12 November 2009

Ocean acidification impacts coastal rivers

Cosmos Online
Ocean acidification, is affecting not only coral reefs, but coastal ecosystems by impacting everything from the oyster industry to the extent of dead zones along the U.S. Pacific coast.
Amazon estuary

The estuary of the Amazon River is (pictured in this satellite image) is one of the largest in the world. Credit: NASA

PORTLAND, OREGON: Ocean acidification, caused by rising CO2 levels, is affecting not only coral reefs, but coastal ecosystems by changing everything from the ability of oysters to adhere to the riverbed to the extent of dead zones along the U.S. Pacific coast.

Ocean acidification is occurring because CO2 from the burning of fossil fuels is dissolving in the seas, creating a weak carbonic acid solution. Much of the attention on the problem so far has focussed on coral reefs, which are particularly vulnerable to changes in pH (see “Oceans of acid”).

Combination of impacts

But riverine and other coastal environments may also be unusually vulnerable because they face a range of additional threats from problems such as pollution and variable levels of acidity and organic matter flowing into them, said Richard Feely, an oceanographer at the National Oceanic and Atmospheric Administration’s Pacific Marine Environmental Laboratory in Seattle, USA.

“Each estuary is different [and]… in coastal ecosystems, we have to think about combined impacts,” he said at a scientific meeting of the Coastal and Estuary Research Foundation in Portland, Oregon earlier this month.

Another danger is that coastal systems are highly variable. Adding the problem of acidification on top of their natural variability might push them over tipping points from which they cannot recover, added Burke Hales, an oceanographer at Oregon State University in Corvallis.

Spiralling out of control

On the Northwest coast of North America, for example, shifting winds can alternately draw deep, more acidic water up to the surface or pull less acidic surface waters into the bays. The result is that the bays can see rapid changes in acidity with changes in the weather.

“This natural variability is pushing these systems close to important thresholds, and the creeping changes that go along with that [from climate change and ocean acidification] might be enough to push these systems over that line,” said Hales.

But, he added, “it’s not all bad news.” Upwelling waters from the deep may be acidic, but they also bring up a lot of nitrates. These can fertilise massive growth of phytoplankton, which form the basis of the coastal ecosystem.

These single-celled plants consume the carbon dioxide in the water, which is then quite suitable for the growth of shellfish when weather conditions cause it to flow into the bays. “Oysters do great [in estuaries],” Hales said, “These conditions are conducive to shell formation.”

That’s good news, he added for the region’s US$100 million oyster industry, which can choose the best times for pumping water from the bays into onshore spawning tanks, where the oysters are protected from subsequent changes in water acidity.

Delicate balance

When the ocean functions properly he said, “it’s a delicately poised system that supports massive productivity.”

But, Hales warned, it doesn’t take a whole lot of ocean acidification to change everything. “The wheels can come off easily,” he said. Overall, he warns, the question is whether coastal systems such as that of the Pacific Northwest are resilient enough to change, or if they can easily spiral out of control.


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