Mangroves constitute a complex socio-ecosystem containing an immense biodiversity integrating specific flora and fauna, as well as indigenous populations circulating along tidal channels and river branches. Mangroves represent only 1% of the surface of the earth and constitute one of the most productive biomes in terms of organic matter. often described as hostile, this biome structures the shores and constitute a real mechanical barrier against natural wave erosion. Moreover, according to a review published in 2011 by Nature Geosciences, mangroves make it possible to capture a large amount of carbon dioxide, up to 1,000 mg of carbon per hectare. This carbon stored in both mangrove mudflats and peats, called blue carbon, represents about 15% of the CO2 dissolved in the ocean. By 2035, around 75% of the population will be present on the coasts, thus it is necessary to look carefully at these very special places. During a report in the Scientific Method section on France Culture, François Fromard and Marie-Christine Comier-Salem revisit their book "Mangrove: a forest in the sea" where they explain in detail how mangroves work and what are their issues.
Photo : © DR Anton Bielousov
Mangroves, or mangals, occur (or grow) in the tidal sway zone of tropical and intertropical areas. They are therefore subject to a to-and-fro movement of salt and freshwater. On Earth, there are two large areas of mangroves that do not communicate with each other and have no species in common: the Eastern zone (Asia and East Africa) and the Western zone (American Pacific coast and Atlantic rim). Mangal flora is essentially made up of mangroves, characteristic trees, but also of ferns, palms and orchids. These alophystes species had to adapt due to particular constraints (or pressure) such as salinity or low oxygen under which they live,. Indeed, thanks to lenticels in its aerial part mangrove are able to breathe during high tides, and thus to fight against low oxygen conditions. In addition, these species have a double capacity to withstand high salinity thanks to the blocking of coarse salt crystals at the roots and the rejection of the finest ones by the foliar part. Furthermore, several symbioses between fauna and flora allow the survival of this particular biome through, for example, the soil bioturbation by crabs, which enriches it in oxygen. Concerning the fauna, many species take refuge in these areas, often uninhabited. Both brackish water and mangroves roots constitute a perfect nesting place for all kinds of fish. Mangroves also represent a migration site attracting many birds thanks to its incredible biodiversity. Indeed, the tropical climatic conditions, the richness and availability of food ensure favourable living conditions for migratory species and thus ensure their reproduction.
Thanks to this richness, man wanted to enhance the mangrove values by exploiting them. Ponds and canals were dug to allow shrimp to leave the sea and be trapped in cultivation ponds.Shrimp are reared to be sold creating a new income for local inhabitants.. Shrimp farming yields are sufficient for five first years. However, after this time iit is necessary to dig other ponds and canals to ensure the economic sustainability of their activity. This phenomenon is mainly visible at the level of the Mekong Delta where the customers’ demand for shrimp is high, leading to a significant decline in mangroves and the total disappearance of natural mangroves in this region. However, the tsunami of 2004, allowed a real awareness of the crucial role of mangroves. Some studies have shown that mangroves attenuate the force of tidal waves caused by tsunamis (about 20 to30%). Consequently, many policies and programmes for the reforestation of these "Green Walls" have been put in place. Unfortunately, these attempts often failed as they were not always carried out in the right areas and with the right plant species.
Despite a global disappearance of the mangrove surface, in some areas, such as in West Africa or Indonesia, there is a natural progression of mangroves. These areas of characteristic richness have a double beneficial impact, both environmental, thanks to its role as a mechanical barrier, a blue carbon sequestration, and socio-economic, through the reasonable exploitation of mangroves by populations to increase their incomes by fishing and seafood sale. It is therefore necessary to preserve mangroves, and to raise the public awareness of the various roles of these forests in the sea.
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