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MANGROVE SWAMP ECOSYSTEMS: Major Characteristics and their Ecological Function BACKGROUND Mangrove is the term denoting any salt intolerant intertidal tree species-often found as four species: Red (Rhizophora mangele) Black (Avicennia germinans) White (Langunecularia racemosa) Buttonwood (Conocarpus erecta) Mangrove swamps generally occur in tropical coastal areas which are subject to tidal fluctuations. Ecologically they develop best in estuaries, behind sand bars, and offshore islands as well as in protected bays with shallow water and mud flats. The soils in mangrove swamps are generally muddy, a common feature of estuarine areas. While some mangrove plant species can be found in fresh water, they develop best in brackish water habitats. FLORA Except for the Button mangrove, all of the mangrove species have special roots which perform mechanical (Red) and aerating (Red, Black and White) functions. In the Red, they take the form of “prop” roots and in the Black and White; they grow upwards from the soil (pneumatophores). Another special feature of the Red and Black mangrove trees is the seedlings (or propagules), which develop on the trees directly after fertilization. Red seedlings are long and torpedo-like, while those of the Black are bean-like. When theses seedlings drop they are either fixed in mud or dispersed by sea currents occurring in the swamp. Generally the mangrove species occupy different habitats with only one or occasionally two species occupying a particular zone. Red mangroves are normally at he seaward margin of the swamp (seaward tidal zone) with the Black on higher ground behind them. White and Button mangroves in turn are present further behind the Black. In Barbados, zonation in mangroves is poor due to the limited quantity and spatial size of that exists. The prevailing mangrove in Barbados is the Red variety. FAUNA Due to the high productivity in mangrove swamps this environment supports a large number of organisms. Mangrove leaves are the foundation of the food web in coastal swamps and account for 90% of the primary productivity. Direct feeders on the mangrove leaves include crabs (e.g. Fiddler crabs), and other detrital invertebrates. Bacteria also degrade the fallen leaves to produce organic rich detritus, a food source for bottom feeders such as starfish and sea urchins and for suspension feeders (e.g. barnacles, oysters, mussels) and certain sedentary worms and small crustaceans. The last two are in turn food sources for many fishes. Bacterial action eventually breaks down all remaining organic material into dissolved nutrients. From this source, mangrove trees again obtain nutrients for further growth. Mangrove fauna can be divided into two groups- animals, which permanently live in the swamp and those which spend only part of their lives there. The permanent dwellers include tree mussels, sponges, oysters and barnacles (which live attached to mangrove prop roots). In Barbados, other residents include several migratory birds (including several species of heron), the Red Seal Coot, Cattle Egret, the Yellow Warbler, several varieties of Tilapia species, and tropical freshwater fish. Other animals, which spend part of their lives in the swamp, include shrimp, mullets, snappers, grunts, tarpon and snook. LOCATION In Barbados, there are few remaining mangrove swamps. The largest viable mangrove stand is found at Graeme Hall Swamp on the south coast. At this location, there is a predominance of Red Mangrove with some White and only a few Button mangroves. There are other locations along the coast where mangroves may be found (e.g. Holetown, Speightstown, and Sandy Lane). However, these are nowhere near as extensive as those found in Graeme Hall Swamp. MAJOR ENVIRONMENTAL FUNCTIONS Mangroves provide four major functions: 1. Aquatic nurseries: Mangrove ecosystems are the breeding ground for many kinds of fish and shellfish. The maze of interlaced roots offers their larvae and juveniles abundant food and protection against larger predators. In addition to the detritus food web mentioned previously, the prop roots of the Red mangroves also develop rich communities of marine invertebrates and algae. 2. Wildlife Habitat: Mangrove wildlife is also abundant and varied. This is a reflection of the complexity and diversity of habitats within a mangrove. Some animals reside in the canopy, the roots, the mud, the associated lagoons and mud flats. Although many of these animals are year round residents, mangroves are also an important habitat for seasonal visitors and others who spend only part of their lives there (e.g. Cattle Egrets nest and roost in mangrove areas but feed inland of mangrove areas). Birds find mangroves rich in food resources and highly suitable for nesting. Wetland birds fees on the great number of crabs, molluscs, fish and other organisms that live there. Each species often specializes in a particular food item or feeding style to utilize the diverse resources available in these environments. 3. Shoreline stabilization: In times of hurricanes and other storm swell activity, mangroves offer good protection to shorelines and anchored vessels. The intricate tangle of springy prop roots and low tangled growth of Red mangrove (the dominant fringe species) makes these trees especially resistant to uprooting by hurricane winds and storm waves. Similarly, they protect coastal areas behind them from severe wave damage and erosion. During quiet weather, the network of prop roots in the shallows accumulates flotsam and fine sediments. These roots slow the flow of water currents, thus tending to cause suspended particles to settle out and deposit in the outer edge of the mangrove fringe. On the landward side the mangroves trap silts that would otherwise make its way to coastal waters. If mangroves are removed the shoreline is eroded by wave action placing large amounts of sediment in suspension therefore increasing water turbidity which can kill or damage coral reefs by both shading and suffocation and they also slow the growth and damage sea grass beds. 4. Water quality: In order to sustain high growth rates mangroves remove large quantities of inorganic nutrients from the “forest floor”. These forests are dependent upon the magnitude of these external sources of minerals to maintain their high productivity. The anaerobic mangrove sediments can also trap heavy metals and pesticides without harm to the trees. As a result mangroves can incorporate, trap or immobilise inorganic nutrients, heavy metals or pesticides that would otherwise flow to the sea, degrading the quality of coastal waters. NATURAL AND HUMAN STRESSES ON MANGROVES Natural stress on the mangrove system is often either by cyclic storms and/or salt accumulation. In both cases in a dry environment, mangrove development is affected. In the former case, the damage encountered is loss of foliage, uprooting and erosion. Full recovery takes several years, however, regeneration is almost immediate due to the mangrove propagules. In the latter case- hypersalinity (where salt accumulates in the soil when the potential for evaporation exceeds the amount of rainfall) the net result is mangrove die off. Human stresses are a result of direct/deliberate human intervention. The recovery from these stresses is much slower and in some cases may not even occur. The principle reason for this is that man’s activity often transforms the physical or chemical characteristics of the site, making it unsuitable for mangrove development or recolonisation. Such stresses include reclamation, charcoal production, overfishing and harvesting, conversion to agricultural lands, coastal development, garbage disposal and pollution. SUMMARY Mangrove swamps are a source of biological productivity in terms of fisheries, aquaculture and forestry as well as ecosystems of biological diversity, since they function as genetic reservoirs and habitats for migratory waterfowl and marine species. Mangroves are great assets which need to be considered for preservation when dealing with coastal development. Mangrove swamps should not be seen as useless areas of vegetation to be cut down indiscriminately for agriculture, housing and industrial development but as viable resources to be developed in a suitable manner. For additional information, contact: The Director Coastal Zone Management Unit Bay Street St. Michael BARBADOS Tel: (246) 228-5950/1/2/3 Fax: (246) 228-5956 Email: email@example.com
"MANGROVE SWAMP ECOSYSTEMS"