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Mangroves: Nurseries of the Reefs

(c) 1999 Jonathan Bird

Mangroves often go unappreciated by the casual observer.   Frequently too dense to get into, we only see these areas from a distance or the edge.   However, the mangroves represent an extremely important part of the equation of life in all of the world's tropical ocean ecosystems.

Mangroves are plants living in the tidal coastal areas between sea and land.   The term has been applied to any and all species of trees which occupy this zone of life.    All share the trait of being able to tolerate partial submersion in high salinity water, and poor oxygen content in the ground where their roots penetrate.   Different kinds of mangrove trees have evolved different ways of dealing with these two limiting factors, but all true mangrove trees must deal with them to survive.   Mangroves grow only in the tropics.   The richest mangrove communities occur in areas where the daytime temperature is greater than 75 degrees F (24 degrees C) and the annual rainfall exceeds 40 inches (100 centimeters.)

The fact that most mangrove plants can survive in a pot watered only with fresh water indicates that mangroves don't actually need salt to survive.   Yet, they only grow in the salty waters of the ocean's edge.   This is not because they need salt, but rather because they have evolved the means of surviving where other plants cannot, thus carving out a niche for themselves with little competition for space and nutrients from other plants.

To deal with salt, many mangroves stop the salt from entering their tissues by filtering it out at root level.   Some mangrove plants can exclude about 90% of the salt in the salt water they absorb with a special filter in the roots.   But some salt still gets in.   So the next trick is to excrete the unwanted salt.   Some plants do this with salt glands in their leaves.   In fact, the leaves of many mangrove plants have the most efficient salt-excreting systems known.   In some species, excreted salt crystals can be seen or tasted on the leaves!   Another technique concentrates the unwanted salt in bark or older leaves.   The plant periodically sheds the bark and leaves, taking the salt away with it.

Mangrove plants produce on average about a quarter of a pound of litter (bark, leaves, twigs, fruit, flowers, etc.) per square foot per year (1 kg/square meter/yr).   Some of this is directly consumed by small animals, like crabs and fishes, but most of it has to be broken down further before the nutrients are available to other animals or plants.   Because of the amount of organic material to be broken down, the mud at the bottom of the mangrove forest (which is underwater much or all of the time) has a very high concentration of bacteria.   In fact, a typical teaspoon of mud from a mangrove has more than 10 billion bacteria in it.   The bacteria consume the litter, digesting it and increasing the amount of protein it makes available to other animals.   When fishes and invertebrates feed on this waste, they produce their own waste which feeds yet other animals.   The smallest remains feed plankton in the open ocean as it washes out to sea every day on the receding tide.   The tides carry a cargo of food out to sea from the mangroves every time they retreat, making the mangrove an important food and nutrient source for animals and plants on the reefs and in the open ocean.

Since the mud in the mangrove is so thick with litter, bacteria, and the end result of lots of decomposition, it has very little oxygen.   So instead of absorbing oxygen through their underground roots, like many plants, the mangrove plant has developed long roots which come up out of the water into the air before joining the trunk of the plant.   The roots have "breathing" cells above water called lenticels which draw in air.   These cells have one weakness, which is that they can be smothered by a light coating of oil.   So an oil spill can very easily kill an entire mangrove forest by suffocation.

Petroleum is the primary mangrove pollutant in the Caribbean.   Drilling and shipping of petroleum products in and around the Caribbean proves a constant peril to mangroves.   In 1962, the Argea Prima, an oil tanker, spilled about 3.7 million gallons (approx. 14 million liters) of crude oil near the coast of Puerto Rico, killing a large amount of mangroves and associated fauna.   Over 25 years later, hydrocarbon residues were still measured at this site in the soil, and the surviving mangroves had numerous mutated seedlings caused by the toxic spill.   This is but one of many examples of how tragically an oil spill can damage mangroves.

Mangroves typically produce fruits or seeds that float.   This makes sense for plants that live at least part of their lives in water.   As the fruit or seeds are dropped, they float away on the tide, to hopefully mature elsewhere, thus spreading the population of mangroves.

Probably because mangrove plants can only thrive in a narrow range of conditions, many species have developed fascinating techniques of reproduction.   While the dispersal of live, germinated seeds (known as vivipary) is very rare in most plants, many species of mangrove plants utilize this technique.

Several species of viviparous mangrove plants produce seeds which have a buoyant outer coating.   The seed floats until it reaches a favored water salinity (not too salty, not too fresh).   When the salinity is right, the coating peels off, and the seed sinks to the bottom.   With luck, it will take hold and grow.   Other species produce seedlings which stay attached to the mangrove plant while a stem and some roots grow out of either side of the seed.   After the seed has developed its "starter" root and stem, it falls into the water.   In the buoyancy of salt water, the whole seedling floats horizontally on the tides and currents.   But when the seedling reaches brackish coastal water, the less buoyant root sinks, flipping the whole seedling to a vertical position where the root can hopefully touch bottom and take hold.   While not all mangrove trees utilize vivipary, they all seem to produce large seeds or fruits which can survive a long time, suggesting that sometimes these seeds or fruits float for quite a while in "suspended animation" until they find a suitable place to germinate.

Perhaps the most important contribution to the ecosystem made by mangroves is as a refuge.   The tangled maze of roots in the mangrove forest creates a confusing shallow water labyrinth.   Taking refuge in this protective maze, many animals survive here when they couldn't survive anywhere else.   The mangrove is a nursery for juvenile fishes and invertebrates of many species.   In the protection of the calm sheltered waters of the mangroves where larger predators cannot find them, the juvenile fishes find ample food and safety.   When they mature, they may head out into the more dangerous reef areas to find mates or pursue their hidden fishy agenda.   Mussels and sponges attach to the mangrove roots, often the only rigid structure to be found for invertebrates requiring a "hand-hold."

Recent research has shown that some sponges have actually formed a mutualistic partnership with certain mangrove trees.   These sponges grow on the roots of the mangroves not only as a place to hold on, but as a way to obtain nutrients.   It seems that when the sponge grows on the root, the root sends out "rootlets" into the sponge for the purpose of exchanging nutrients.   The tree provides carbon to the sponge and the sponge provides inorganic nitrogen to the tree.   Sponges engaged in such an association grow 2 to 10 times faster than the same species growing somewhere else, and the roots of the mangrove trees grow two to four times faster than bare roots without sponges.

Currently, the Caribbean is losing mangroves at a rate of about one percent per year.   Most of the loss is on mainlands (like Venezuela, Columbia and Panama) rather than islands, although the Bahamas have lost nearly half their mangroves in the past ten years.   People sometimes lament the loss of pretty mangroves, but rarely do they realize just how hard the destruction impacts the reef and the shoreline.   Loss of mangroves effects not just mangrove fishing, but productivity on the reefs as well.

Furthermore, mangroves are slow to recuperate from cutting.   Several species, including Red mangrove, do not re-sprout after being cut.   So, once they have been cut down, they will never return unless replanted.   When a large section of mangrove forest is cut, the roots can no longer oxygenate the oxygen-deficient soil, and the large amount of bacteria in the soil begins to churn out hydrogen sulfide.   This makes the soil extremely acidic.   After that, the soil may not support any mangroves, even if it is replanted.  

Mangroves serve as a buffer zone between the ocean and the shore.   Their roots hold the shoreline together, limiting erosion and attenuating the waves.   Clearing the mangrove forests makes the shoreline vulnerable to the erosive effects of the sea, and significantly hinders the life cycle of fishes and invertebrates which need the mangroves to survive or breed.   The colorful coral reefs and pretty fishes of the Caribbean may get more people's attention than the mangroves, but without the mangroves, life cannot continue to flourish on the reefs as it does now.   It is imperative that as development of coastal areas continues, care is taken not to overlook the importance of the mangrove forests, the nurseries of the reefs.

A mangrove in Utila, Honduras.

The brackish water of a mangrove.

Mangrove roots.

A mangrove seed, often called a "cigar." The root is at the bottom. It will fall off the tree and float root down until it floats into the right depth water.

Seeds which have sprouted.

A leaf showing crystals of excreted salt.

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update 10/22/08