However, these habitats are regressing at an alarming rate: 20 to 35% have already disappeared since the 1980s.
The cause ? The generalization of human activities both on the coast (urban and tourist development, intensive aquaculture, overexploitation of wood, fish, crustaceans and shellfish) and inland (intensive agriculture, mining, deforestation, etc.) (6).
The balance of these ecosystems is disturbed by agricultural runoff and sewage, which greatly increase the amount of nitrogen in the estuaries when they reach the coast. Mangroves can then become net emitters of N₂O (7). Furthermore, an estimated 10% of GHG emissions associated with deforestation would be related to the intentional destruction of mangroves, which releases CO₂ and other GHGs trapped in the sediments. These phenomenons participate in the retroactive loop of climate change, and thus aggravate the global warming of our planet. In the longer term, their territory are likely to be strongly reshaped by rising water levels, temperature changes (air and water), and depending on their resilience to extreme climate events.
The consequences of mangrove destruction are numerous and worrisome. Studies have shown a direct link between the degradation of mangroves and the decrease in surrounding fisheries resources, which could lead to greater food insecurity for surrounding populations (8).
In the Gulf of Mexico and the Caribbean, Sargassum (brown seaweed) blooms have been directly linked to the destruction of mangroves on the Brazilian coast, much further south. Nutrients carried by large rivers, previously filtered by the mangroves, flow out to sea and feed the algae which then form large drifting rafts. Once deposited on the coasts, the sargassum decomposes and rapidly produces (within 48 hours) hydrogen sulfide (H₂S) and ammonia gas (NH₃), in mechanisms comparable to those of green algae in Brittany. These gases are toxic for both humans and biodiversity (9).