A Blue View: Floating Forests

by John Racanelli, Chief Executive Officer

Published April 16, 2014

A Blue View is a weekly perspective on the life aquatic, hosted by National Aquarium CEO John Racanelli.

From the smallest plants and animals invisible to the human eye to entire ecosystems, every living thing depends on and is intricately linked by water.

Tune in to 88.1 WYPR every Tuesday at 5:45 p.m. as John brings to the surface important issues and fascinating discoveries making waves in the world today.

April 16, 2014: Floating Forests

Click here to listen to John discuss the important role mangrove forests play in the health of our oceans!

Gardeners in Maryland know that most trees in our temperate climate don't like having wet feet. And water that's salty? Forget about it. Around here, having tree roots submerged in saltwater is guaranteed to kill off your landscaping.

But far south of the Chesapeake, fringing tropical and subtropical coastlines, there exist floating forests of mangroves, whose roots grow in a luxuriant tangle at the ocean’s edge. And there, they thrive.

Botanists call the 50 species of mangroves halophylic, or "salt loving." Mangroves have adapted to putting down roots where other plants can't: in areas inundated daily by the tide; in thin, nutrient-poor, low-oxygen soils; and in water that varies from fresh to brackish to salty. Just how much salt can mangroves tolerate? Well, typical seawater has a salinity of 35 parts per thousand; in other words, about 35 grams of salt for every liter of sea water. Some species of mangroves can survive in salinities of more than 90 parts per thousand!

To thrive in this salty abundance, these plants need strategies to clear the excess salt. Some species excrete it through glands in their leaves. Others use their roots.

The weird, knobby roots of mangroves actually make traveling to paradise for a tropical vacation possible—tough, woody evergreen mangroves stabilize the soil and prevent many islands from simply washing away.

Thank the mangroves, too, for the colorful diversity of fish and invertebrates you see on your next coral reef dive. Many oceanic and coral reef fish—including snapper, tarpon and lobster—spawn in the nursery provided by the mangrove’s submerged tangle of roots. A mangrove forest is a rich hub of biodiversity, supporting a unique ecosystem of bacteria, plants, mammals, amphibians, invertebrates and birds—some found nowhere else.

Earth’s largest mangrove forest—the Sunderbans of India and Bangladesh—is a UNESCO World Heritage Site, home to at least 250 species of birds, endangered estuarine crocodiles and even Bengal tigers!

In North America, mangrove swamps are found throughout the Gulf Coast, from Florida to Texas. The largest mangrove forest in the United States is in Florida’s aptly named Ten Thousand Islands National Wildlife Refuge. This special place is home to many endangered species, including West Indian Manatees, and clouds of scarlet ibis and white pelicans.

The scarlet ibis typically inhabits mangrove swamps ranging from northern South America southward along the coast of Brazil, occasionally making visits in Florida.

The scarlet ibis typically inhabits mangrove swamps ranging from northern South America southward along the coast of Brazil, occasionally making visits in Florida.

Although mangrove forests host so-called “charismatic megafauna” like manatees and tigers, truly their greatest treasure may be the thick mud of mangrove leaf litter—fertile with bacteria and fungi—that accumulates in the water below the trees. There, detritivores, like crabs and other animals, feed on decaying leaf litter and contribute to a complex food web that begins, literally, in the mud.

Other microfauna encrust the mangrove's submerged roots, including a profusion of filter feeding mussels and barnacles. Like Chesapeake’s oysters, mangrove barnacles efficiently filter pollutants from the water.

These crustaceans and mollusks in turn support populations of shrimp and fish that are economically important to Gulf of Mexico fisheries. So, the next time you’re dining on sustainably-caught shrimp, take a moment to thank a mangrove for your meal.

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