Ocean acidification, global warming, and the Great Barrier Reef

The decline in the rate of reef-building along the Great Barrier Reef is severe, sudden, and unprecedented in at least 400 years. Ocean acidification is a leading suspect.

Image courtesy of Jurgen Freund of Freund Factory
These Porites and other corals provide a critical nursery and long-term habitat for tens of thousands of marine species that rely on coral reefs

Perhaps it's time to begin talking about global warming and acidifying oceans in the same breath, rather than as related-but-separate issues.

In mid-December, The Monitor ran a story on research showing that some areas of the world's oceans are acidifying faster than marine scientists had predicted even three years ago.  The culprit: the excess carbon dioxide that human industrial activity and deforestation are pumping into the atmosphere -- and that the oceans are absorbing. (That article came on the heels of another, more-general Monitor article on the topic a month earlier.)

Now comes word that corals along Australia's Great Barrier Reef have been growing at an increasingly slow pace since about 1990. The process coral colonies use to build their crusty superstructures is called calcification; it's fallen off by some 14 percent since '90, according to a study published in the journal Science on Jan. 2. (You need a subscription to get the full research paper.)

The decline is severe, sudden, and "is unprecedented in at least 400 years," according to Glenn De'ath, a scientist at the Australian Institute of Marine Science outside of Townsville, Australia, who led the research team.

Dr. De'ath's team says more work is needed to pin down the relative contributions among several possible causes, including pollution, warming ocean temperatures, and ocean acidification.  But the group says it sees warming temperatures and ocean acidification as leading candidates.

The reason? Declines in coral far offshore were comparable to those near shore, where reefs would be more strongly affected by nutrient or soil run-off from land.

Researchers are concerned about the issue worldwide because it undermines the ability of shell-forming marine creatures -- many of which are key links in the marine food chain -- to build their homes. And reefs are nurseries and havens for a range of creatures.

Rock candy and calcium carbonate

To paint a picture of the process, Dwight Gledhill, a reef scientist with the US National Oceanic and Atmospheric Administration turns to kids making rock candy as an analogy. During a phone chat, he recalls the recipe: 1) Add lots of sugar to boiling water -- more than you'd think the water could take and still stay dissolved. The water in effect becomes "supersaturated" with dissolved sugar.  2) Suspend a string in the water, which gives the dissolved sugar a "nucleus" on which to gather back into crystal form. 3) Wait for a few days. Viola: an array of sugar crystals -- the rock candy -- clings to the string.

There is still sugar left in the water once the crystals have grown. But try to make another batch from the same sugary water and the process is likely to be slower, if not a dud, without replenishing the sugar concentration.

Generally speaking, Dr. Gledhill explains, shell-making follows a similar process. The oceans are supersaturated with dissolved calcium carbonate. The carbonate comes from the erosion of rocks on land over tens of thousands of years and longer. Shell-making creatures -- from tiny plankton to giant clams -- use some of the carbonates to make shells.

So do coral reefs. And their need for carbonates -- particularly a form called aragonite -- is endless. Even without destructive fishing practices from humans, reefs erode naturally as they are clobbered by storms or crumble under assaults from other forms of marine life, such as parrot fish.

(By the way, once the creatures die and sink to the sea floor, the carbon that once came from the atmosphere but now resides in their shells gets sequestered for very long periods of time. Think limestone deposits on land where prehistoric seas once spread.)

As humans add more CO2 to the atmosphere, however, the oceans take it up. So far, the oceans have absorbed roughly a third of the CO2 humans have added to the atmosphere since the dawn of the Industrial Revolution. These days, some of the carbonates that would have been available for shell- or reef-building serve as a buffer to neutralize the weak carbonic acid that forms as CO2 and seawater mix. This makes less available for shell and reef-building -- slowing the calcification process. At worst, once waters become undersaturated with carbonates existing shells and reefs begin to dissolve.

Researchers working in the Southern Ocean and off the tip of Washington State's Olympic Peninsula have started to see some of this erosion locally. In the case of the Southern Ocean, at least, long-term increases in acidification are reinforced by natural swings in seawater chemistry that occur with seasonal changes in ocean circulation.

What Dr. De'ath actually saw 

For the Great Barrier reef, the slowdown appears to have begun with 1990 as the "tipping point."

Coral builds annual layers, similar to tree rings. The Doc and his team examined samples from 328 coral colonies. The colonies come from 69 reefs, covering most of the Great Barrier Reef's length. Coral ages ranged from 10 to 436 years.

By looking at how dense the coral skeletons are and at how much they grew from year to year, the team calculated a "calcification rate." The group found that calcification rates increased from the late 1500s through the early 1600s, then increased only gradually (with a few ups and downs) until 1990. The rate then fell significantly.

The group acknowledges that the causes for the Great Barrier Reef-wide decline aren't known. But the broad reach of the corals' equivalent of a housing slump weighs against a range of explanations -- including disease -- leaving warmer ocean-surface temperatures and acidification as the last co-conspirators standing.

From the standpoint of environmental "stress," the Great Barrier Reef faces several. The team argues that it's time to systematically sort through them to identify the most influential "straws" that threaten to break the proverbial camel's back.

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