Mysteries of the mangrove swamp

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"They look like Ents, don't they?" whispered one of the tourists in the kayak next to mine. For some reason, completely unbidden, we'd all taken to whispering once we'd gotten deep into the forest. "You know, the living trees from Lord of the Rings."

With their crooked roots reaching out like arthritic fingers, the mangrove trees—at least, those of the species our guide called white mangrove—did look as though they might be a touch on the sentient side. To the best of my knowledge, though, none of the mangroves lining the shores of Isla Damas, Costa Rica has ever freed its roots from the gloppy mud, and taken off tromping through the brackish water.

That said, this mangrove swamp—and others like it all around the world—is hardly idle. Really just a catch-all term for any tree or shrub that can survive and thrive in the briny water along river mouths and coastlines, mangroves play important roles as an animal habitat, a natural filtration system and even a source of protection against some of nature's more destructive powers.

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Salt is, generally, bad news for plants. But the 54 different species of mangroves all evolved to survive in water that, while not 100 percent saline, is certainly much more salty than any other plants can stand. In a mangrove swamp, there are only mangroves. Fascinatingly, different types of mangroves developed different strategies for dealing with the problem.

"They can exclude it at the root surface, using some sort of filter that we really don't understand yet. It's odd because larger molecules than salt still get in," said Peter Hogarth, Ph.D., Fellow in the Department of Biology, University of York.

"Another solution is that they tolerate it. Salt negatively affects enzymes, but within some mangrove cells there's a mechanism for keeping salt away from cell machinery," he said.

"They can also just get rid of it. Mangroves tend to shed bark and leaves. Some plants deposit salt in the leaves and bark, and then just get rid of them. Others actually secrete salt through specialized salt glands om the leaf surface. You'll see salt crystals on those leaves, and if you lick them, they taste quite salty."

Because of that regular shedding of leaves and bark, mangroves are also particularly good at putting nutrients from the water to practical use, said Aaron Ellison, Ph.D., Senior Ecologist at the Harvard Forest. That means a mangrove swamp can help clear things like sewage effluent and fertilizer runoff from river water before it reaches the sea.

In fact, Hogarth has seen this effect in action on the heavily polluted Indus River. What comes out of this delta into the Indian Ocean is in a lot better shape than the water upstream, he said. Unfortunately, mangroves worldwide are often given more nutrients than they can handle.

"The system only works up to a point," Ellison said. "It's a matter of finding the right level of indignity we can visit on the system. Right now, runoff, particularly from over-application of fertilizer, is so large that it just overwhelms the filtering capacity of most mangrove forests."

Sewage and fertilizer are a threat to the mangroves' survival. So is unfettered development. In Costa Rica, coastlines are in demand for hotels and other tourist amenities. All over the world, the swamps are frequently seen as dangerous places that would be better off filled in and put to use. That's a problem, said both scientists, because when that happens, you lose the important benefits mangroves provide.

It's not just about cleaner water. It's also about habitat. Fish spawn and spend their early years in the safety of tangled mangrove roots. So local families fish in the swamps of Isla Damas. You'd see mom, dad, three kids and a dog in a small motorboat—the dog was the only one without a line in the water.

Further away, commercial fisheries benefit from the breeding ground the swamp provides.

Then there are the crabs.

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I almost screamed the first time I reached out to touch a tree, in an effort to turn the kayak around a sharp corner, and realized that the trunk was alive with dozens of small, scuttling crabs. They climb over the trees in search of food, Hogarth said. And, like the mangroves, the crabs have an ingenious adaptation to their tidal environment--almost like backwards scuba gear.

"They're basically amphibious," Hogarth said. "As with all crabs they have a gill chamber within the shell, evolved for extracting oxygen from water.

But what happens is that they come out of their burrows with a gill chamber full of water, spurt it out an aperture and allow it to trickle over the upper section of their exoskeleton. It spreads out in a thin sheet, picks up oxygen and loses carbon dioxide and then flows into another aperture and back to the gill chamber. They just keep recirculating the same water."

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Mangrove swamps can also protect people, as well as crabs and fish. On the Isla Damas tour, our guide said that New Orleans would have been better protected against Hurricane Katrina, if the mangrove swamps of southern Louisiana hadn't been demolished.

That particular claim isn't true, Ellison said, because there were never a lot of mangroves near New Orleans and a Category 5 hurricane isn't going to be slowed much by trees. However, mangroves really do offer a layer of protection against both smaller hurricanes and tidal waves.

The key is at the roots. Under the mud, mangrove roots intertwine with one another, Hogarth said, forming a strong, natural wall that prevents erosion and can absorb some of the force of wild wind and water.

When the 2004 Indian Ocean Tsunami struck, areas of southern India where mangroves had been cleared out suffered more damage than those that still had a mangrove barrier between them and the ocean, Hogarth said. Mangroves can even mean the difference between a profitable harbor and an unusable one.

"Port Bin Qasim harbor, near Karachi in Pakistan, is protected by a mud bank topped with mangroves," Hogarth said. "Outside the bank, the waves are 5 meters high. Inside it, they're only 1 meter. The bank wouldn't survive without the mangroves to hold it in place, and you couldn't have a major harbor with 5 meter waves."

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