The coming changes in ocean pH may well occur too rapidly for some marine species to adapt to and survive – warns Professor Lloyd Peck
The perceived big problem that past and predicted ocean acidification has for marine animals is that it makes extracting calcium carbonate from seawater to make skeletons more difficult. Many experiments have shown that exposure to reduced pH has negative consequences for most, if not all marine groups. The problems with the vast majority of studies is that they are perforce short-term and do not replicate the slower natural rates of change seen – and to come in the world’s oceans.
They also do not allow for inter-generational adaptation responses. Therefore, our understanding of longer-term ecologically realistic effects is limited. In a recently published paper in the journal Global Change Biology, a study lead by the British Antarctic Survey but including others from Great Britain, Singapore and Australia measured the size and composition of skeletons of four very different groups of animals across the globe. In all four groups – clams, predatory snails, sea urchins and lamp shells skeletons were smaller towards the poles and at lower temperatures. However, the best fit came when skeleton size was compared with the availability of calcium carbonate in seawater – which is also affected by temperature, salinity and pressure.
In one group, the clams, they need extra protection at high latitudes in Antarctica to protect them from being hit by icebergs. In this group their shells were thicker in Antarctica than elsewhere, which we might expect would buck the trend. But these clams change shape and become rounder in the cold polar waters, which keeps their skeletons as a smaller part of their total biomass, therefore staying within the trend of smaller skeletons in areas where calcium carbonate is less available.
The availability of calcium carbonate is measured as how saturated the seawater is and this varies across the globe. The lowest values on earth are in polar waters and these are similar to those predicted for temperate and tropical latitudes by the end of this century or in the early parts of next century. This work shows that for many groups over multi-generational evolutionary timescales, marine animals can adapt to reduced levels of calcium carbonate availability. It should, however, be noted that some groups such as lobsters and crabs with large crushing claws do not exist in Antarctica. And it may be that a greater difficulty in building calcium dense crushing structures limits these species.
The real question now is whether the coming changes in ocean pH will occur too rapidly for some or many groups to adapt to and cope. A further question is – will some of those that fail be in critical positions in the ecosystem, causing wider scale problems? In this respect, much depends on how fast each species reproduces and how many offspring it produces. These factors affect how quickly a beneficial genetic change can be made and how long it takes to spread through the population.
In terms of direct importance to human populations, some of the species studied here are eaten at some sites around the world. Urchins are eaten widely around the world for their roes. The clams are consumed in several tropical Asian sites such as Singapore, where they are known as mud clams. The snails studied here were whelks and the European species, the common dog whelk is eaten in the United Kingdom and several other countries across the European Union. But because such widely different groups were evaluated in the study, there is confidence that the response seen are widespread – which throws a spotlight on other groups consumed by humans including mussels, oysters, crabs and lobsters.
Professor Lloyd Peck is principal investigator in marine science at the British Antarctic Survey
Lloyd Peck, Public Service Europe, 6 August 2012. Article.