Helen Scales, National Geographic News 24 Nov 08;
Increasing levels of carbon dioxide in the atmosphere may make Earth's oceans more acidic faster than previously thought—unbalancing ecosystems in the process, a new study says.
Since 2000, scientists have measured the acidity of seawater around Tatoosh Island off the coast of Washington state. The acidity increased ten times quicker than climate models predicted.
The research also revealed the corrosive effect of acidic oceans could trigger a dramatic shift in coastal species and jeopardize shellfish stocks.
"The increase in acidity we saw during our study was about the same magnitude as we expect over the course of the next century," said study co-author Timothy Wootton, a marine biologist from the University of Chicago.
"This raises a warning flag that the oceans may be changing faster than people think," he said.
Increased carbon dioxide emissions from human activities have led to a 30 percent rise in ocean acidity in the past 200 years.
(Related: "Acid Oceans Threatening Marine Food Chain, Experts Warn" [February 17, 2007].)
When atmospheric carbon dioxide dissolves in the oceans it forms carbonic acid, which in turn has a negative impact on marine life.
Laboratory studies show that as seawater acidity increases, the calcium carbonate shells and skeletons of many marine species, such as hard corals, sea urchins, and stony seaweeds.
A Shifting Balance
Wootton and colleagues built models of an ecosystem based on field data of how species interact along Tatoosh Island's rocky shores.
Surprisingly, in a scenario of increasing acidity, not all species with calcium carbonate shells faired badly.
Instead, a shift took place: Larger mussels and barnacles suffered, leaving smaller barnacles and some calcium-based seaweeds better off.
In nature, "species are competing for space, they are eating each other, it's an incredibly dynamic system," said James Forester, a Harvard University ecologist who co-authored the study in this week's journal Proceedings of the National Academy of Sciences.
"When you change the playing field—in this case by altering acidity—you can get unexpected responses," he said.
"Mussels usually dominate the ecosystem because they are good at overgrowing and crushing out other species that grow on the rocks," said co-author Wootton.
"But when the mussels decline, it means other species—no matter whether or not they have a shell—can do better," he said.
An acidity-driven shift in coastal ecosystems could spell disaster for shellfish industries that rely on mussels and other similar species, Wootton warned.
Nancy Knowlton is a marine biology professor at the Scripps Institution of Oceanography in La Jolla, California, who was not involved in the study.
She pointed out the importance of adopting an "enemy of my enemy is my friend" approach when trying to understand the effects of ocean acidification on whole ecosystems.
While the field surveys did show an overall decline in mussels, the predictive models were needed to hunt for longer-term changes.
"There is inertia in the system because many of these species live for a long time," said co-author Wooton.
"The little changes we see in the dynamics of the ecosystem may magnify over time."
A Wider Pattern?
These are the first data on ocean acidity from temperate—rather than tropical—waters. No one knows whether similar rapid changes are taking place elsewhere.
"The rules might be quite different on Tatoosh Island," Wootton suggested.
"There could be mechanisms going on in the waters around our island that are unique.
"We really need to get more data from other sites away from the equator to see what patterns are there," he added.
Marine biologist Knowlton said, "This is typical of so many climate studies—almost without exception things are turning out to be worse than we originally thought."
Marine life faces 'acid threat'
BBC News 25 Nov 08;
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.
Ocean pH
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.
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.
Calcium carbonate
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.
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.
Chemical oceanography
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.
Acidic seas threaten coral and mussels
Impact of rising carbon dioxide levels far worse than previously thought
Steve Connor, The Independent 25 Nov 08;
Rising carbon dioxide levels are increasing acidity in the oceans 10 times faster than scientists thought, posing a greater threat to shell-forming creatures such as coral and mussels.
An eight-year project in the Pacific has found that rising marine acid levels will challenge many organisms, because their shell-making chemistry is critically dependent on a less acidic, more alkaline environment. The study monitored seawater pH levels at the north-east Pacific island of Tatoosh off Washington state in the United States.
Timothy Wootton, from Chicago University, said scientists found that acidity levels increased at more than 10 times the rate predicted by computer models designed to study the link between atmospheric concentrations of carbon dioxide and ocean acidity.
Atmospheric carbon dioxide levels have increased by about 100 parts per million since the start of the industrial revolution and are now at their highest point in at least 650,000 years.
About a third of man-made carbon dioxide emissions has dissolved into the oceans. As carbon dioxide dissolves in seawater, it forms carbonic acid, which lowers the ocean's alkalinity and pH level, making it more acidic.
The Intergovernmental Panel on Climate Change (IPCC) predicted last year that most coral reefs would disappear by the century's end because of rising temperatures and ocean acidity.
However, this latest study, published in the journal Proceedings of the National Academy of Sciences, suggests that the rate of ocean acidification may be far higher than the rate used by the IPCC scientists in their assessment of future prospects for shell-forming marine creatures such as corals.
Professor Wootton said: "An alarming surprise is how rapidly pH has declined over the study period ... These data point to the urgency of obtaining a globally extensive set of ocean pH data through time, and suggest that our understanding of ocean pH may be incomplete.
"The results showed that variation on ocean pH through time was most strongly associated with increasing atmospheric carbon dioxide, which supports the prediction that increasing release of CO2 to the atmosphere leads to ocean acidification."
The study was unusual in that it looked at acidity in the ocean's intertidal region, inhabited by shell-forming creatures such as barnacles and mussels. Professor Wootton said there was a shortage of data on ocean acidification, especially in non-tropical regions, which this study addressed.
"Our study reveals the strongest negative impacts of declining pH are on several species of particular importance – large calcifying mussels and goose barnacles. This finding illustrates several reasons why the effects of declining ocean pH are of general concern, as these species create critical habitats for other coastal species, are important players in coastal nutrient processing, and reflect the more general risks to shellfish harvesting."
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