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Grief on the coral reef

POSTED 21 SEPTEMBER 2006

Fertilizer: Too much of a good thing?
Coral usually live in tropical or subtropical seas with low levels of the major nutrients phosphorus and nitrogen. When a coral's symbiotic algae grow in highly fertile conditions, they cease providing nutrition to the coral animal, which can starve. But high fertility poses another problem: It can fuel the growth of multi-cellular algae -- seaweed -- that can smother coral.

Brian Lapointe of Harbor Branch Oceanographic Institution surveyed the waters along Florida's Atlantic coast, and found a direct correlation between high levels of phosphorus and the absence of coral. beautifully detailed illustrations of different types of coralPhosphorus seems to be the villain in many of the algal "blooms" that smother coral, he says. He thinks natural phosphorus levels had previously kept corals out of northern Florida. But ocean concentrations of phosphorus are rising, and coral is also suffering further south. (However, while Lapointe blames phosphorus for the absence of coral in northern Florida, other marine biologists attribute the absence to cold water, not phosphorus.)

These plates help document the oldest studies of the Florida Reefs. Figures 1-4, Oculina varicosa Leseuer. Figure 5, Oculina implicata, Ag. ms. Figure 6, Oculina arbuscula, Ag. ms. (see "Report on the Florida Reefs" in the bibliography). Image: NOAA

Nitrogen is the other major fertilizer of coral. In the Florida Keys, Lapointe says, the concentration of biologically active nitrogen rose 100 percent in the 1980s and 1990s, while seaweed was exploding and the reef was declining. Data from the world's longest continuous water-quality monitoring on a coral reef linked the smothering mat of algae to the rising nitrogen.

Through isotopic analysis, Lapointe's research group identified the sources of the excess nitrogen. "In the wet season and wet periods, there is a lot of runoff from land, from agriculture," he says. In dry periods, he adds, more nitrogen comes from sewage, in particular from leaking septic tanks and injection wells that receive hundreds of millions of gallons of partly treated sewage daily.

Even tiny concentrations of nitrates in water can spark algal overgrowth, and Lapointe says that saving the reefs will require restrictions on nitrogen and phosphorus. Areas with coral reefs that suffer from severe overfertilization, like South Florida and Hawaii "need to really move toward a zero-discharge goal, to remove nutrients from sewage and storm water."

Seen from above, a rocky coast meets a blue ocean, but for the small bleached coral
A satellite image of sea-surface temperatures at Australia's Great Barrier Reef shows hot water over inshore reefs, where coral bleaching is most severe. Photo: University of Queensland/NASA

Reefs: In hot water?
When water temperatures get too high, corals bleach as they expel their brightly colored algae. Bleached coral eventually starves and dies, unless it contains lingering algae that can revive, or it can adopt new algae. Could swapping out algae help coral survive a period of global warming? Some marine ecologists have proposed the "adaptive bleaching hypothesis": Bleaching followed by recolonization by algae suited to warmer water could allow coral to survive changing water temperature.

"It's intriguing," says Tamar Goulet in the department of biology at the University of Mississippi. "As coral is facing global climate change, what if they could survive by expelling the algae that are no longer suitable, and getting new algae?"

But she warns that adaptive bleaching may not save corals, since most coral seems quite choosy in terms of algae. In a recent study (see "Most corals ..." in the bibliography), Goulet gathered 15 years of data on corals to see how many could host multiple clades (groups of related algae) and found that only 23 percent of corals definitely housed more than one clade. Further, the different clades were "not necessarily within the same colony, maybe in the same reef or in a different geographic area." The other 77 percent of species always appear with the same algae: "They don't seem to switch their clades at all," she says.

Overfertilization, bleaching and disease are three major causes of coral reef destruction.In the face of some encouraging reports that some coral can adapt by adopting new algae, says Goulet, "The point of my paper was to put these scientific studies in perspective; they apply to 23 percent. The rest of the coral species, 77 percent, host a single clade. They do not appear to change that clade due to disease or transplantation to a novel environment. This implies that those 77 percent of coral species may survive global climate change only if their existing algae survives."

A sickness in the reef
Like humans, coral can get sick, mainly due to "the usual range of suspects that affect most organisms, says C. Drew Harvell, professor of ecology at Cornell University. The list includes "quite a few bacterial agents," fungi and perhaps some viruses, she says.

The field of coral epidemiology is not far advanced, but it should get a boost from the Coral Reef Targeted Research and Capacity Building project, which is supported by the Global Environment Fund and the World Bank. Harvell is leading the section on coral disease.

What is the relative importance of disease in the decline of coral? It's hard to isolate, says Harvell. "From my perspective, disease tends to be quite episodic. When it's bad, it can be very bad. During an outbreak, it can rival any other threat to coral," she says.

In understanding any infection, she adds, we must consider "the host, the pathogen and the environment. If all three are changing, it's hard to know what triggers the outbreak." A good example of that complexity came in 2005, she says. The record number of hurricanes wracked coral, and "the warmest thermal anomaly in 100 years in the Caribbean caused massive bleaching" and widespread disease. "We lost a lot from disease that followed the hurricanes and the warmth, but it's a little bit difficult to identify which factor accounts for the large-scale loss," Harvell says.

Patch of spindly, crooked finger like corals, one bleached white
White syndrome on branching Acropora corals bears a strong resemblance to scars of crown-of-thorns starfish feeding. The cause may be white band disease or something else. Photo: Australian Institute of Marine Science.

Coral do have an innate immune system that helps fight some infections, Harvell says, but when "corals are under a lot of stress, that can compromise their natural immunity."

Unless you know the source of an epidemic, whether human or coral, "it's difficult to think about any kind of control," Harvell says. In the ocean, control does not mean vaccines or antibiotics, but rather trying to halt outbreaks at their sources. Some ocean pathogens are known to originate on land, she adds. For example, California sea otters have been infected with a protozoan called toxoplasmosis that originates in cat feces and causes brain disease. Infected otters "are susceptible to sharks or die from toxoplasmosis in the brain. There seems to be a pretty clear link with land."

While no coral pathogens has unequivocally been linked to land sources, the fungus Aspergillus (which attacks corals called sea fans) lives on land and does not make spores (the fungal equivalent of seeds) in the ocean.

Coral reefs: Hope on the horizon?

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