Neotropical Mangroves

TROPICAL BIOLOGY AND CONSERVATION MANAGEMENT - Vol. IV - Neotropical Mangroves - L.D. de Lacerda

NEOTROPICAL MANGROVES L.D. de Lacerda Instituto de Ciências do Mar, Universidade Federal do Ceará, Fortaleza, Brazil Keywords: Mangroves, tidal environment, biogeochemistry, human impacts, climate changes

adaptations,

biogeography,

Contents

N E rE S S di PE CO to C E r D s/A IM OL R u E S AF th N S T ors on ly

1. Mangroves extension and distribution in the world 2. Mangrove flora and the origin of the Neotropical mangroves 3. Mangrove associated fauna 4. Ecology 5. Mangrove products and services 6. Environmental impacts on mangroves 7. Conclusions Glossary Bibliography Biographical Sketch Summary

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The last few decades have seen a rapid growth of interest in mangroves. This results from the perception that mangrove ecosystems are valuable resources and provides many environmental services at the local and regional scales, and at a global level in a scenario of climate changes. Mangroves are adapted to the rash conditions of the intertidal frame and therefore are ideal models to the study of different plant physiologies and adaptative mechanisms; also mangroves respond rapidly to global climate change and may be used as proxies to its impacts on the coastal region. Unfortunately, there is an accelerated rate of destruction of mangroves worldwide and nearly all maritime countries in the tropics and subtropics are concerned with the conservation and sustainable management of this natural wealth. Surreptitiously, however, many nations permit the misuse of mangroves for quick monetary returns. The scientific community and decision makers worldwide are now in a state of alert, requesting sound directives for the sustainable use of mangroves. The present chapter is an attempt to present a concise information of the botany, ecology, threats and management of mangrove ecosystems, aiming a rapid and useful source of major facts of interest to decision makers and to all those who see the ecological and economical value of mangroves. 1. Mangroves extension and distribution in the world Mangroves are the dominant vegetation form colonizing protected coastal areas and estuaries throughout the tropics and sub-tropics. Mangroves are restricted to tidally influenced wetlands. However, they can also occur in areas without a tidal regime e.g. in some choked coastal lagoons and along rivers upstream to the saline intrusion. Some few examples of inland mangroves have been reported for areas where carbonate

©Encyclopedia of Life Support Systems (EOLSS)

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geology (e.g. some Caribbean islands) allows underground saline waters to penetrate far from the tidal fringe. Generally speaking, mangroves refer to salt-tolerant marine tidal evergreen forests that include trees, shrubs, palms, epiphytes and ferns in most areas in tropical and subtropical latitudes.


Mangroves provide important services and goods, mostly related to coastal protection, conservation of biological diversity and provision of habitat, spawning grounds and nutrients for a variety of fish and shellfish. In certain areas mangroves are significant sources of construction wood, fodder and wood products, as well as charcoal and fuel wood, medicine, honey, tannins, dye and other forest products. Also, mangroves are important carbon sinks in coastal areas, mostly associated with their high standing biomass and the accumulation of carbon-rich sediments, where sub-oxic to anoxic conditions inhibit complete oxidation of organic matter. The high concentration of tannins in mangrove biomass and high sedimentation rates typical of these ecosystems, also favour the rapid burial and slow decomposition of organic matter augmenting their role as long term carbon sink (Lacerda, 2002). Below sectors of the Great Barrier Reef in Australia, large deposits of poorly decomposed mangrove peat has been recently found. This material was buried and eventually covered by more recent coral reef sedimentation following past sea level rise about 9,000 year ago. The degree of preservation of this peat material suggests very slow decomposition confirming the role of long term sink played by mangroves. Notwithstanding their ecological significance, mangrove forests throughout the world are suffering impacts from many anthropogenic activities, which resulted in a steady decrease in their extension in most countries, reaching a world average forest loss of about 20% during the past 25 years (FAO, 2007).

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Mangroves occupy about 70% of tropical and subtropical coasts between latitudes 35ºN and 38ºS, although their best development in terms of biomass, productivity and biodiversity occurs between latitude 10ºN and 10ºS, where climate conditions and freshwater supply are most favourable. Global extension of mangrove forests reaches about 16 million hectares, with about 29% in the Americas, including the Caribbean, 20% in Africa and 51% in the Indo-Pacific region. Global extension 106 ha Neotropical mangroves Atlantic coast 2.52 Pacific coast 1.21 Caribbean 0.81 Americas 4.54 Africa 3.24 Asia 6.05 Oceania 2.02 Total 15.85

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Region

% of the world total

Source

Lacerda (2002) 28.7 20.4 38.2 12.7 100

Spalding et al. (1997) FAO (2007) FAO (2007) -

Table 1. Extension of Neotropical mangroves and the global distribution of mangrove forests area based on the available most recent and reliable estimates obtained by remote sensing mapping.


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In the Americas abundant rainfall and extensive fluvial systems favoured the development of extensive and broad forests along coastal plain estuaries of the Pacific coast, from Honduras to northern Ecuador, and also in the Atlantic coast, along what is probably the world’s most extensive near-continuous mangrove coastline extending for nearly 3.000 km, including the Orinoco Delta, the entire coastline of the Guyana, Sironame and French Guyana and the coastline of northern Brazil to Maranhão State, which includes the world’s largest continuous mangrove stand located along the Maranhão-Pará coast in Northern Brazil covering about 0.7 x 106 ha, being considerably larger than the Sundarbans of Bangladesh, the second larger continuous mangrove belt in the world, with about 0.5 x 106 ha (Spalding et al., 1997).


Mangrove area in the neotropics is largest along the Atlantic coast (2.52 x 106 ha) where nearly optimal environmental conditions (climate and freshwater supply) occur. The Pacific coast with 1.21 x 106 ha harbors less than 50% of the total area along the Atlantic Ocean. This more restricted distribution is caused by climatic constrains generated by singular oceanographic conditions resultant from the Humboldt Current suppressing connective activity and creating an arid climate sector southward from 3o30’ S of latitude in northern Peru. High soil salinity and nearly no freshwater continental contribution stops mangrove distribution southward to this latitude, at the Tumbes River estuary. The Caribbean region, in particular the larger islands of Cuba, Jamaica and Haiti/Santo Domingo, has about 0.81 x 106 ha (Table 1). Countries with largest forests are Brazil with 1.38 x 106 ha; followed by Cuba (0.53 x 106 ha); Mexico (0.52 x 106 ha); Colombia (0.39 x 106 ha) Venezuela (0.25 x 106 ha) and Honduras (0.23 x 106 ha). These seven countries respond for over 72% of the total mangrove area in the American continent (Lacerda, 2002). 2. Mangrove flora and the origin of the Neotropical mangroves

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From nearly half a million species of vascular plants only about 70 species and a few hybrids are recognized as true mangrove species, meaning those species with occurrence restricted to mangrove habitats, not to be found anywhere else. Among these only two families Rhizophoraceae and Avicenniaceae present a pan tropical distribution, even extending to higher sub-tropical latitudes such as south Japan and southeast USA, in the northern hemisphere and to south Brazil, Australia and New Zealand in the southern hemisphere (Tomlinson, 1986). Depending on region and costal characteristics within regions, over one hundred additional plant species also occur within mangroves, including epiphytes migrated from landward forests and sea grasses and algae from the seaward edge. At higher latitudes salt marsh floral components are also common.

Most of the true mangrove plant species occurs in the Indo-Pacific region, where plants are believed to have first acquired the mangrove habitat and evolved the physiological and structural adaptations to the brackish water about 100 to 80 million years ago. Through radiate dispersal during the Pliocene, including a littoral drift along the Tethys Sea coastline and what is presently the Mediterranean Sea, mangroves were dispersed through most of the tropics. Mangroves thence crossed through the still opening Atlantic Ocean to the Caribbean by the early Eocene (55-50 million years), also jumping the then flooded Panama isthmus to the Pacific coast of the Americas. About


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40 million years ago mangrove pollen from Nypa, Rhizophora, Avicennia and Pelliciera were already spread through the Atlantic and Pacific coast of the Americas and the Caribbean.


From the original Indo-Pacific pool, eleven species belonging to five genera presently constitute the true mangrove flora in the neotropics. A twelfth species, the palm Nypa is recorded from the fossil record in South America during the Paleocene, but disappeared from the region in more recent periods. Recently it has been introduced in Panama, but as a farmed species for fiber production. Among present day mangrove taxa, the Avicenniaceae is represented by four species: Avicennia germinans (L.) Stearn., the one with largest distribution along both American coasts, and A. schaueriana Stapft & Leechm., restricted to the Atlantic coast, but occurring from Florida in the USA to south Brazil. The last two species A. bicolor Standl. and A. tonduzii Moldenke, are restricted to the Central American Caribbean and Pacific coasts.

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The Rhizophoraceae, generally dominating the areas facing open water, is represented by four species: Rhizophora mangle L., shown in Figure 1 colonizing river bancs in northern Brazil, the widest distributed species, occurring practically along the entire extension of mangroves in the Americas; and the rarer R. harrisonii Leechm.; R. racemosa G. F. Mayer and R. samoensis (Hochr.) Salvosa., restricted to equatorial latitudes. It is possible that the more restricted distributed species of the Avicenniaceae and Rhizophoraceae are varieties and/or hybrids of the wider distributed species and studies on the genetics of these two genera are still required.

Figure 1. A preserved Rhizophora mangle riverine forest in northern Brazil. R. mangle, known as the red mangrove due to the high tannin content of its bark, is one of the widest distributed mangrove species in the neotropics.


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The family Combretaceae has two widely distributed species Laguncularia racemosa (L.) Gaertn. and Conocarpus erectus L., both occurring along the Pacific and Atlantic coasts. The two species are better developed along higher, lower salinity areas, but individuals have been reported even at the seaward edge of forests. At least one subspecies of C. eretus, has been suggested, but again lack of genetic studies hamper its full acceptance.


The last family is the Pellicieraceae, represented by a single species, Pelliciera rhizophorae Pl. & Tr., more abundant on the more consolidated sediments, generally occurring higher in the tidal frame. This species once widespread throughout the Caribbean and the equatorial coast of the Pacific of Central America is today extremely restricted to isolated sites in Panama and Costa Rica. However, similar to Nypa, it has recently been re-introduced in reforestation experiments along the Caribbean coast of Colombia (Lacerda, 1993).

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Among the most abundant and diversified associated flora, i.e. non-obligatory mangrove species, are the fern Acrosticum aureum, Mora megistosperma and M. oleifera, frequently present along the gradation to terrestrial vegetation. These species are so typical of some Neotropical mangroves, which are considered as part of the mangrove formation proper. Still landward and in upstream locations, mostly under humid climate, mangroves typically grade into swamp forests and palm forests dominated by Symphonia globulifera, Pterocarpus officinalis and palms such as Euterpe oleracea. There are also open areas of herbaceous marshes where the mangrove fern A. aureum is mixed with salt marsh species (e.g. Portulaca oleracea, Batis maritima, Sporbolus virginicus, Cyperus sp. and occasional small shrubby mangroves as well as bare salt pans in many areas dominated by herbaceous succulent communities formed by Aizoaceae, Amaranthaceae, Chenopodiaceae and Portulacaceae. Along the humid Pacific coasts of Ecuador and Colombia and also in south eastern Brazil, mangrove canopy is invaded by a high diversity of epiphytes, particularly from the families Bromeliacea, Orchidacea and Aracea. Interesting to note is the frequent visits to theses epiphytes by local dwelling fauna in particular land and tree crabs, although the plant-animal relationship in these cases are still to be understood (Lacerda, 1993). Along the tectonically passive coast of the Atlantic Ocean, relic sand dunes and sand ridges left by the last changes in sea level during the Holocene may be frequently seem within the coastal plain. In such cases typical coastal forests and dune vegetation may occur mixed with mangrove vegetation, resulting in patchy, highly biodiversity forests. The sea grasses Ruppia maritima L.; Halodule wrightii Achers; Thalassia testudinum Konig, Halophila baillonis and Syringodium filiforme are found in the lower intertidal and into the sub-tidal areas along most of the seaward fringe of the Caribbean and Atlantic mangroves. These sea grasses are the preferred fodder for the manatees, which find shelter in mangrove tidal creeks. Apart from sea grasses, benthic seaweeds found in mangroves include over 110 species, mostly from the Rhodophyceae. Diversity is higher in the highly transparent Caribbean waters and lower in continental mangroves receiving large continental contribution. About 50% of the species were recorded growing on roots and trunks, the remnants colonizing rocks, stones and shell fragments (about 30%) or directly growing on sand or mud substrates (about 20%). A few associations of macroalgae are found in most mangroves worldwide; the Bostrichietum,


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typically colonizing trunks and roots, which is formed by a dozen Rhodophyceae including the genera Bostrichia, Caloglossa and Catenella. Another association, the Rhizoclonietum is formed by over 10 species of the Chlorophyceae, mostly from the genera Rhizoclonium, Enteromorpha and Cladophora. Additionally, about 60 species of fungi belonging to all groups of warm waters infest submersed roots, stems and twigs, only a few, however, are host-specific. This group of organisms plays a central role in biomass decomposition and nutrient cycling within mangroves.

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Benthic micro flora is dominated by diatoms of the genera Auliscus, Cyclotella, Cocconeis, Fragilaria, Melosira, Navicula, Bidulphia, Thalassionema, Thalassiosira, Actynophycus. These plants are extremely important to the biogeochemical characteristics of mangrove sediments. Biofilms, formed at the sediment-water interface, mostly by micro algae, fungi and bacteria, promotes the rapid formation of bio-mineralized minerals, in particular framboidal pyrites with plays a key role in nutrient and trace element geochemistry in mangrove sediments. Two stages of the formation of biogenic framboidal pyrites are shown in Figure 2. In figure 2A aggregations of pyrite crystals are easily seem forming the typical framboidal structure. In figure 2B fromboids are covered by a clay-organic thin film, helping preserving them into the anoxic sediment. This biomineralization processes is eventually responsible for the capacity of mangroves in immobilizing pollutants, in particular heavy metals of environmental significance, decreasing their bioavailability to mangrove plants and animals. This non-obvious service is of increasing use in developing nations as an alternative to much more expensive pollution mitigation procedures.

Figure 2. Framboidal pyrites formed on the biofilm present in the surface sediments of mangroves. 2A. Framboid clear of organic layer; 2B. Framboid covered by a clayorganic layer. These structures forms rapidly by the activity of sulfate reducing bacterial and are able to efficiently trap trace elements.


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3. Mangrove associated fauna


Mangrove fauna, similarly to the flora presents much higher biodiversity in the IndoWest Pacific than in Neotropical mangroves. There is no “true mangrove fauna” as is the case for plants, animals occurring in mangroves also dwell in other protected coastal shores. However, a typical mangrove dwelling fauna can easily be identified constituted of several dozen species of crustacean and molluscs. Among the most representative are the tree and land crabs from the Ocypodidae and the Grapsidae (e.g Aratus pisonii, Cardisoma guanhumi, Goniopis cruentatata, Ucides cordatus, and several species of the genus Uca), as well as mud lobsters and many species of shrimps and barnacles. Mussels (Mytella spp.) and several species of clams inhabit the mud substrate, whereas oysters (Crassostrea spp.) are abundant colonizing tree trunks and aerial roots and snails (Littorina spp.) are also frequent on trees. The benthic fauna is the first step in decomposing the tannin rich mangrove organic matter. Crabs, in particular, are believed to respond for nearly 25% of the C cycling in mangrove soils, by physically breaking and cutting mangrove litter and digesting it within burrows in the soil, facilitation further colonization of decomposing litter by microorganisms. Apart from the obvious role as decomposers, facilitating further colonization of deposited litter by other invertebrates as well as fungi and bacteria, burrowing animals also promotes sediment oxygenation and their faecal pellets are an important source of nutrients to roots.

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A large diversity of birds spends part of their life cycles in mangroves. For example, estimates from the 1980s suggested some 326,000 Nearctic migrant shorebirds were present on the mangrove coast between Belém and São Luis, while in the early 1990s some 150,000 were recorded only from the Maranhão Gulf, making this region of northern Brazil of global importance for its bird-life. In French Guiana up to 1.2 million migratory shore-birds were reported from the Coppename Monding Nature Reserve, including a colony of 20,000 scarlet ibis which nest in the mangroves. Around the Cayapas-Mataje forests, Ecuador, 173 bird species, from 45 families were recorded. At the eastern deltaic mangroves of the Orinoco Delta a remarkable diversity of about 178 bird species have been recorded. Aside from water birds, characteristic mangrove species include the spotted toddy-flycatcher, the rufous crab-hawk and the dark-billed cuckoo. Resident birds include the herons, cormorants, greater flamingos, egrets, spoonbills, scarlet ibis, and wood stork. Some species are thought to be largely mangrove-restricted, such as the threatened sapphire-bellied hummingbird (Lepidopyga lilliae) which is endemic to Colombia (Kjerfve et al., 1997). Birds are particularly important in bringing nutrients to mangrove environments and significant differences in mangrove biomass and productivity have been reported between mangroves hosting bird nests and colonies and those without a significant presence of avifauna. On the other hand a study on the effects of metal (Hg) contamination on mangroves in Trinidad & Tobago, suggested that observed higher mutation rates in trees harbouring migrating bird colonies relative to control areas without colonies, were related to higher Hg content in sediments imported by migrating birds and deposited as content in feathers and faecal material from birds’ nests. Mangrove roots, trunks and branches attract rich epifaunal communities, including sponges, hydroids, anemones, polychaetes, bryozoans and ascidians, apart from the bivalves and crustaceans (Macintosh and Ashton, 2002). These organisms are important


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food sources for fish, thus mangrove fish biodiversity is also very large. Some 200 fish species, 66 of commercial importance have been recorded only from the Paranaguá Bay, in southern Brazil, including many that use the bay and its bordering mangroves for spawning or juvenile life-history stages, but others such as mullet, snook and catfish are considered residents. Surveys of the fish fauna in the mangrove bordered Cananéia lagoon in south-eastern Brazil found 68 fish species from 23 families, mostly representing marine or brackish species. Some 52 fish species have been recorded from the Tacarigua Lagoon, Venezuela, from 27 different families including marine estuarine and freshwater species, whereas 97 fish species have been recorded from the Orinoco Delta (Seeliger & Kjerfve, 2001).


Numerous reports have discussed the role played by the root-colonizing fauna in the augmenting of aerial root structure of mangroves. Spheronema terebrans, a root boring isopod, has been reported as inducing aerial roots ramification in many mangrove areas. Oysters and barnacles may play similar roles. Since, increasing complexity of root architecture is fundamental for enhancing mangrove resistance to wave action and erosion at the intertidal zone, the present of abundant root-colonizing fauna may have a positive impact on mangrove development. There is a rich mammalian fauna and over 50 mammals have been recorded in Neotropical mangroves, including fishing bats, three-toed sloth, red howler monkey, ocelot, jaguarondi and even the giant otter, tucuxi dolphin and manatees. Crab-eating raccoons occur in most of the continent’s mangroves. American crocodiles and caimans are abundant at the Caribbean and Pacific coast of South and Central America. The beaches in north-western French Guiana include some of the most important areas in the world for nesting leatherback turtles. 4. Ecology

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The dominance of the intertidal habitat by mangroves is due to adaptations to the waterlogged conditions and tidal regime, mostly including well-known and repeatedly described features of the anatomy and histology of mangrove plant organs. Mostly widespread adaptations are: a) complex architecture of aerial roots shown in Figure 3, mostly from the Rhizophoraceae, gives physical support for the tree in a waterlogged and tidal environment submitted to wave and current action. Also they increase sediment trapping capacity resulting in positive sedimentations rates of a few millimeters per year. These accretion rates help mangroves in coupling with natural sea level rise. In extreme cases such as along the coast of the Guyanas and oriental Venezuelan coast in northern south America, mangroves fringes, mostly formed by monospecific stands of Avicennia, trap a large quantity of continental sediments brought to the Atlantic Ocean by the Amazon and Orinoco rivers respectively, resulting in a progradation seaward of the mangrove fringe of a few meters per year; b) the abundant distribution of the aerenchyma in stilt roots, knee roots, pneumatophores (per se a morphological adaptation resulting in development of geotropic-negative roots), aerial roots, and all roots aboveground in general. The abundant aerenchyma facilitates transfer of oxygen from the photosynthetic canopy to roots imbibed in the anoxic soil. c) all aerial structures close to water level including trunks and roots, as well as the leaf bases of Nypa fronds, are covered with lenticels, group of cells forming pores that


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essentially breathe for the tree, supplying oxygen to the underground roots trapped in waterlogged (and therefore oxygen deprived) soil, these ubiquitous and numerous largecelled lenticels, are present even on propagules of many mangrove species; d) wax covered and/or succulent leaves, enriched in tannins contribute to diminishing water loss and UV irradiation. Tannins may also keep low insect herbivory rates in these evergreen trees.

Figure 3. A preserved mangrove basin forest area in southeastern Brazil during low tide showing the complex architecture of aerial roots which allows tree stability in a changing physical environment due to tide and wave action and contributes with increasing sediment trapping and sedimentation.

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Physiological adaptations were also developed in face of the brackish to saline water environment and the nearly absence of oxygen in sediments and pore water, these include: a) viviparity, which is most developed in the species of Rhizophoraceae, but not exclusive of this family. In Avicennia, Nypa and Pelliciera, all described as semiviviparous (crypto-viviparity); seeds have cotyledons and the beginning of a plumule or embryo contained in the testa, but germination happens only after the testa or shell of the seed rots and ruptures on the substrate. Viviparity grants mangroves a pantropical distribution, since they may still be viable after 6 to10 months floating in seawater; b) excess salt exclusion or secretion organs/mechanisms. These adaptations make mangroves capable to surviving water and\or soil salinities up to 5 times higher than normal seawater; according to species mangroves can exclude salt uptake at the root level (e.g. in Rhizophora), remove excess salt at leaf level by salt excretion glands (e.g. in Avicennia), by cuticular transpiration at the leaf level (e.g. in Avicennia and Laguncularia) or by accumulating salt in the leaf tissue and then shedding the leaves; c) forming oxidized rhizospheres. Additionally to the stressful conditions of the physical environment of the intertidal zone, the chemical conditions developed within the soft, muddy substrate often results in sub-oxic or anoxic environments at the root level in the soil, due to fast consumption of oxygen by bacterial decomposers and decreased downward flux of oxygen through the fine-grained sediment. Mangrove roots generally exude oxygen in the soil creating oxidized rhizospheres, in manner similar to grasses inhabiting waterlogged environments. As a result, iron-plaques form around the external cortex of roots due to precipitation of Fe and Mn. These iron plaques impede the uptake of soluble toxins present in the anoxic environment. Incidentally, these mechanisms


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probably originally selected for salt tolerance and anoxia, also make these plants tolerant to many environmental toxins, including sulfide, metals and micro organic pollutants (Lacerda, 1998).

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Mangrove forests can be included in at least 3 different forest structure types, and two sub-types according to location and hydrology, which present different morphologies. Riverine forests, as these tall Rhizophora mangle forests in northern Brazil, shown in Figure 1, are generally the best developed with the forest canopies reaching 30 m or more along low-salinity river margins in humid equatorial latitudes. Trees with height of up to 50 m and 1.0 meter in diameter, have been reported in some places in humid Pacific coast of northern Ecuador (over 3.000 mm/yr) and in northern Brazil (1.200 to 2.800 mm/yr). Riverine forests extend along rivers following the intrusion of salt water. Freshwater and nutrient inputs are generally high, but salinity and nutrient concentrations vary seasonally following river hydrology. Elsewhere in humid coast of Pacific Colombia, for example, riverine forests also show trees reaching 40 m in height, with prop roots extending up to 10 m from the forest floor. These forests my extend up to 20-40 km inland or more depending on tidal amplitude and coastal morphology (Lacerda, 1993). Basin forests, such as the one showed in Figure 3, occur in interior areas flooded only in the highest tides remaining flooded for longer periods, favouring evapo-transpiration. As a result these forests’ soil may present high salinity, being diluted only during the rainy season, under theses conditions some species are exclude from basin forests, which are generally dominated by Avicennia and Rhizophora. Notwithstanding, basal area, canopy height, timber volume and biomass may only be a little smaller than in riverine forests. In areas were basin forests are only flooded in spring tides or where arid to semi-arid climates dominate, soil salinity may reach 2-3 times seawater normal values. Under these environmental conditions Dwarf forests, mostly of stunted Avicennia shrubs, are found with adult tree height reaching one to two meters only, as shown in Figure 4.

Figure 4. Dwarf mangroves in hipersaline areas. Increasing soil salinity areas are natural components in many mangroves, particularly under semi-arid climates. However, these areas may also result from changing hydrology due to road/pond construction close to aquaculture sites.


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Fringe forests occur at the sea edge submitted to daily tidal washing which also brings water and nutrients. These forests however, are particularly sensitive to erosion, mostly due to an unbalance between erosion/sedimentation rates and wave action at the coast, as many mangroves in semi-arid coastlines such as those at the Jaguaribe river estuary, in north-eastern Brazil shown in Figure 5. These forests, in general, present relatively smaller trees compared to riverine forests with typical canopies reaching less than 20 m and shorter, even in equatorial latitudes, depending on hydrodynamics (Seeliger and Kjerfve, 2001). Again sub-types occur in most areas. For example lagoon fringe mangroves, generally submitted to very small tidal amplitude are generally restricted to a narrow band of a few meters wide, and rarely reach 10 m high. Similarly island and overwash mangroves are frequently washed completely by tides resulting in short residence time of nutrients and decomposing litter thus hampering a full structure development (Lacerda, 1993).

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Figure 5. Erosion of fringe mangroves due to disequilibrium between continental and marine sediment supply in northeastern Brazil, caused mostly by river damming and water diversion. These effects may augment due to global climate changes due to increasing sea level and decreasing rainfall.

Variability of morphological characteristics of these forests types is very large and due to the natural dynamics of the coastal areas, substitutions of one forest type by another frequently occur through time. However, in many areas a zonation pattern of monospecific bands generally parallel to shore or river bank may be distinguish. Zonation is more frequent when local topography, soils characteristics are proportionally affects by distance from shore or river bank. However, with the exception of large, smooth coastal plains, major factors controlling mangrove forest structure and species occurrence varies not only with regional variables such as, latitude, wave action, rainfall and freshwater runoff, but also with small scale variation


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in hydrology, micro topography and saline intrusion. Small scale variations, on the other hand, will control important environmental parameters such as rates of erosion and sedimentation, aridity, soil and pore water salinity, nutrient inputs, and soil fertility, which eventually will dictate which species will develop in a given area, thus making clear zonation patters very difficult to distinguish (Kjerfve et al., 1997).


Mangrove biomass varies according to forest type, nutrient and freshwater supply ranging form 200 to 280 t/ha in riverine forests at the humid coast of equatorial latitudes to a few tons per hectare in arid and semi-arid coastlines an at the latitudinal extremes of mangrove distribution. Litter fall also vary according to forest type and availability of nutrients and freshwater, ranging from 122 g.m-2.year-1 in Florida, USA, the north extreme latitudinal extent of Atlantic coast mangroves, to more than 1,300 g.m-2.year-1 in lower latitudes along the humid coasts of Central America and northern Ecuador and Colombia and the northern Atlantic coast of South America (see Lacerda, 2002 for a compilation).

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Above ground primary productivity of mangroves, meaning the sum of wood growth and total litterfall are among the largest reported for terrestrial ecosystems, and is also influenced by latitude and the availability of freshwater and nutrients. Neotropical fringe and riverine mangroves, the most productive types, show primary productivity ranging from 781 g.m-2.year-1 in Puerto Rico, with precipitation of about 800 mm.yr-1 to 2,457 g.m-2.year-1 in the humid (1.680 mm) Términos Lagoon in southern Mexico, a clear example of the influence of freshwater availability on primary production (Day et al., 1988). Most of the existing data on mangrove productivity and biomass measurements however, refers to above ground production. Very few studies detailed belowground production, which is believed to be even higher than aboveground as suggested by the biomass allocation of mangrove trees with belowground biomass generally accounting for 40 to 60% of the total forest biomass, whereas leaves accounts for less than 10% and the remaining being represented by trunks and branches.

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A significant proportion of mangrove productivity is exported to coastal areas as macrodetritus and particulate organic carbon (POC), these two fractions responds to about 40 to 60% of the productivity not directly consumed by herbivores or buried in the mangrove soil. A larger proportion, about 50%, suffers partial decomposition in the mangrove environment and is exported as dissolved organic carbon (DOC). As mentioned before, the sub-oxic to anoxic conditions prevailing in mangrove soils impedes the total decomposition of deposited organic matter. Although a larger portion is buried and accumulated in sediments for very long periods, the anaerobic decomposition of mangrove organic matter, mostly performed through dissimilatory sulfate reduction by anaerobic or facultative anaerobic sulphate reducing bacteria, results in high concentrations of (DOC) in pore waters, which are exported through tides to adjacent coastal waters. As such, dissolved organic carbon from mangroves is believed to be the basis for large detritus-based food chains, responsible for the majority of the commercial fisheries in most tropical areas. In certain areas where oceanographic conditions permit, mangrove carbon, as DOC, may be exported to the continental shelf and slope, although its importance to the trophic ecology of these areas are virtually unknown.


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5. Mangrove products and services Mangroves have been a significant natural resources since humans established coastal nomads gathering and hunting groups foraging shallow coast waters for fish and shellfish, as attested by 7.000 year old ceramics found alongside their piles of shellfish remains in what is now northern Colombia and the Orinoco Delta of Venezuela and the abundant “sambaqui” (detritus moulds) left by pre-historical populations along most the Brazilian coast, dating as old as 4.500 years old (Lacerda, 2002).

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Today, mangroves are still of great economic importance to mankind, in particular in coastal developing nations in the tropics. Economic use can be divided in two categories. One category of exploitation which requires capital investment and maintenance cost, which includes the direct and indirect use of mangrove parts and products, such as extraction of tannins, pulp for paper, fuel wood, construction timber, fodder and honey, for example. This category also includes indirect products related to fish and shellfish capture and traditional aquaculture, such as the tambak in Southeast Asia, which uses mangrove waters to fertilize their culture ponds (FAO, 2007). Figure 6 shows a typical daily catch of cockles gathered from mangroves in Caribbean Colombia. This source of protein is still the most important for many indigenous populations in the tropics. The other category includes mangroves services, which are for free but difficult to quantify and even harder to create public awareness about them. Mangroves may preserve biodiversity and wildlife; provide nurseries and spawning grounds for juveniles of fish and shrimps; moderate local climate and protect shore from erosion (Lacerda, 1993; 2002; FAO, 2007).

Figure 6. A day catch of cockles in Colombian mangroves. In many tropical and subtropical countries, mangroves are still the major source of animal protein to indigenous populations. Apart form humans, the enormous densities of invertebrates in mangroves support large populations of fishes, birds, mammals and reptiles.


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Many endangered species dwell in mangrove areas where they found sanctuary, including the Royal Bengal Tiger (Pantera tigris); manatees, reptiles and numerous bird species. Marine food chains are also linked to mangroves. Mangrove rehabilitation in urban areas in south America, but also in the southern Japanese Island of Okinawa, have brought back a diversity of birds and wildlife and has been successfully used as an amenity to the urban environment. Fisheries in most part of the tropics depend on mangrove export of nutrients and organic matter and/or protection for their juveniles. For example, in Panama, the five most commercially important shrimp species spend their juvenile states in mangroves. In north and northeaster Brazil, mangrove crabs, Ucides cordatus and Cardisoma guaiumi support thousands artisan fishers with an average weekly catch of about 200.000 individuals during the peak of the fishing season. Fisheries catch in most of mangrove-lined coasts, depend up to 70% on mangroves both as shelter and food source. After mangrove felling decrease of fisheries has been reported in many areas. It is accepted that for each hectare of mangrove felled, nearly 0.5 tons of fisheries is lost every year. Mangroves also help protecting sea grass beds and coral reefs by trapping continental sediments avoiding their negative impact on these suspended sediment sensitive ecosystems. Many offshore coral reef areas in northern South America, where continental suspended matter transport to the sea in enormous, only exist due to this “service” given by mangroves.

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Apart from sediments, mangroves can also filter and immobilize pollutants strongly decreasing their bioavailability to plants and animals and serving as natural waste water treatment in many underdeveloped coastal areas in the tropics and subtropics (Lacerda, 1998). For example, rehabilitated mangroves have been successfully used as barriers to pollutant transfer from landfill disposal sites and adjacent coastal waters in Guanabara Bay, Brazil. They promote metal immobilization through the rapid precipitation of metal sulphides (see Figure 2) and further burial due to high sedimentation rates. Similar results have been obtaining by planting mangroves around mining and smelter tailings also in south-eastern Brazil.

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Another environmental significant service recently proved from the unfortunate disaster of the 2004 Indian Ocean Tsunami, is the protection of the coastal region from extreme climatic and oceanographic events. In most countries affected by the tsunami wave mangrove fringed coasts resulted much more protected and suffered much less impacts. The reduction rate of the tsunami force was determined mainly by the density and width of the mangrove fringe, being reduced to a level less than 15% of the original force behind forests with total number of trees per 10 m of shore line of 400 or more. In the Colombian Caribbean, at the Corales del Rosario Park, planting mangrove along exposed shores of islands has also been proved very efficient in preventing erosion due to strong wave action during hurricane season. These services requiring require no capital or maintenance expenditure. If the mangroves would not have been there it would cost billions of dollars to achieve these objectives (FAO, 2007). A recent comprehensive worldwide estimate of natural capital of ecosystems has rated mangroves as one of the most valuable of the world.


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6. Environmental impacts on mangroves While the importance of the sustainable utilization of mangrove products and services is increasingly realised, particularly with the decreasing of world capture fisheries, failing aquaculture activities in converted mangrove areas and the breaching of artificial sea defences in some countries, many people still regard mangroves as wastelands. Major drivers, pressures and environmental impacts on mangroves are summarized in table 2. Sources of impacts are related to anthropogenic activities acting on the regional and global scales resulting in significant impacts discussed below in detail. Presures Increasing CO2 concentration

Impacts on mangroves - Increasing photosynthesis - Increasing water use efficiency

Increasing seawater temperature Sea level rise

- Expanding latitudinal distribution - Increasing Carbon accumulation in biomass and sediments

Decreasing water flow to estuaries Decreasing sediment flow to the ocean Increasing saline intrusion

- Increased soil salinity makes difficult osmotic balance - Migration inland - Erosion at the coastline Competitive exclusion of freshwater plants - Erosion of old mangrove deposits enriched in trace metals

Augmenting water withdraw Increasing nutrient inputs and concentration Increasing soil erosion and sediment load to estuaries Increasing nutrient inputs and concentration Increasing sediment load to estuaries from ponds

- Increasing primary production Increasing events of euthrophication - Expansion on new formed island and river banks in estuaries

Global Climate Change


Drivers Changing atmospheric conditions

-

-

Changing oceanic conditions

-

-

Damming

-

Agriculture & urbanization

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Regional land Use Changes

-

-

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-

Aquaculture

-

-

- Increasing deforestation - Appearance of exotic species Increasing events of euthrophication - Expansion on new formed island and river banks in estuaries

Table 2. Major anthropogenic drivers, pressures and environmental impacts on mangroves at the regional and global scales.


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The scale of human activities responsible for mangrove deforestation has increased dramatically, with many countries showing losses of 50-80% or more, compared to the mangrove forest cover that still existed even 50 years ago (FAO, 2007). As a response to this a FAO/UNEP initiative reviewed databases and literature on mangroves in 121 countries and areas providing an updated list of the most reliable, recent estimate for each country, mostly based on inventories or analysis of remote sensing imagery (FAO, 2007). Regression analyses based on these data provided reliable estimates for 1990 and 1980 and an extrapolated estimate for 2000 for each of the 121 countries and areas.

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The preliminary results of this initiative showed that mangrove deforestation continues, albeit on a slightly lower rate in the 1990s than in the 1980s. Mangroves have been converted into agriculture, aquaculture, industrial or urban development and/or degraded by the careless release of pollutants of various anthropogenic activities. For example, the Philippines has lost 75% of the mangrove area that existed in the 1950s (Primavera, 2000), mostly due to conversion to agriculture and aquaculture, as well as unsustainable timber exploitation. In eastern Guyana conversion of mangroves to prime agricultural land has been one major source of significant forested area decrease. More recently however, aquaculture has been the major cause of mangrove loss in many countries due to the increasing demand for aquaculture products, in substitution of the more and more scarce natural fisheries. For example, loss of up to 90% mangrove cover in some parts of Ecuador (e.g. around the Guayaquil Gulf and Guayas River estuary), occurred to aquaculture (Bodero, 1993). In niorth eastern Brazil, notwithstanding the legal protection in this country which regards mangroves as permanent preservation areas, about 10% of the total shrimp farm area has been originally converted from mangrove forests, as shown in figure 7. Sometimes, conversion is not necessary to destroy mangroves, changing hydrology due to access road construction to aquaculture farms results in changing salinity patterns and eventually mangrove death. Also releasing effluents, in particular during drying of ponds for harvesting, may cause significant damages in mangrove fringe forests surrounding aquaculture ponds, as clearly shown in figure 8.

Figure 7. Aquaculture pond invading a fringe mangrove forest in north-eastern Brazil. One of the most recent and fast cause of mangrove deforestation, in particular in Asia and the Pacific coast of Latin America.


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Figure 8. Erosion of riverine mangroves due to the releasing of aquaculture ponds effluents in northeastern Brazil. Apart from the physical impact, these effluents are highly enriched in nutrients and sediments causing euthrophication in many estuaries and sedimentation of tidal creeks..

The livelihoods of the local coastal communities have been diminished/or totally lost by the destruction or degradation of mangroves and conflicts between aqua-farmers and local human groups and traditional populations has increased exponentially throughout the tropics. Unfortunately, for many countries with small areas of mangroves, virtually no information exists on the present state of mangrove forests. However, some of these countries, in particular island nations, may have mangroves as the sole sources of timber and wood products, as well as protection from climatic events.

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A more recent concern regarding alterations of mangrove forested areas is the recognizance that mangroves respond very rapidly to coastal changes induced by basinharbored activities even when far from the coast, and are also affected by global climate change. Erosion of fringe mangroves is occurring in most arid coastlines receiving freshwater and continental supply from rivers which have been dammed to provide water supply to a growing population and agriculture. Similarly, changes in rainfall patterns and consequently in continental runoff resulted in erosion of mangroves areas worldwide (Figure 5). Mangrove distribution changes are also associated with changing extension of salt water intrusion and increasing soil salinity as well as river transport capacity decrease, resulting in the trapping of the continental sediment supply in estuaries. Detailed descriptions of changing mangrove area and distribution patterns have been recently published for many areas in the world, in particular in semi-arid coastlines. The example shown in figure 9, from a small estuary in the semi-arid coast of northeastern Brazil, shows the different mechanisms involved in the process. Damming of the river resulted in cutting off the seasonal high water flow in the rainy season. This annual washout of the estuary avoided accumulation of sediments. After dam construction, peak rainfall season flow decreased 20 times, reducing the capacity for sediment transport. Saline intrusion increased as a result of lesser freshwater input and mangroves


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first advanced over bare areas substituting existing salt marshes, as shown from 1968 to 1999 in the maps of figure 9. Simultaneously, enlargement of beaches along river banks as well as creation and augmenting of islands in the estuary also occurred being more evident in recent years. These new areas are fast colonized by seedlings of Avicennia and Laguncularia, whereas Rhizophora propagules also colonized newly expanded river beaches. As a final result, mangrove area extension increased by 43% in the past 40 years. Significant changes, due to similar changes in basin morphology and land use have been reported as far apart countries as in New Zealand, Australia, Venezuela and northern Brazil.

Figure 9. Changes in mangrove extension in northeastern Brazil. From 1968 to 1999 major changes occurred by mangrove occupying old salt pans. From 1999 to 2004, river damming resulted in enlargement of river banks and island formation and enlargement at the estuary. Cumulative to these regional or local originated impacts on mangrove distribution,


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global climate change also acts upon regional and local forces. Augmenting carbon dioxide and temperature will lead to increasing primary production of mangroves, which will increase carbon accumulation in biomass and burial in mangrove sediments. Therefore in this aspect mangrove may counter act the global effects of atmospheric composition changes.

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On the other hand, reducing rainfall in many parts of the tropics will enforce river damming and water withdraws for irrigated agriculture and human use, therefore accelerating the changes such as those described above for northeastern Brazil. Other effects are not so obvious; changing the intensity of ENSO (El Niño Southern Oscillation) events may affect many regions of the Americas by changing intensity of rainfall and droughts. For example, it has been demonstrated that there is a significant correlation between SOI, the atmospheric signature of ENSO, sand dune displacement in many arid sectors of the Atlantic coast of South America. As a result, increasing the intensity of ENSO phenomena will induce faster displacement of sand dunes, which may burry large sectors of natural vegetation including mangroves. Figure 10 shows mangroves in the Ceará coast in Brazil, being buried by fast moving natural dunes, and examples like this will certainly increase in the near future.

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Figure 10. Mangroves being buried by moving sand dunes in north-eastern Brazil. Magnitude of dune displacement in the region is a function on the intensity of ENSO events and may respond to global climate change.

Contrary to the local, direct impacts on mangroves, impacts from global climate change are much more difficult to forecast, modeling and, therefore, minimize. Also, generic environmental legislation is not applicable at this scale and mangroves have been nearly absent from the international global discussion panels on climate change. Besides, reliable results on this subject are still scarce creating further difficulties in understanding the magnitude of the consequent impacts. 7. Conclusions Notwithstanding the harmful anthropogenic impacts on mangroves in several areas in the world, overall, however, Neotropical mangroves remain still abundant in most areas.


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The largest mangrove areas of Central America, Pacific coast of Colombia and northern Ecuador, the Orinoco Delta in Venezuela, as well as the immense tract of forests along the northern Brazilian coast, are still well preserved. Additionally there has been increasing public awareness that mangroves bring considerable benefits to coastal populations, augmenting the degree of legal protection and the promotion of coastal management and sustainable use strategies. In many countries serious efforts on mangrove rehabilitation and silviculture projects are recuperating many areas previously degraded by human action. Responsible reaction from aquaculture products consumers also triggered better regulation of the activity, also resulting in less impact and even in recuperation plans in many traditional aquaculture site.


On the opposite, however, both decision makers and the public in general, are still unaware of the potential impacts of global climate change on mangroves. Including the sustainable conservation of this important ecosystem should be a priority in the forthcoming panels and discussions regarding global climate changes. Building human capacity for facing the impacts of global climate changes on mangrove dominated coasts is of extreme and urgent importance and will be a sine qua non goal to achieve the desirable sustainable livelihood of the immense fraction of humankind living in tropical and subtropical coast. Glossary

Lenticels:

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Iron-plaques:

Pneumatophores:

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Viviparity:

Dissimilatory sulfate reduction:

ENSO:

SOI:

Group of cells forming pores that essentially breathe for the tree. Located on aerial roots, trunks and even base of leaves, they supply oxygen to the underground roots imbibed in waterlogged (and therefore oxygen deprived) soil. Deposits of Fe3+ and Mn4+ oxy-hydroxides on the external surface of roots due to exudation of oxygen by plant into the anoxic or sub-oxic interstitial environment where reduced dissolved Fe2+ and Mn2+ ions are present. Pencil-like (in Avicennia and Sonneratia) or knee shaped structures (in Bruguiera and Ceriops) extensions of the subterranean root system rising from the ground and extending a long distance from the parental tree Seeds display no dormancy, germinating while still attached to the mother tree. Viviparity, together with high resistance and floating capacity maximize migration and colonization of new areas. Is the most typical metabolism occurring in the anoxic environment of mangrove soils. Sulfate reducing bacteria uses sulfate as electron donors replacing the absent free oxygen. El Niño-Southern Oscillation; is a global coupled oceanatmosphere phenomenon involving important temperature fluctuations in surface waters of the tropical Eastern Pacific Ocean Southern Oscillation Index; is the atmospheric signature


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of ENSO and reflects the monthly or seasonal fluctuations in the air pressure difference between Tahiti, in Central Pacific and Darwin, in Australia. Bibliography FAO (2007). The World’s Mangroves. FAO Forestry Paper 153, FAO, Rome 89 p. [The most update survey on mangrove areas and their recent changes around the word, based on the best and more reliable estimates]. Kjerfve, B.; Lacerda, L.D. & Diop, E.H.S. (1997). Mangrove Ecosystem Studies in Latin America and Africa. UNESCO, Paris, 297pp. [A collection of original research papers on mangroves of the two continents, mostly written by local specialists].


Lacerda, L.D. (1993) Conservation and Sustainable Utilization of Mangrove Forests in Latin America and Africa Regions. Part I - Latin America. International Society for Mangrove Ecosystems, Okinawa, Japan, 272 pp. [This book includes chapters on most countries bearing mangroves in the Americas, with chapters written by local specialists of each country, thus providing a complete picture of mangrove importance to these nations]. Lacerda, L.D. (1998). Biogeochemistry of Trace Metals and Diffuse Pollution in Mangrove Ecosystems. International Society for Mangrove Ecosystems, Okinawa, 64 p. [A review on contaminants biogeochemistry, mobility and bioavailability within mangroves and its applicability in pollution control]. Lacerda, L.D. (2002). Mangrove Ecosystems: Function and Management. Springer Verlag, Berlin, 292 p. [A worldwide evaluation of the status of mangroves with emphasis on ecosystems functioning, environmental impacts and sustainable use. Includes a comprehensive chapter on mangrove phenology]. Macintosh, D.J. & Ashton, E.C. (2002). A Review of Mangrove Biodiversity Conservation and Management. Final Report. Univesity of Aarhus, 133 pp. [A review of the present status of mangrove use, management and conservation, with emphasis on Indo-West Pacific mangroves]. Seeliger, U. & Kjerfve, B. (2001). Coastal Marine Ecosystems of Latin America. Springer-Verlag, Berlin. 332 pp. [The most complete description of nearly all types of littoral plant communities of Latin America and the Caribbean].

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Spalding, M.; Blasco, F. & Field, C.D. (1997). World Mangrove Atlas. International Society for Mangrove Ecosystems, Okinawa, 178 pp. [The most complete mapping of mangrove forests extension around the world]. Tomlinson, P.B. (1986). The Botany of Mangroves. Cambridge University Press, Cambridge, UK, 678 pp. [This book is the most comprehensive treatise on the true mangrove flora, its anatomy and provides a detailed discussion of most significant mangrove plant adaptations to the coastal environment] Biographical Sketch

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Professor Luiz Drude de Lacerda has a Graduation in Biology (1977), MSc (1980) and PhD (1983) in Biophysics from the Federal University of Rio de Janeiro. He is a Titular Professor at the Fluminense Federal University, Niteroi, and Head of the Graduate Program in Marine Sciences of the Federal University of Ceará, in northeastern Brazil, and coordinator of the national Institute of Science and Technology on Continent-Ocean Materials Transfer. He has acted as visiting professor at the universities of Hamburg, Germany; Nice and Toulon, France. Professor Lacerda is presently a CNPq senior researcher (1A) and a Scientific Steering Committee member of the IGBP-LOICZ program (International Geosphere-Biosphere Program - Land Ocean Interactions in the Coastal Zone) and of the ISME (International Society for Mangrove Ecosystems). Under these programs he has coordinated several international research projects on mangrove ecology, management and conservation in Asia, Africa and Latin America. Prof. Lacerda has authored over 170 journal papers and 18 books and 68 book chapters, on mangrove subjects and on the biogeochemistry of tropical ecosystems in general, environmental pollution and coastal zone management.


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