St. John USVI Environment: Mangroves
How Mangroves Depend Upon the Environment
The term mangrove loosely describes those tropical trees or shrubs that are specially adapted to grow in salty, wet and muddy environments, such as the shallow waters of calm bays, the periphery of salt ponds, and within marshes and wetlands that are exposed to flooding and salt water intrusion. This is an extraordinary adaptation. Salt is generally abhorrent to plants of any variety. For example, when Union Civil War General William T. Sherman made his infamous "march to the sea", he salted the fields of southern farmers, thus destroying the crops and rendering the farms useless for years to come. Mangroves not only have to withstand the rigors of a saltwater environment, but they also have to be able to hold firm in the loose and oxygen-poor soils characteristic of these locations.
The red mangrove proliferates along the shorelines of shallow calm bays, both on the muddy shore and in the water itself. The red is the classic mangrove characterized by its numerous arch-shaped roots that start at the base of the tree and arch out and down into the water and mud. It also has distinctive seeds that at maturity look something like foot-long red pencils, which emerge prominently from the center of the mangrove's leaf clusters.
When the red mangrove seedling matures, it falls from the tree into the water. Here, it can stay alive and afloat for up to a year. As the seedling moves at the mercy of the winds and gentle currents, it begins to develop. Tiny side roots emerge from one end of the seed and small leaves on the other. The root end of the seed absorbs water and becomes heavier than the leaf end. The added weight tips the long seed, turning it leaf end up and root end down.
In order to survive and begin to reproduce, the root end of the seed needs to reach shallow enough water so as to obtain at least a tenuous foothold in the mud. Then it must enjoy calm enough water conditions so that it will not be moved until its roots have a chance to secure themselves to the soil. To achieve these conditions the mangrove is dependent on the marine environments of the coral reef and underwater grasslands as well as upon the geography of the region.
The sticky mud that serves to catch the root end of the seed is a mixture of dirt, organic debris that is washed down to the bay from land during heavy rains, and sand, which is a product of the coral reef. The sediments and organic material provide nutrients and stickiness while the sand provides stability and substance for the mixture.
The necessary ultra-calm water conditions are also produced by a combination of environmental factors. In the Virgin Islands, bays are formed at the bottoms of valleys by the mountain ridges that line the sides of the valleys. These ridges extend further out to sea than the center of the valley and form the rocky headlands that we call points. The headlands protect the bay from the full force of the ocean. Coral reefs, growing along the edges of the headlands where the bay is open to the sea, provide an effective barrier against waves and currents. Sea grass beds growing within the center of the bay further calm the water. The numerous blades of grass present a large surface area through which bottom currents must pass, thus slowing them considerably. The proper combination of all these environments may result in a body of water so tranquil that barely a ripple can be seen for days at a time - conditions quiet enough to allow the red mangrove's pencil-sized seed the time it needs to root and mature.
Observing the developments of the mangrove seedling, we can see how much the mangrove depends upon its neighboring terrestrial and marine environments to reproduce. Correspondingly, these environments also depend on the mature mangrove for their survival.
How the Environments Depends Upon and
Benefits by the Mangrove Forest
In addition to preserving the coastline, mangroves promote the health of the coral reef and seagrass beds by protecting them against their most insidious enemy, turbidity or water cloudiness. During heavy rains, water flows down the slopes of mountain valleys into rocky streambeds that we call guts. The guts channel the water directly into the sea or sometimes into marshes and salt ponds near the coast. The water carries earth, pebbles, organic debris, like old dead leaves and twigs as well as whatever else is in the way of the stream. This is a potential problem because this debris-laden water, called runoff, can make the seawater turbid and coral reefs and sea grass cannot long survive in cloudy water.
Here, mangroves come to the rescue. Their roots act like a filter, trapping the runoff debris within their thick and tangled web. This prevents the sediment-laden runoff from flowing directly into the ocean where it would cause widespread turbidity and reef damage. Moreover, the mangrove turns this potential problem into a vital resource. The runoff organic matter meets up with literally tons of mangrove leaves that have fallen in the water. Trapped within the tangle of mangrove roots, this organic stew are broken down by microorganisms and turned into suitable food for other creatures that feast on the rotting debris as well as upon the microorganisms themselves. Thus, the underwater mangrove forest becomes a world in itself, providing a rich and plentiful habitat for countless species of baby fish and tiny sea creatures that also find sanctuary amidst the intricate maze of protective roots.
Another interesting result of the debris-filtering nature of mangrove roots is that not only do they prevent loss of land due to coastal erosion, but they actually cause the shoreline to expand thereby creating more land. As the trapped sediments and debris are broken down and stabilized, they build up and gradually rise above the surface of the water. This new dry land will eventually be colonized by other plant species that, although not as salt-tolerant as the red mangrove, are better suited for life on dry land. The red mangrove responds to this inland competition by simply moving farther out to sea, and little by little the size of our island increases.
In these numerous ways, mangroves are truly guardians of the shoreline. They protect the coast from erosion and hurricanes, the coral reef and the sea grass beds from turbidity, the tiny sea creatures from large predators, and turn potentially harmful runoff into essential nutrients for the marine community and into solid land for the expansion of St. John.
How the Mangrove Adapts to Its Harsh Environment
The red mangrove, or Rhizophora mangle, usually grows in shallow salt water. Their arch-shaped prop roots, in conjunction with drop roots that descend from the branches, provide firm support in the loose underwater soil. These roots are also the mechanism for this plant's ability to tolerate salt water. They act like a living desalination system; they have the ability to allow water in while filtering the salt out. Although this process is more than 99% effective, the red mangrove is not entirely independent of the need for fresh water. Eventually, the small amounts of salt that pass through the filtration system build up within the plant tissue and can become toxic. Thus, the red mangrove, as well as other mangrove varieties, needs an occasional flood of fresh water, which could come from a prolonged rain, a gut wash, or even a hurricane.
The underwater soil in which the red mangrove grows is notoriously oxygen-deficient. The action of bacteria and other microorganisms on the vast accumulation of organic material trapped within the root system uses up almost all of the available oxygen. This is a condition that would present an insurmountable problem for other plants, because roots must have oxygen in order to survive. Undeterred, the red mangrove adapts to this situation by utilizing the portion of its roots that lie above the water's surface to draw the necessary oxygen directly from the air itself. Air enters the roots through small openings called lenticles, finding its way down to the lowest portions of the roots by traveling through specialized air ducts. When the roots lie beneath the surface, however, a specialized greasy coating keeps the water out.
The black mangrove, Avicennia germinans, is easily identified by little sticks, called pneumatophores, coming out of the mud around its trunk. These are actually part of the black mangrove's roots and serve two purposes. Most importantly, they act like snorkels bringing fresh air to the majority of the root that exists in the oxygen-depleted environment underwater and underground. Secondly, the pneumatophores help to anchor the black mangrove to its tenuous foundation of loose mud. The red mangrove's lenticles and the black's pneumatophores are extremely sensitive to greasy contaminants, which can clog up the openings. They are, therefore, at particular risk from oil spills.
The black mangrove is less tolerant of salt than is the red and cannot live its entire life in salt water. It is, therefore, usually found behind the red mangrove or on the shore side of salt ponds or marshes. Like the red, this mangrove excludes salt at the roots, but at a 90% efficiency instead of the 99% capability of the red. The salt that enters the black mangrove tissues is excreted by salt glands on the upper surface of the leaves. If you hold a black mangrove leaf up to the sunlight, you will see the salt crystals on the leaf.
white mangrove, Laguncularia racemosa, is the least salt-tolerant
of the three mangrove varieties and cannot withstand prolonged
periods in flooded conditions. For this reason white mangroves
are usually found on drier land than are its red and black cousins.
On St. John white mangroves are particularly plentiful around
the edge of salt ponds and along guts where they open to the
The next time you pass the mangroves along the Leinster Bay Road or in town around Frank Bay Pond or elsewhere on the island, take a brief pause. Have a good look at these guardians of the shoreline, which quietly and modestly provide for the orderly flow of life between land and sea and are such an integral part of both our marine and terrestrial environments.