Mangroves are woody plants that grow at the interface between land and sea in tropical and sub-tropical latitudes where they exist in conditions of high salinity, extreme tides, strong winds, high temperatures and muddy, anaerobic soils. There may be no other group of plants with such highly developed morphological and physiological adaptations to extreme conditions.

Because of their environment, mangroves are necessarily tolerant of high salt levels and have mechanisms to take up water despite strong osmotic potentials. Some also take up salts, but excrete them through specialized glands in the leaves. Others transfer salts into senescent leaves or store them in the bark or the wood. Still others simply become increasingly conservative in their water use as water salinity increases Morphological specializations include profuse lateral roots that anchor the trees in the loose sediments, exposed aerial roots for gas exchange and viviparous waterdispersed propagules.

Mangroves create unique ecological environments that host rich assemblages of species. The muddy or sandy sediments of the mangal are home to a variety of epibenthic, infaunal, and meiofaunal invertebrates Channels within the mangal support communities of phytoplankton, zooplankton and fish. The mangal may play a special role as nursery habitat for juveniles of fish whose adults occupy other habitats (e.g. coral reefs and seagrass beds).

Because they are surrounded by loose sediments, the submerged mangroves' roots, trunks and branches are islands of habitat that may attract rich epifaunal communities including bacteria, fungi, macroalgae and invertebrates. The aerial roots, trunks, leaves and branches host other groups of organisms. A number of crab species live among the roots, on the trunks or even forage in the canopy. Insects, reptiles, amphibians, birds and mammals thrive in the habitat and contribute to its unique character.

Living at the interface between land and sea, mangroves are well adapted to deal with natural stressors (e.g. temperature, salinity, anoxia, UV). However, because they live close to their tolerance limits, they may be particularly sensitive to disturbances like those created by human activities. Because of their proximity to population centers, mangals have historically been favored sites for sewage disposal. Industrial effluents have contributed to heavy metal contamination in the sediments. Oil from spills and from petroleum production has flowed into many mangals. These insults have had significant negative effects on the mangroves.

Habitat destruction through human encroachment has been the primary cause of mangrove loss. Diversion of freshwater for irrigation and land reclamation has destroyed extensive mangrove forests. In the past several decades, numerous tracts of mangrove have been converted for aquaculture, fundamentally altering the nature of the habitat. Measurements reveal alarming levels of mangrove destruction. Some estimates put global loss rates at one million ha y−1, with mangroves in some regions in danger of complete collapse. Heavy historical exploitation of mangroves has left many remaining habitats severely damaged.

These impacts are likely to continue, and worsen, as human populations expand further into the mangals. In regions where mangrove removal has produced significant environmental problems, efforts are underway to launch mangrove agroforestry and agriculture projects. Mangrove systems require intensive care to save threatened areas. So far, conservation and management efforts lag behind the destruction; there is still much to learn about proper management and sustainable harvesting of mangrove forests.

Mangroves have enormous ecological value. They protect and stabilize coastlines, enrich coastal waters, yield commercial forest products and support coastal fisheries. Mangrove forests are among the world's most productive ecosystems, producing organic carbon well in excess of the ecosystem requirements and contributing significantly to the global carbon cycle. Extracts from mangroves and mangrove-dependent species have proven activity against human, animal and plant pathogens. Mangroves may be further developed as sources of high-value commercial products and fishery resources and as sites for a burgeoning ecotourism industry. Their unique features also make them ideal sites for experimental studies of biodiversity and ecosystem function. Where degraded areas are being revegetated, continued monitoring and thorough assessment must be done to help understand the recovery process. This knowledge will help develop strategies to promote better rehabilitation of degraded mangrove habitats the world over and ensure that these unique ecosystems survive and flourish.

Biology of mangroves and mangrove Ecosystems

Mangrove communities are recognized as highly productive ecosystems that provide large quantities of organic matter to adjacent coastal waters in the form of detritus and live animals (fish, shellfish). The detritus serves as a nutrient source and is the base of an extensive food web in which organisms of commercial importance take part. In addition, mangrove ecosystems serve as shelter, feeding, and breeding zones for crustaceans, mollusks, fish of commercial importance, and resident and migratory birds. Although mangroves in the United States are protected, the systematic destruction of these ecosystems elsewhere is increasing. Deforestation of mangrove communities is thought to be one of the major reasons for the decrease in the coastal fisheries of many tropical and subtropical countries. There is evidence to propose a close microbe-nutrient-plant relationship that functions as a mechanism to recycle and conserve nutrients in the mangrove ecosystem. The highly productive and diverse microbial community living in tropical and subtropical mangrove ecosystems continuously transforms nutrients from dead mangrove vegetation into sources of nitrogen, phosphorus, and other nutrients that can be used by the plants. In turn, plant-root exudates serve as a food source for the microorganisms living in the ecosystem with other plant material serving similarly for larger organisms like crabs. This overview summarizes the current state of knowledge of microbial transformations of nutrients in mangrove ecosystems and illustrates the important contributions these microorganisms make to the productivity of the ecosystems. To conserve the mangrove ecosystems, which are essential for the sustainable maintenance of coastal fisheries, maintenance and restoration of the microbial communities should be undertaken. Inoculation of mangrove seedlings with plant-growth-promoting bacteria may help revegetate degraded areas and create reconstructed mangrove ecosystems.

The role of sediment microorganisms in the productivity, conservation, and rehabilitation of mangrove ecosystems: an overview

Rafting of marine and terrestrial organisms has been reported from a variety of substrata and from all major oceans of the world. Herein we present information on common rafting organisms and on ecological interactions during rafting voyages. An extensive literature review revealed a total of 1206 organisms, for which rafting was confirmed or inferred based on distributional or genetic evidence. Rafting organisms comprised cyanobacteria, algae, protists, invertebrates from most marine but also terrestrial phyla, and even a few terrestrial vertebrates. Marine hydrozoans, bryozoans, crustaceans and gastropods were the most common taxa that had been observed rafting. All major feeding types were represented among rafters, being dominated by grazing/boring and suspension-feeding organisms, which occurred on all floating substrata. Besides these principal trophic groups, predators/scavengers and detritus-feeders were also reported. Motility of rafting organisms was highest on macroalgae and lowest on abiotic substrata such as plastics and volcanic pumice. Important trends were revealed for the reproductive biology of rafting organisms. A high proportion of clonal organisms (Cnidaria and Bryozoa) featured asexual reproduction, often in combination with sexual reproduction. Almost all rafting organisms have internal fertilisation, which may be due to the fact that gamete concentrations in the rafting environment are too low for successful fertilisation of external fertilisers. Following fertilisation, many rafting organisms incubate their offspring in/on their body or deposit embryos in egg masses on rafts. Local recruitment, where offspring settle in the immediate vicinity of parents, is considered an important advantage for establishing persistent local populations on a raft, or in new habitats. Some organisms are obligate rafters, spending their entire life cycle on a raft, but the large majority of reported rafters are considered facultative rafters. These organisms typically live in benthic (or terrestrial) habitats, but may become dispersed while being confined to a floating item. Substratum characteristics (complexity, surface, size) have important effects on the composition of the rafting community. While at sea, ecological interactions (facilitation, competition, predation) contribute to the community succession on rafts. Organisms capable to compete for and exploit resources on a raft (space and food) will be able to persist throughout community succession. The duration of rafting voyages is closely related to rafting distances, which may cover various geographical scales. In chronological order, three features of an organism gain in importance during rafting, these being ability to (1) hold onto floating items, (2) establish and compete successfully, and (3) develop persistent local populations during a long voyage. Small organisms that do not feed on their floating substratum and with asexual reproduction or direct development combine all these features, and appear to be most suited for long-distance dispersal on rafts and successful colonisation after reaching new habitats. All available evidence suggests that rafting is an important process for the population dynamics of many organisms and that it also has had and continues to have a strong influence on coastal biodiversity.

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The Ecology of Rafting in the Marine Environment. II. The Rafting Organisms and Community

Mangrove forests dominate the world's tropical and subtropical coastlines. Similar to other plant communities, nutrient availability is one of the major factors influencing mangrove forest structure and productivity. Many mangrove soils have extremely low nutrient availability, although nutrient availability can vary greatly among and within mangrove forests. Nutrient-conserving processes in mangroves are well developed and include evergreeness, resorption of nutrients prior to leaf fall, the immobilization of nutrients in leaf litter during decomposition, high root/shoot ratios and the repeated use of old root channels. Both nitrogen-use efficiency and nutrient resorption efficiencies in mangroves are amongst the highest recorded for angiosperms. A complex range of interacting abiotic and biotic factors controls the availability of nutrients to mangrove trees, and mangroves are characteristically plastic in their ability to opportunistically utilize nutrients when these become available. Nitrogen and phosphorus have been implicated as the nutrients most likely to limit growth in mangroves. Ammonium is the primary form of nitrogen in mangrove soils, in part as a result of anoxic soil conditions, and tree growth is supported mainly by ammonium uptake. Nutrient enrichment is a major threat to marine ecosystems. Although mangroves have been proposed to protect the marine environment from land-derived nutrient pollution, nutrient enrichment can have negative consequences for mangrove forests and their capacity for retention of nutrients may be limited.

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Nutrition of mangroves

Rafting has been inferred as an important dispersal mechanism in the marine environment by many authors. The success of rafting depends critically on the availability of suitable floating substrata. Herein currently available information on floating items that have been reported to carry rafting organisms is summarised. Floating items of biotic origin comprise macroalgae, seeds, wood, other vascular plants, and animal remains. Volcanic pumice (natural) and a diverse array of litter and tar lumps (anthropogenic) are the main floating items of abiotic origin. Macroalgae, wood, and plastic macrolitter cover a wide range of sizes while pumice, microlitter, and tar lumps typically are <10 cm in diameter. The longevity of floating items at the sea surface depends on their origin and likelihood to be destroyed by secondary consumers (in increasing order): nonlignified vascular plants/animal carcasses < macroalgae < driftwood < tar lumps/skeletal remains < plastic litter < volcanic pumice. In general, abiotic substrata have a higher longevity than biotic substrata, but most abiotic items are of no or only limited food value for potential rafters. Macroalgae are most abundant at mid-latitudes of both hemispheres, driftwood is of major importance in northern and tropical waters, and floating seeds appear to be most common in tropical regions. Volcanic pumice can be found at all latitudes but has primarily been reported from the Pacific Ocean. Plastic litter and tar lumps are most abundant near the centres of human population and activities. In some regions of abundant supply or zones of hydrography-driven accumulation, floating items can be extremely abundant, exceeding 1000 items km –2 . Temporal supply of floating items is variable, being seasonal for most biotic substrata and highly sporadic for some items such as volcanic pumice. Most reported velocities of floating items are in the range of 0.5–1.0 km h –1 , but direct measurements have shown that they occasionally are transported at much faster velocities. Published trajectories of floating items also coincide with the main oceanic currents, even though strong winds may sometimes push them out of the principal current systems. Many studies hint toward floating items to link source regions with coastal sinks, in some cases across long distances and even entire ocean basins. Fossil evidence suggests that rafting has also occurred in palaeooceans. During recent centuries and decades the composition and abundance of floating items in the world’s oceans have been strongly affected by human activities, in particular logging, river and coastline regulation, and most importantly oil exploitation and plastic production. The currently abundant supply and the characteristics of floating items suggest that rafting continues to be an important dispersal mechanism in present-day oceans. 182 M. Thiel & L. Gutow

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The ecology of rafting in the marine environment - i - the floating substrata

. Experiments were conducted to determine the decomposition rate of mangrove wood in two areas of differing water nutrient concentrations. Stakes were prepared from prop roots of Rhizophora inangle and from branches of Avicennia. Conocarpus and Lagguncularia. and tied in the natural habitat at two sites—Man-of-war Cay (high nutrient concentrations) and Twin Cays (low nutrient concentrations) — off the Belize coast. The stakes were retrieved after 4–24 months and the vertical zonation and succession of higher marine fungi was recorded. Consumption of wood by shipworms (Teredo bartschi), the major decomposers, was measured by digital analysis of the area of wood consumed by these boring organisms.



Summary

A total of 20 species of marine Ascomycotina, 2Basidiomycotina, and 6 anamorphic fungi were identified from the experimental stakes. Differences in species composition between the two sites of Twin Cays and Man-of-war Cay (Belize) were observed, as well as a certain degree of patterning in the vertical distribution of fungi. Among Ascomycotina, members of Halosphaeriales show a definite tendency to thrive at greater depths than other species. Mangrove decomposition by shipworms was clearly higher in the nutrient-rich waters of Man-of-war, where the stakes were already heavily riddled after 8 months and had mostly disappeared after 2 years, while they were still intact at the other site. No significant difference in degradation of the 4 species of mangroves was noted.

B. Bebout

Papers

Decomposition of Mangrove Wood by Marine Fungi and Teredinids in Belize