Observational and experimental studies in natural systems suggest that ocean acidification will alter the structure of seaweed communities.
Large quantities of carbon dioxide generated as a biproduct of human industry and the use of fossil fuels, have been entering our atmosphere, leading to rising global temperatures and ultimately climate change. Yet while awareness of the potential impacts of climate change is now widespread amongst the scientific community and beyond, the consequences of a parallel phenomenon are less well known. The ocean is a natural sink for CO2 and has to some extent mitigated the impacts of increasing atmospheric CO2 by absorbing it as part of a natural cycle, but such are the quantities of the gas entering the ocean that chemical changes are predicted, which in turn will impinge upon marine ecosystems.
In a process known as ocean acidification CO2 combines with seawater to lower the ocean pH, in effect a move along the scale towards acidity. Even small changes in pH can affect the ways that some organisms use calcium carbonate to build skeletons and shells, used for support and protection, but until recently most experimental studies on impacts were short term and carried out in laboratory conditions, with few studies looking at what actually happens in nature.
Now scientists based at PML, the University of Bristol and the University of Chicago have discovered that as seawater in the Northwestern Pacific has been acidifying, some species of marine algae are showing significant changes over time. The seaweeds, known as coralline algae because they build calcium carbonate skeletons to provide protection in the high energy habitats near to shore, have suffered a decline in the thickness of their thalli (the leaf like growths of seaweeds). Using measurements collected during experiments in the period from 1981 to 1997 and comparing them to measurements of the same species from the same area collected in 2012, the scientists found that some algae are more than twice as thin as they used to be. Not all species had thick thalli however, so increasing acidification affected thinner algae in a different way – reducing the thickness of the cell walls. Either way it appears that the various species of coralline algae are trading off calcium carbonate skeleton production in the face of increasing ocean acidification, demonstrating a real-time, real-world impact of the chemical changes now taking place in our seas. According to lead author Dr Sophie McCoy, now based at PML, this is evidence of a genuine change brought about as a result of our activities, and it is a change we should be concerned about:
“This is strong evidence of a significant impact on these important seaweed species and the same story is likely to be found elsewhere around the world as our seas become more acidic. The coralline algae are important in marine ecosystems providing settlement places and nursery grounds for many other organisms including starfish, corals and fish. Anything that makes them more stressed and less able to survive could have ramifications across coastal ecosystems and affect other organisms along marine food chains”.
Dr Sophie McCoy is a Marie Curie research fellow at PML.