Mangrove Learning Center

     When many Americans are asked to describe the seashore, they often illustrate a windswept sandy beach or cobble stone and rock-strewn shore. Indeed, this is a common perception of the seashore for people living in the northern and southern parts (latitudes) of the world. If, on the other hand, this question is asked of a local of the Caribbean, parts of South America, or even the Florida Keys (USA), they may give a very different answer. People living in these areas may instead describe a dense ring of forest hugging the coastline and reaching all the way to the water. The trees that make up these strange beach-front forests are collectively called mangroves.
     Mangrove forests make up one of the most unique ecosystems on earth in that they thrive where no other trees can survive, the transition zone between the ocean and land. The mangroves, in turn, provide shelter for an incredible variety of terrestrial and aquatic organisms. This strange idea of a saltwater forest brings up three questions: (1) What exactly is a mangrove? (2) How can mangroves survive in saltwater? and (3)What role do mangroves play in the ocean environment? Before we tackle the last two questions, lets answer the first one.
     What exactly is a mangrove? Mangroves are salt tolerant (halophytic) dicot trees that live in areas saturated with salt water at least part of the time. A dicotyledenous tree is one that has two seed leaves instead of one. An oak tree is an example of a dicot tree while a palm tree is an example of a monocot or one seed leaf tree. Mangroves are found on coastlines and islands all over the world between the latitudes of 32 degrees North and 38 degrees South. In these areas mangroves cover about 25% of the coast line in the Caribbean and up to 75% of the coastline in other parts of the world such as Asia and South America. There are many different kinds of mangroves. There are 16-24 families and 54-75 species of mangroves, depending on which scientist you ask. Asia lays claim to the greatest diversity, or total number of species, of mangroves. The New World (North and South America) has only 12 species of mangroves. If we focus on North America and the Caribbean, the number drops even more. The U.S. and Caribbean have only four species, three if you get picky about it (more on that later).
     Now that we have some idea of what a mangrove is and where we can find it, lets answer those other two questions; (2) How do mangroves survive in saltwater? and (3) What role do mangroves play in the ocean environment? Along the way we can also look at the different species of mangroves found in the U.S. and Caribbean.
      How do mangroves survive in saltwater? Ever wonder why normal trees only grow well away from the beach, much less in the water? The answer is salt, oxygen (O₂), and water. First, let us focus on salt. Obviously, saltwater is very salty and normal trees cannot tolerate being soaked in it. Perhaps less obvious is the fact that the soil near the sea may be saltier than the ocean water itself! How can that be? Simple: During each high tide, some water soaks into the soil, evaporates and leaves salt. During the next tide, the same thing happens and more salt is deposited. After several high tides, salt in the soil reaches very high levels. The only way that this salt can be removed from the soil is through rainfall. As a result, in periods of low rainfall, the amount of salt in these soils can reach extremely high levels. If the tree takes up too much salt from the water or soil, this salt can damage and dehydrate the cells inside the roots, branches, and leaves. Any tree growing in these conditions will need to either keep salt out or get rid of excess salt. Most trees cannot do this.
     Mangroves have several adaptations that allow them to live under high salt conditions. Mangroves possess salt pores on their leaves that can excrete salt that is taken up by the roots and trunk. Some mangroves avoid the salt problem all together and have roots and a trunk that are impermeable to salt. Finally, all true mangroves display a unique reproductive mechanism known as viviparity. Instead of releasing seeds, which would soon get soaked with salt and die, the seeds of mangroves germinate while still attached to the tree. The fully developed seedlings are then dropped from the trees. These baby mangroves float root side down in the water until they reach the shallows near shore and take root.
     Now lets look at oxygen. The soil near the shoreline is very low in oxygen. This soil is full of decaying organisms and plant leaves. The bacteria and other microfauna that feed on this decaying material use up most of the available oxygen in the soil. At this point, you may ask what's the problem? Trees produce oxygen! True, trees produce oxygen as a byproduct of photosynthesis that occurs in the leaves. The problem is that it is a long way down from the leaves to the roots! As a result, the oxygen produced during photosynthesis does not get to the roots. Therefore, roots from normal trees would not get enough oxygen in this habitat and would die. Any tree living in this zone has to possess some mechanism to allow extra oxygen to get to the roots.
     Mangroves are well adapted to areas with low oxygen, including the shallow water where mangroves first appear and the land beyond the shoreline. Mangroves possess several structures that compensate for low oxygen in the soil. Some, such as the Red Mangrove, have structures called prop roots to collect oxygen. Prop roots are large wide roots that begin above the surface of the water and soil. Others, such as the Black Mangrove, possess straw-like roots (pneumatophores) that penetrate the soil and are exposed to the air. Most mangroves also possess slits in the bark and roots called lenticils. These slits, coupled with airways inside the trunk and roots (aerenchyma), allow oxygen to get into the root system while keeping salt out.
     Now we can consider water. Although there is lot of water available along the coast, little of it is fresh water. Fresh water input into this habitat includes rainfall, floods, and river input. All of these sources provide fresh water to the coast, but much of the freshwater is quickly lost to the ocean. Indeed, much of the river input is actually brackish water. Any tree living in this zone must therefore be able to conserve fresh water. Fresh water can also be obtained from filtered seawater, but this is difficult and takes a great deal of energy. Mangroves are well adapted for water conservation. The leaves of mangroves are very tough and have a waxy covering. This waxy covering minimizes evaporative water loss from the leaves.
     Many mangroves are specialized to live in specific areas along the coastline. Some mangroves are better at handling soaking with water, while others are better at handling drier and saltier soil. In other words, some mangroves grow best in the water while others grow better away from the water. Therefore, along the water you may see only one kind of mangrove. If you were to look deeper (landward) into the mangrove stand you would see another type of mangrove take over as the soil becomes drier. Beyond this second type of mangrove, another type that is more salt tolerant would grow and so on. This layering of different mangroves suited to different water/soil conditions is known as zonation. We will come back to this topic soon.
     Now lets address the third question. What role do mangroves play in the ocean environment? Mangroves are important in island formation, serve as a buffer zone between the ocean and land, and support a wide array of life. Many island chains around the world contain mangroves. These islands are also undergoing constant changes, some growing smaller and some growing larger. Many people are well aware that some islands can grow through volcanic eruptions and the movements of underwater plates on the ocean floor. You might be surprised to know that mangroves can also make islands!
     As we have already discussed, mangrove seedlings drop from their parents and can float in water. In a process called dispersal, some of these seedlings can float between islands. In some areas, wave action results in the temporary accumulation of sand bars. When mangrove seedlings reach these areas they can take root. When the roots of these mangroves grow, they help stabilize the sand bar. Over time, many mangroves may occupy this sand bar and prevent the waves from washing the sand bar away. The end result of this process is a small island that may continue to grow with additional mangrove colonization.
     Mangroves also serve as important buffers of wave action and weather. In both island and continental areas, mangroves cover much of the waterfront. These forests protect the inner parts of the islands from wave action caused by regular storms and even hurricanes. Mangroves are so efficient at protection against weather, that boats are regularly placed in mangrove lagoons when there is the threat of a hurricane.
     Mangrove forests provide both shelter and a food source for an incredible diversity of life. This process begins when mangrove leaves fall into the water or soil. These fallen leaves are then colonized and decomposed by bacteria. In turn, the decaying leaves and bacteria serve as a food source for plankton. The decaying leaves also release nutrients that support the growth of marine algae. These plankton and algae serve as a food source for many small marine and terrestrial organisms.
      The organisms living in association with mangroves can be divided into two basic groups, air dwellers and water dwellers. First, let us consider the underwater dwellers living among the prop roots of the red mangrove. The organisms that live in association with mangroves include members of several different taxonomic groups ranging from bacteria to algae and animals. Some of these organisms settle on and attach to the mangrove roots. Once attached, these animals cannot move about and are called sessile. Examples of these are algae, encrusting sponges, feather duster worms, anemones, oysters, barnacles, and tunicates. Mobile organisms living among the mangroves include worms, jellyfish, periwinkles, crabs and shrimp, sea cucumbers, and many juvenile fish species.
      Now let us look at the air dwellers that live in association with the mangroves. Terrestrial organisms living among the mangroves include various arthropods (including insects and crabs) and several species of birds. Many insect species frequent the cover provided by mangroves, including our friend the mosquito. Several terrestrial crab species also frequent the mangroves. Some, like the mangrove tree crab (Aratus pisonii), are arboreal species specialized for climbing trees. Others, such as the White or Land Crab (Cardisoma guanhumi), build burrows in the soil beyond the waters edge. Various bird species also take advantage of the food and cover provided by the mangroves. Some birds are so commonly seen in the mangroves that the word mangrove is incorporated in their names. One such bird, the Mangrove Cuckoo (Coccyzus minor), is a common visitor to the mangrove forests, feeding on the readily available insects. Other common denizens include the Belted Kingfisher (Ceryle alcyon), several heron and egret species (Family Ardeidae), and the occasional ibis (Family Threskiornithidae).
     With these two questions answered, we can now learn about the four different kinds of mangroves in North America and the Caribbean and also look at zonation more closely (in case you are still confused). Imagine you are in a boat docked right next to a mangrove stand in the Caribbean. Let us take a hypothetical walk from the waterfront towards dry land. The first mangrove that you will see is the Red Mangrove (Rhizophora mangle). This mangrove is very easily identified by the large prop roots that stick out of the water. These prop roots look like very tall, arched versions of the roots you see on big old trees. Prop roots, because they stick out of the water, allow the tree to get oxygen. As an added bonus, they are also impermeable to salt. This species is more tolerant of submersion than any of the other mangroves. It is therefore more suited to living right on and next to the water. The leaves of this tree, like all mangroves, are well suited for keeping water in. This species has elliptical waxy green leaves that limit evaporative water loss.
      As we wade through the shallow water and onto the wet soil, the Black Mangrove (Avicennia germinans) replaces the Red Mangrove. This tree is less tolerant of water and is found in areas that only get wet during high tide. It is, however, more tolerant of the lower oxygen levels found in the soil beyond the waters' edge. This species is easily identified by the presence of many specialized roots (pneumatophores). Pneumatophores resemble thin straws stuck in the soil. The many pneumatophores that each tree has serve as airways to provide oxygen to the roots. The leaves of the Black Mangrove are dark green on top and pale grey underneath. The leaves on this species are often encrusted with salt. In fact, if you wipe off the salt, new salt will be excreted within minutes. This species is not impermeable to salt like the Red Mangrove, and must therefore excrete salt from the leaves and roots.
     As we move even farther inland, the soil becomes too salty for the Black Mangrove. It is here in the muddy soil beyond the high tide line that the White Mangrove (Laguncularia racemosa) grows. This species does not need specialized roots to get extra oxygen, but instead needs to get rid of the large amount of salt that accumulates inside the roots and trunk. This tree has two salt glands at the base of every leaf and excretes large amounts of salt. This tree can be identified by its lack of specialized roots, salt glands, and green velvety flowers that look like ribbed bags. The leaves of this species, like the first two, are dark green and oval shaped.
     Finally, as we walk past the White Mangrove stands we reach the Buttonwood (Conocarpus erectus). This unfortunate tree is considered a mangrove imposter by many scientists because it is not very tolerant of salt, is not viviparous, and lacks specialized roots. Instead, it is considered a mangrove associate. Despite this cruel treatment, this tree is commonly found in mangrove stands and seems to be tolerant of the low nutrient soils near the shoreline. This tree looks very similar to the other three species. The leaves are, however, different. First, while similar in color, the leaves are very narrow and pointed. In addition, the leaves on this species extend from the branches in an alternate pattern. In the three other species, leaves on either side of the branch extend directly across from each other. Finally, the flowers of this tree resemble little buttons, hence the name buttonwood.
     We now have an understanding of how interesting and important mangrove forests are. These trees are important in island formation, serve as a buffer against hurricanes and wave action, and are a haven for many species of organisms. Unfortunately, these important ecosystems are threatened by various activities of man. The two major threats to the mangrove ecosystem are deforestation and pollution.
     Mangrove deforestation has many causes. They include coastal development, timber harvesting, shrimp farming, and flooding. Development of hotels, resorts, and water front properties results in the clearing of mangrove forests during construction. Many mangrove species are also sought after as exotic woods for furniture and other uses. This too, results in the cutting down of many mangrove forests. In many parts of the world, especially South America, shrimp farming is a large enterprise. Unfortunately, many mangrove forests are cleared to make space for large shrimp ponds. This practice not only results in the cutting down of mangroves, but also results in the production of vast quantities of waste from the shrimp farming itself. This can result in large algal blooms that are harmful to nearshore animals. The construction of dikes, ditches, canals, and causeways can result in flooding that prevents mangrove roots from obtaining oxygen.
     Pollution is the other major threat to the mangrove ecosystem. There are two types of pollution that can affect mangroves. They are point-source pollution, and non point-source pollution. Point-source pollution is any discharge that can be isolated to a specific source. This includes industrial discharges and oil spills. Mangroves have been found to be particularly sensitive to oil. This is most likely because oil fouls the specialized lenticels, aerenechyma, and pneumatophors of mangroves. Non point-source pollution includes discharges that come from multiple sources. Examples of non point-source pollution affecting mangroves include agricultural, shrimp farming and pesticide runoff.
     Luckily, there are many initiatives aimed at maintaining the mangroves we have. Many countries have initiated management plans that make it illegal or very difficult to cut down mangroves. Some countries have also established initiatives that address several factors affecting mangroves. These include rebuilding canals and storm water drains, minimizing agricultural runoff, installation of pollution control devices, and regular monitoring of water quality. In addition, alternatives to shrimp farming have been initiated. One of the most promising alternatives is silvofisheries. This kind of mariculture actually takes advantage of the ability of mangroves to support many species of organisms. Instead of cutting down mangroves, fish and shrimp are grown within existing mangrove stands. Another alternative includes using shrimp ponds that have been abandoned due to shrimp disease or loss of productivity. Here, other animals such as grouper are grown instead. Other initiatives include efforts to restore the mangroves that have been lost to deforestation. Many countries have initiated reclamation projects in which mangroves are planted in abandoned shrimp ponds and other areas where mangroves have been cut down.
     Perhaps the most important way to protect mangroves is through education. Hopefully this article has helped to dispel the notion that mangrove forests are bug infested, useless swamps. They are instead a fascinating and very important resource for us to study, appreciate, and preserve. They are an important nursery for fish and other organisms, process nutrients, and protect islands from wave and storm action. We need to make the effort to find ways to live alongside this natural resource.

 


                David M. Cabrera
                Marine Science Instructor
                SCUBA Instructor
                Odyssey Expeditions
                12804 Goldstone Ct
                Silver Spring MD 20904
                e-mail: