Dec 3, 2008
Sour future for the oceans?
Earlier this month conservation group Oceana predicted that ocean acidification could lead to enormous economic losses, including damage to the coral reefs off the Hawaiian Islands that produce around $360 million in economic benefits each year, and harm to the $10 billion global shellfish-farming industry. Deep-sea corals such as those in Alaskan waters provide sites for fish to feed, reproduce and hide from predators – in an area that produces more than half of US commercial fish landings. And such corals are likely to suffer before their tropical counterparts as cold waters absorb more carbon dioxide.
Recently, Australian scientists predicted that the “tipping point” for harm to the Southern Ocean could be 450 ppm. Oceana, meanwhile, proposes that stabilization of atmospheric carbon-dioxide levels at 350 ppm is the key to prevent significant damage from acidification. But, given that levels have already reached 385 ppm, this will require massive reductions in carbon emissions.
A couple of weeks ago, fellow US-based conservation organization the Center for Biological Diversity notified the US Environmental Protection Agency (EPA) of its intent to file a lawsuit against the body for failure to respond to the threat of ocean acidification by imposing stricter pH standards for ocean-water quality. The EPA has 60 days to correct the alleged violations before the Center may pursue legal action. (As an aside, the scale of that suit pales in comparison to that of environmental activist Dan Bloom who, according to Reuters, plans to lodge a claim at the International Criminal Court for $1 billion damages against all world leaders for failing to prevent global warming.)
Oceana’s figures are particularly troubling given findings from a recent study of acidity levels and marine ecosystems at Tatoosh Island (48.32 °N, 124.74 ° W) off the north-western tip of Washington State, US. Ocean-pH measurements are relatively sparse around the globe, but Tim Wootton and colleagues from the University of Chicago, US, have data covering the last eight years. These show that the acidification rate in this temperate region was an order of magnitude faster than predicted by models of ocean pH.
“There is very little data published on changes in ocean pH through time, and none that we know of for areas outside the tropics,” Wootton told environmentalresearchweb. “Our data show that declining ocean pH is a more urgent problem than has previously been suggested, at least in some areas of the world, but we lack widespread data series to know for sure how general these results may be.”
The team also found that pH level fluctuated on a number of timescales, from hours to years, which is attributed to processes such as photosynthesis and respiration by ocean-dwelling lifeforms, increased carbon-dioxide levels and upwelling. Scientists had previously believed that the buffering effect of the ocean would prevent such swings in pH.
“This variability provides clues to major drivers of pH in the ocean, which fluctuate over different time scales,” said Wootton. “We were able to explain about 70% of the variability we observed, with a combination of eight principal factors with known mechanisms that could affect ocean pH. The largest fraction of variability was associated with increases in atmospheric carbon dioxide.”
According to Wootton, carbon dioxide appeared to be the most important driver of long-term changes in pH because, unlike other factors associated with pH change – such as water temperature, upwelling, salinity or alkalinity – it exhibited a consistent directional change through time. A large component of the pH variability was attributed to changes in biological processes such as photosynthesis by marine plants, which uses up carbon dioxide, and organismal respiration, which releases carbon dioxide.
“This finding is significant because current models of ocean pH largely leave out biological activity,” said Wootton. “If environmental changes such as declining pH or increasing temperature alter the abundance and species composition of the ocean biota, these changes may feed back to further affect ocean pH. If so, then we will need to account for these changes in our efforts to project ocean conditions into the future.”
So pH levels in this particular region of the ocean are constantly in flux, with a long-term trend towards higher acidity. What does this mean for local ecosystems? Shoreline species with calcium-carbonate shells, such as mussels, are faring less well than (or, more technically, “showing a general decline in ecological performance”) than species without calcium-carbonate skeletons such as foliose algae – sheet-like seaweeds. That’s probably because lower pH levels in the surrounding water make it more of a challenge to secrete and maintain a calcium-carbonate shell or skeleton.
To find out what the future holds for sea-dwellers, Wootton and colleagues Catherine Pfister and James Forester have used multispecies Markov-chain models to predict the long-term effects of the acidity changes on the ecosystem. This analysis indicated that mussels and stalked barnacles, which are currently the large, dominant shelled species, will dwindle. But species such as non-calcified algae and calcified-acorn barnacles will increase, as their enemies, whether competitors for space or shelled consumers, are affected.
“The ongoing pH changes in the ocean have detectable ecological effects in nature, including impacts on important habitat-forming and harvested species,” said Wootton. “Hence, changing ocean pH should be taken as a serious threat to ocean ecosystems, and its ecological impacts merit increased study.”
With carbon-dioxide levels continuing to rise, more longer-term data on both pH levels and species dynamics will be key to predicting the future of the oceans under different scenarios of climate change.