SCIENTISTS STUDY CORAL REEFS AND OCEAN ACIDIFICATION

FROM:  NATIONAL SCIENCE FOUNDATION
Coral reefs defy ocean acidification odds in Palau
Palau reefs show few of the predicted responses

Will some coral reefs be able to adapt to rapidly changing conditions in Earth's oceans? If so, what will these reefs look like in the future?

As the ocean absorbs atmospheric carbon dioxide (CO2) released by the burning of fossil fuels, its chemistry is changing. The CO2 reacts with water molecules, lowering ocean pH (making it more acidic) in a process known as ocean acidification.

This process also removes carbonate, an essential ingredient needed by corals and other organisms to build their skeletons and shells.

Scientists are studying coral reefs in areas where low pH is naturally occurring to answer questions about ocean acidification, which threatens coral reef ecosystems worldwide.

Palau reefs dodge ocean acidification effects

One such place is Palau, an archipelago in the far western Pacific Ocean. The tropical, turquoise waters of Palau's Rock Islands are naturally more acidic due to a combination of biological activity and the long residence time of seawater in their maze of lagoons and inlets.

Seawater pH within the Rock Island lagoons is as low now as the open ocean is projected to reach as a result of ocean acidification near the end of this century.

A new study led by scientists at the Woods Hole Oceanographic Institution (WHOI) found that coral reefs in Palau seem to be defying the odds, showing none of the predicted responses to low pH except for an increase in bio-erosion--the physical breakdown of coral skeletons by boring organisms such as mollusks and worms.

A paper reporting the results is published today in the journal Science Advances.

"This research illustrates the value of comprehensive field studies," says David Garrison, a program director in the National Science Foundation's Division of Ocean Sciences, which funded the research through NSF's Ocean Acidification (OA) Program. NSF OA is supported by the Directorates for Geosciences and for Biological Sciences.

"Contrary to laboratory findings," says Garrison, "it appears that the major effect of ocean acidification on Palau Rock Island corals is increased bio-erosion rather than direct effects on coral species."

Adds lead paper author Hannah Barkley of WHOI, "Based on lab experiments and studies of other naturally low pH reef systems, this is the opposite of what we expected."

Experiments measuring corals' responses to a variety of low pH conditions have shown a range of negative effects, such as fewer varieties of corals, more algae growth, lower rates of calcium carbonate production (growth), and juvenile corals that have difficulty constructing skeletons.

"Surprisingly, in Palau where the pH is lowest, we see a coral community that hosts more species and has greater coral cover than in the sites where pH is normal," says Anne Cohen, co-author of the paper.

"That's not to say the coral community is thriving because of the low pH, rather it is thriving despite the low pH, and we need to understand how."

When the researchers compared the communities found on Palau's reefs with those in other reefs where pH is naturally low, they found increased bio-erosion was the only common feature.

"Our study revealed increased bio-erosion to be the only consistent community response, as other signs of ecosystem health varied at different locations," Barkley says.

The riddle of resilience

How do Palau's low pH reefs thrive despite significantly higher levels of bio-erosion?

The researchers aren't certain yet, but hope to answer that question in future studies.

They also don't completely understand why conditions created by ocean acidification seem to favor bio-eroding organisms.

One theory--that skeletons grown under more acidic conditions are less dense, making them easier for bio-eroding organisms to penetrate--is not the case on Palau, Barkley says, "because we don't see a correlation between skeletal density and pH."

Though coral reefs cover less than one percent of the ocean, these diverse ecosystems are home to at least a quarter of all marine life. In addition to sustaining fisheries that feed hundreds of millions of people around the world, coral reefs protect thousands of acres of coastlines from waves, storms and tsunamis.

"On the one hand, the results of this study are optimistic," Cohen says. "Even though many experiments and other studies of naturally low pH reefs show that ocean acidification negatively affects calcium carbonate production, as well as coral diversity and cover, we are not seeing that on Palau.

"That gives us hope that some coral reefs--even if it is a very small percentage--might be able to withstand future levels of ocean acidification."

Along with Barkley and Cohen, the team included Yimnang Golbuu of the Palau International Coral Reef Center, Thomas DeCarlo and Victoria Starczak of WHOI, and Kathryn Shamberger of Texas A&M University.

The Dalio Foundation, Inc., The Tiffany & Co. Foundation, The Nature Conservancy and the WHOI Access to the Sea Fund provided additional funding for this work.

-NSF-

OVERFISHING AND CORAL KILLING-SPONGES

FROM:  NATIONAL SCIENCE FOUNDATION 
Overfishing of Caribbean coral reefs favors coral-killing sponges
Caribbean-wide study shows protected coral reefs dominated by sponges with chemical defenses

Scientists had already demonstrated that overfishing removes angelfish and parrotfish that feed on sponges growing on coral reefs--sponges that sometimes smother the reefs. That research was conducted off Key Largo, Fla.

Now, new research by the same team of ecologists suggests that removing these predators by overfishing alters sponge communities across the Caribbean.

Results of the research, by Joseph Pawlik and Tse-Lynn Loh of the University of North Carolina Wilmington, are published this week in the journal Proceedings of the National Academy of Sciences (PNAS).

"In fact," says Pawlik, "healthy coral reefs need predatory fish--they keep sponge growth down."

The biologists studied 109 species of sponges at 69 Caribbean sites; the 10 most common species made up 51 percent of the sponge cover on the reefs.

"Sponges are now the main habitat-forming organisms on Caribbean coral reefs," says Pawlik.

Reefs in the Cayman Islands and Bonaire--designated as off-limits to fishing--mostly have slow-growing sponges that manufacture chemicals that taste bad to predatory fish.

Fish numbers are higher near these reefs. Predatory fish there feast on fast-growing, "chemically undefended" sponges. What's left? Only bad-tasting, but slow-growing, sponges.

Overfished reefs, such as those off Jamaica and Martinique, are dominated by fast-growing, better-tasting sponges. "The problem," says Pawlik, "is that there are too few fish around to eat them." So the sponges quickly take over the reefs.

"It's been a challenge for marine ecologists to show how chemical defenses influence the structure of ocean communities," says David Garrison, a program director in the National Science Foundation's (NSF) Division of Ocean Sciences, which funded the research.

"With this clever study, Pawlik and Loh demonstrate that having--or not having--chemical defenses structures sponge communities on Caribbean coral reefs."

The results support the need for marine protected areas to aid in coral reef recovery, believes Pawlik.

"Overfishing of Caribbean coral reefs, particularly by fish trapping, removes sponge predators," write Loh and Pawlik in their paper. "It's likely to result in greater competition for space between faster-growing palatable sponges and endangered reef-building corals."

The researchers also identified "the bad-tasting molecule used by the most common chemically-defended sponge species," says Pawlik. "It's a compound named fistularin 3."

Similar chemical compounds defend some plants from insects or grazers (deer, for example) in onshore ecosystems, "but the complexity of those ecosystems makes it difficult to detect the advantage of chemical defenses across large areas," says Pawlik.

When it comes to sponges, the view of what's happening is more direct, he says. "The possibility of being eaten by a fish may be the only thing a reef sponge has to worry about."

And what happens to reef sponges may be critical to the future of the Caribbean's corals.

-NSF-

THE SPONGE AND THE CORAL REEF

Puff Sponge.  Credit:  Wikimedia.

FROM: NATIONAL SCIENCE FOUNDATION
Life on a Coral Reef: Insult Is (Sometimes) Added to Injury
When is insult added to injury for a Caribbean coral reef?

When overfishing removes predatory fish that feed on sponges, according to results reported this week in the journal PLOS ONE.

Using the undersea habitat Aquarius--moored on Conch Reef off Key Largo, Florida--marine scientist Joseph Pawlik of the University of North Carolina Wilmington (UNCW) and colleagues found that these predator-fish are the same brightly colored angelfish and parrotfish that attract scuba divers and glass-bottom boat tourists.

Pawlik is first author of the PLOS ONE paper; co-authors, all from UNCW, are Tse-Lynn Loh, Steven McMurray and Christopher Finelli.

Chemical warfare beneath the waves

The fish prey on sponges without chemical defenses--sponges missing what might be called the "yuk factor."

"Sponges that manufacture metabolites that are distasteful to fish are largely left alone," says Pawlik.

"That being said, when overfishing by humans removes these predatory fish, reefs shift toward faster-growing sponges that can out-compete reef corals for space.

"That further hinders corals' chances of recovery."

Coral cover on Caribbean reefs is at historic lows due to disease, heat stress from warming waters and waves from storms.

Undersea garden of sponges

"Coral reefs, especially in the Caribbean, have undergone many changes in the past few decades," says David Garrison, program director in the National Science Foundation's (NSF) Division of Ocean Sciences, which funded the research.

"With the decline of reef-building corals, sponges are becoming the main organisms on many reefs. These findings provide important information about interactions between sponges and predatory fish in coral reef communities."

Previous research showed that Caribbean sponge communities were primarily structured by the availability of plankton, or tiny floating plants and animals, rather than by predators.

But sponge growth experiments performed by Pawlik and colleagues--research that used cages to exclude predators--show the opposite.

"Overfished reefs that lack spongivores [sponge-eating fish] soon become dominated by faster-growing, chemically undefended sponge species, which better compete for space with reef-building corals," says Pawlik.

Endangered corals: threatened by 'new game in town'?

That has implications for fisheries management throughout the Caribbean.

"Some coral species are listed as critically endangered on the IUCN [International Union for Conservation of Nature] Red List, with four reef-building corals on the top ten list for risk of extinction."

Sponges are already overrunning certain coral reefs.

"As the effects of climate change and ocean acidification disrupt marine communities," says Pawlik, "it's likely that reef-building corals will suffer greater harm than sponges, which don't form at-risk limestone skeletons [as corals do]."

Hence, he believes, Caribbean reefs of the future are likely to be made up increasingly of sponges.