These are considered some of the most extensive and structurally complex mangrove ecosystems in South America, covering over 14,000 square kilometers (km2)and reaching heights of 35-40 meters (m). Each stand is continuous, interrupted only by streams and rivers; some of the most well preserved mangrove forests in South America. A intricate preserve network has made conservation of these diverse ecosystems possible with preserves even stretching between countries such as those crossing from Suriname to French Guiana. There are 8-9 distinct forest types each with a distinctive dominating mangrove species or a different combination of mangrove species throughout the stand. The highest biomass is found among Rhizopora species found throughout the ecoregion including the Orinoco Delta area.
Location and General Description
The mangroves of this Guianan mangrove ecoregion lie along the north coast of South America running through southeastern Venezuela, Guyana, Surname and French Guiana between the Oyapok and Orinoco River Deltas. Also found within this ecoregion are the Gulf of Paria and the San Juan River Delta. The great length of coastline covered by this ecoregion leads to variation in climate and rainfall between the extreme ends. For example, rainfall in the Venezuelan section ranges from about 960 millimeters (mm) annually just east of the Gulf of Paria to 3054 mm in the wetter parts of Suriname, with the dry season decreasing from 4 to 0 months accordingly. The mean monthly temperature in the Orinoco Delta ranges from 25.4° C to 27.2° C.
Ranging in elevation from sea level to about 4 m this ecoregion is mainly flat following a narrow strip of Atlantic Ocean coastline. Rises in elevation result from hills formed from sediment deposition and build up; creating sand and shell ridges as well as elevated mud flats which eventually form the clay flats that some mangrove species inhabit. Sandy beaches and mudflats drift westward at the rate of 1.5 kilometers (km) a year in Suriname. Marine currents are primarily east to west motivating the deposition processes along the coast of this ecoregion. Various habitats are present in this coastal ecoregion providing diversity in species with the availability of mangrove ecosystems, salt to brackish lagoons, brackish herbaceous swamps, swamp woods and swamp forests. These swamps are a very important source of freshwater to the mangrove vegetation and other living species of both flora and fauna.
Mangrove forest types and dominant species vary depending on the salinity of the water they grow in or near. Mixed stands of Rhizopora spp. and Avicennia spp. are found specifically on the eastern slopes nearer the far northwestern reaches of this ecoregion; however, in general they grow throughout the ecoregion where there is greater marine influence. Avicennia spp. and Rhizopora spp. are found on more western coasts that have a fluvio-marine influence. Rhizopora spp. and Laguncularia spp. are in the intermediate channels with higher freshwater inputs and Rhizopora spp., Avicennia spp. and Laguncularia spp. grow where there is greater organic accumulation in the soils. Avicennia spp. dominated forests grows in the higher zones while herbaceous swampy vegetation grows in the depressions where marine influence is greatest. A significant patch of mangroves (45,000 ha) is found in the Forest Reserve of Guarapiche in the San Juan River, where mangroves are found also in association with palms and swamp forests.
There is great landscape diversity among the eastern deltaic plains. The Gulf of Paria is an alluvial plain with sedimentary marine deposition while the San Juan River and Gulf of Paria are alluvial plains characterized by deltaic swamps with marine-fluvial deposition. The Orinoco Delta, is an alluvial plains swamp delta in which deposition is also predominantly fluvial.
The Orinoco Delta is a very diverse, large and active sedimentary fluvial plain within this ecoregion. Rhizopora mangle, R. harrisonii and R. racemosa associations are found in the mid-delta where the marine influence is greatest and salinity the highest. While Rhizopora spp. dominate the lower delta, in areas of low marine influence, where they are associated with the swampy palm (Mauritia flexuosa) in recently deposited peat bogs. Other habitats in the ecoregion with which they are associated are swampy forests that include Pterocarpus officinalis, Symphonia globulifers, Euterpe oleracea and Roystonea sp. In the inner estuary, where there is greater marine influence, Rhizopora forms wide belts, Avicennia is found on higher ground either alternating with Rhizopora or in pure stands where the muddy areas are associated with Pterocarpus officinalis, Symphonia globulifera and Vitex sp.
In eastern Venezuela, herbaceous muddy vegetation grows in flooded areas, close to mangroves. In the muddy vegetation associated to these mangroves, big hydrophilous herbs, mainly grasses and forbs, and musaceae (Heliconia latispatha) predominate. Costus arabicus, Cyperus giganteus, Eichornia crassipes, Heliconia psittacorum and the palms Roystonea regia and Mauritia flexuosa also are dominant species. There are also extensive muddy forests, structured by middle height (from 15 to 25 m) individuals, with only one dense arboreal stratum, on almost permanently flooded ground. Pterocarpus officinalis, Symphonia globulifera and Tabebuia aquatilis, an endemic species of the coastal zone between the Amazon River and the Orinoco River, predominate. Other remarkable species are the palms Euterpe oleracea, Manicaria spp., and Mauritia flexuosa, which are exploited semi-industrially by extraction of economically valuable palm hearts.
Tidal range in the Orinoco Delta is up to 1.3 m at Pedernales. Mean annual fluvial discharge of the Orinoco River is 36,000 m3 each second, which transports 200,000 tons of sediment yearly. Associated habitats depending on the terrain are deciduous forests, herbaceous muddy vegetation (or muddy meadows), and muddy forests. All of the Guianan mangroves are found on a highly dynamic coast resulting in their instability due to strong coastal currents that carry heavy sediment loads from the Amazon River along the coast. Occasional mass mortality of mangrove vegetation has been known to occur for unknown reasons, followed by heavy coastal erosion. The mangrove belts are up to a few meters width; containing 2 species, well-developed stands of Avicennia germinans exists behind Laguncularia racemosa on the seaward edge, mixed with the salt marsh grass species of Spartina.
Along the Orinoco Delta Rhizopora spp. are found in flooded acid soils, with high organic matter content and low salt; however, they are better developed on slightly elevated silty-clay-loam textured soils. Mixed species are found in higher salinity areas. In the transition zone, they are found in association with the muddy palm.
The biodiversity of this ecoregion is very rich with its unique species of both flora and fauna. The ecoregion is highly noted in its role as a wintering area for migrating shorebirds from Nearctic regions, the most in all of South America. In Suriname and Venezuela alone there are more than 118 species of birds, including more than 70 species of waterfowl and as many as 5 million individuals. The shorebirds however, whether breeding, nesting, passing through or staying for the winter, out number all other species of fauna. The South American endemic, scarlet ibis (Eudocimus ruber) breeds in this ecoregion along the coast of Suriname. Notable species of birds also found in this ecoregion are black skimmer (Rhynchops niger), gull-billed tern (Sterna nilotica), shot-billed dowitcher (Limnodromus griseus), lesser and greater yellowlegs (Tringa flavipes and T. melanoleuca), black-bellied whistling duck (Dendrocygna autumnalis) and tricolored egret (Egretta tricolor) just to name a few from the list of species named as internationally important for the Suriname coastal area.
Mammal, reptile and amphibian species also add to the outstanding diversity of this ecoregion while utilizing the resources present in the array of habitats within and surrounding these mangrove forests. There are over 50 species of mammals present including opossums, bats, primates such as capuchin monkeys (Cebus apella), squirrel monkeys (Saimira sciureus), howler monkey (Alouatta seniculus) and Guianan saki (Pithecia pithecia), giant ant-eater (Myrmecophaga triactyla), carnivores including jaguar (Panthera onca), puma (Puma concolor), and ocelot (Leopardus pardalis), ungulates and rodents including the capybara (Hydrochaeris hydrochaeris). Sea turtles including the endangered olive ridleys turtle (Lepidochelys olivacea) nest on the sand and shell beaches. Other reptiles that frequent this ecoregion are the green iguana (Iguana iguana), spectacled caiman (Caiman crocodilus) and anaconda (Eunectes murinus). Two very unique amphibians manage the saline conditions of this ecoregion including the paradoxal frog (Pseudis paradoxa) and the pipa frog (Pipa pipa).
The most extensive areas of mangrove stands are found in the Venezuelan part of this ecoregion around the Orinoco Delta (46,802 km2) while French Guiana holds 55 km2 but that varies as does the coastline, Surinam holds 1,150 km2 and Guyana holds 800 km2. There are a lot of protected areas in this ecoregion, which has resulted in large intact blocks of mangrove forest and other natural ecosystems. Some fragmentation has occurred due to the alteration of habitat for agriculture. Most fragmentation in this ecoregion however, is due to the special needs of mangrove ecosystems and uncontrollable natural forces such as the coastal currents.
Suriname has large intact mangrove ecosystems with at least 7 protected areas, many of, which have some connectivity increasing their contributions to the environment as a whole. The mangrove trees and shrubs are found in the fringes along the coast, behind wide mudflats and are well-developed estuaries. Some of the protected areas of highest importance include Coppename Monding, the Wia-Wia nature reserve; Bigi Pan multiple-use management area recently proposed for protection. Coppenamemonding, a wetland of international importance in northern Suriname, is noted for breeding, nesting, passage and wintering waterbirds. An occasional breeding area for Eudocimus ruber, up to 3 million individuals of Calidris Pusilla found during northern winter; mixed breeding colonies of herons and egrets. Suriname/Ramsar site is one other protected area of great importance due to the amount of diversity it is capable of holding due to its variation of vegetation including a wetland complex that consists of mudflats, mangrove forests dominated by Avicennia germinans, sand and shell ridges alternating with swamps, narrow sandy beaches, saline and brackish lagoons.
In French Guiana mangroves are intact but spread out due to sedimentation and erosion along the coast which forms many different habitat types. Mangroves are found in fringe communities as a thin band along the coast, interrupted by sandy beaches and rocky headlands. Laguncularia racemosa is found along the seaward edge mixed with the salt marsh species such as Spartina brasiliensis and Avicennia germinans. Along the riverbanks, Rhizopora mangle with Pterocarpus officinalis indicates the change to freshwater swamp forest.
In Guianan mangroves, ecosystems are significantly depleted but large stands are still found between Pomeroon and Waini Rivers where population density is lower. Avicennia germinans is dominant on the coast, growing up to 25m high while Rhizopora mangle persists in the more sheltered areas.
Venezuela holds the Orinoco Delta one of the largest and most diverse mangrove tracts within this ecoregion, which comprises 30,000 km2 of wetlands in Venezuela 77% of the nation’s wetland areas: Parque Nacional Mariusa (2,650 km2) and the Orinoco Delta Biosphere Reserve (8,765 km2).
Types and Severity of Threats
Destruction of mangrove ecosystems including the cutting and removal of all vegetation is the greatest threat to this ecoregion. Unfortunately people remove the mangrove ecosystems to use the wood, leaves and fruits for building material, firewood, charcoal, tanning leather and medicinal purposes. Trees are also removed to free up land for agricultural crops such as rice which when added to additional threats including high input of fertilizers and pesticides to the environment ecosystems throughout the area are affected. Removal of vegetation actually adds pressures to downstream vegetation due to the influx of sediments that result from the erosion that follows land clearing in this water oriented ecoregion. Tourism and urban expansion are increasing the pressures put on the limited resources of these countries increasing reliance on the mangrove habitats for food including the hunting of wildlife, grazing of domestic animals. The mangrove ecosystem has long been a provider of economic income to the people that harvest from its waters however, with the beginning of shrimp farming production is up along with income but this practice is not sustainable as it changes and degrades the natural habitat for all species including the shrimp.
Threats to the Orinoco Delta mangroves are a great concern as the largest intact tract of mangroves within this ecoregion. As early as 1926, it was pointed out that mangrove extraction in this area was very active and that it was desirable to adopt conservation measures. During the 70’s, the Venezuelan Corporation for Guayana Development (CVG), a state-owned company executed hydraulic works to reclaim flooded grounds for agriculture, although according to other sources, the real purpose was related to the exportation of minerals. Engineering works were not very effective due to the complex hydrodynamics of the Orinoco Delta waterways. Additional works solved such a problem; however, soils have been modified to such a extent that now they are not fit for agriculture. The present hydraulic regime could have unpredictable effects on mangroves. Oil prospecting could constitute the main hazard for those mangroves in the future.
Additional information on this ecoregion
- For a shorter summary of this entry, see the WWF WildWorld profile of this ecoregion
- To see the species that live in this ecoregion, including images and threat levels, see the WWF Wildfinder description of this ecoregion.
- World Wildlife Fund Homepage
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