Since the birth of the industrial revolution, ocean pH has dropped by 0.1 units. That might not sound like much until you realise that a 0.1 unit fall is a 30% increase in acidity. And, with predictions that ocean pH will continue plummeting, ecologists are becoming increasingly concerned about the impact of ocean acidification on marine populations. Brian Gaylord and his colleagues from the University of California at Davis explain that the open-coast mussel, Mytilus californianus, is a foundation species for many coastal ecosystems on the exposed northwestern coasts of North America, yet no one knew how ocean acidification might affect this keystone organism, so the team decided to find out how a fall in pH might impinge on the earliest settlers to colonise an exposed rocky outcrop: M. californianus larvae (p. 2586).
Growing freshly fertilized M. californianus larvae in seawater laced with carbon dioxide ranging from the modern level of 380 p.p.m. CO2 up to a ‘fossil-fuel intensive’ scenario of 970 p.p.m. CO2, the team allowed the larvae to develop for 8 days (p. 2586). Then they analysed the strength, size and thickness of the larvae's shells and found that acidification of the mollusc's seawater has a strong impact on shell strength. Shockingly, the shells of 5 day old larvae raised in 970 p.p.m. CO2 were 20% weaker than those of larvae reared at the current CO2 level, while the shells of larvae reared at 540 p.p.m. CO2 were only 13% weaker. The team also found that after 8 days at 970 p.p.m. CO2 the shells were up to 15% thinner and 5% smaller, and the body masses of the molluscs within the shell were as much as 33% smaller than those of mussels grown at modern CO2 levels.
‘The observed ocean acidification-induced decrease in shell integrity in M. californianus represents a clear decline in function,’ say Gaylord and his colleagues, who also warn that, ‘Such reductions may in fact be common in bivalves.’ Outlining the potential ecological consequences of ocean acidification, the team suspects that larvae weakened by rising CO2 levels could develop more slowly or, alternatively, they could be more vulnerable to predation, more susceptible to stress and at greater risk of desiccation. Ultimately these factors could conspire to reduce the mussel's survival and destroy the delicate balance that exists in today's coastal ecosystems.