By Julian Siddle
Science Reporter, BBC News
Mussels can become stressed by increases in acidity
Man-made pollution is raising ocean acidity at least 10 times faster than previously thought, a study says.
Researchers say carbon dioxide levels are having a marked effect on the health of shellfish such as mussels.
They sampled coastal waters off the north-west Pacific coast of the US every half-hour for eight years.
The results, published in the journal PNAS, suggest that earlier climate change models may have underestimated the rate of ocean acidification.
Professor Timothy Wootton from the department of ecology and evolution, University of Chicago, in Illinois, says such dramatic results were unexpected as it was thought that the huge ocean systems had the ability to absorb large quantities of CO2.
"It's been thought pH in the open oceans is well buffered, so it's surprising to see these fluctuations," he said.
The findings showed that CO2 had lowered the water pH over time, demonstrating a year-on-year increase in acidity.
Increasing quantities of dissolved CO2 will make sea water more acidic
The research involved taking daily measurements of water pH levels, salinity and temperature, off the coast of Tatoosh island, a small outcrop lying in the Pacific Ocean, just off the north-western tip of Washington state, US.
As well as measuring physical factors, the health of marine life present in the coastal ecosystem was also tracked.
Professor Wootton says biological factors were missing from previous models of ocean climate systems - and that life in the ocean, or in this case on the ocean edge, can also affect seawater pH.
"Over a short time, biology is affecting pH, through photosynthesis and respiration, but current models don't include biological activity as part of the story," he explained.
Every summer, Professor Wootton returned to the same sites on Tatoosh island's windswept coasts, to look at the abundance and distribution of life at the water's edge. He was especially interested in barnacles, algae and the dominant species, the Californian mussel.
Seawater sampling was conducted for eight years
The mussel has a calcium carbonate -based shell, which can be weakened or even dissolved by exposure to acid. Professor Wootton says the increase in acidity may be responsible for the decline in mussels noted in the study.
"Patterns show the chances of mussels being replaced are higher than for species without calcified shells," he said.
Other species quickly move into the space previously occupied by the mussels - though one of these species, the barnacle, also has calcified shells.
To explain this apparent anomaly, Professor Wootton says the decline of the dominant species allows a window where another species may thrive - though he expects this to be temporary as the interloper too will eventually be affected by the increasing acidity.
"In the short term, the long term decline is offset by the release from competition," he explained.
It's going down 10 to 20 times faster than the previous models predicted
Professor Timothy Wootton, University of Chicago
The researchers say they were surprised that the plants and animals in their study are so sensitive to CO2 changes. These organisms live in the harsh inter-tidal zones, they may be submerged under water, exposed to the sun, then lashed by waves and storms.
Professor Wootton says the most troubling finding is the speed of acidification, with the pH level dropping at a much greater rate than was previously thought.
"It's going down 10 to 20 times faster than the previous models predicted," he says.
The research team are now working together with chemical oceanographers to see how their coastal observations can be matched with large scale observations, to try to explain why the decline in pH levels seems to be happening so quickly.
"We actually know surprisingly little about how ocean acidity is changing over time, we need a broader network of measurements," said Professor Wootton.