more acidic, according to new research.

While these animals are primarily viewed by humans as pests – removal of biofouling organisms costs about $22 billion annually – they also play an important role in marine environments, primarily as food sources for larger organisms.

The researchers, from the University of Cambridge, British Antarctic Survey and Centro de Ciências do Mar, found that as acidity increased, organisms with shells, such as tube worms, saw their numbers reduced to just one-fifth their current numbers, while animals without shells, such as sponges and sea squirts, doubled or even quadrupled in number. The results, published today (28 January) in the journal Global Change Biology, show how these communities may respond to future change.

There is overwhelming evidence to suggest the world’s oceans are becoming, and will continue to become more acidic in the future, but there are many questions about how it will affect marine life. The biofouling community affects many industries including underwater construction, desalination plants and ship hulls.

For the first experiment of its kind, over 10,000 animals from the highly productive Ria Formosa Lagoon system in Algarve, Portugal were allowed to colonise hard surfaces in six aquarium tanks. In half the tanks, the seawater had the normal acidity for the lagoon (pH 7.9) and the other half were set at an increased acidity of pH 7.7. The conditions represented the Intergovernmental Panel on Climate Change’s (IPCC) prediction for ocean acidification over the next 50 years.

The researchers expected that the calcium carbonate shells would be broken down by the acidic environment, but scanning electron microscopy found that it was the organic ‘glue’ holding the calcium carbonate crystals together that was ‘eaten’ by the acid, causing the shells to disintegrate.

There is a great deal of competition for hard surfaces in the ocean – for example, a clean piece of tile placed in the sea would very quickly become covered in biofouling organisms. When organisms with hard shells, such as tube worms, die, they leave behind their shells, giving the next generation of organisms an additional surface to cling onto.

“These environments are almost like mini-reefs, and if you lose some of that three-dimensional complexity, you reduce the space and opportunities for some types of marine life – it becomes harder for some organisms to take their space,” said co-author Dr Elizabeth Harper of Cambridge’s Department of Earth Sciences.

“Our experiment shows the response of one ‘biofouling community’ to a very rapid change in acidity, but nonetheless shows the degree to which these communities could be impacted by ocean acidification, and to which its associated industries may need to respond,” said Professor Lloyd Peck from British Antarctic Survey (BAS), the paper’s lead author. “What’s interesting is that the increased acidity at the levels we studied destroys not the building blocks in the outer shell itself, but the binding that holds it together. Many individuals perish, but we also showed their larvae and juveniles are also unable to establish and create their hard exoskeleton.”

Peck continues, “Although a pH reduction of 0.2 is less than the IPCC’s ‘business as usual’ scenario of pH 0.3 – 0.4 in ocean surface waters by 2100, it will likely be achieved between 2055 and 2070.”

“Taking into consideration the importance of the Ria Formosa lagoon as a natural park the modified community structure driven by a reduction in PH, while potentially reducing biofouling issues, will almost certainly affect lagoon productivity and impact on biodiversity,” said co-author Dr Deborah Power, from Centro de Ciências do Mar.

The study was carried out by scientists from British Antarctic Survey, Centro de Ciências do Algarve, Instituto Portugues do Mar e da Atmosfera and University of Cambridge, and was funded by the Natural Environmental Research Council (NERC) and an EU Research Infrastructure Action under the FP7 ‘Capacities’ Specific Programme.