What are mangroves?

Mangroves are forests of trees called mangroves that can grow in salt water. They are mainly found in the intertidal zone, i.e. the tidal zone, or in estuaries.

There are between 60 and 70 species worldwide. The species that inhabit the mangrove depend on the habitat in which they are found. Different mangrove species have developed different adaptations to the conditions of very specific environments and are often less suited to more diverse vegetation. Some species can tolerate saltier environments than others, soils and waters that are less oxygenated, and can grow in muddy soils. To do this, these trees have developed impressive features such as stilt-like roots to capture oxygen, or the ability to remove salt through their leaves.

And in Mauritius?

There are only 2 species in Mauritius: Rhizophora mucronata, which is the most common on the island's coasts, and Bruguiera gymnorrhiza, which is only found in the north-east and east. In the past, mangroves covered almost 20 km2 of the island's coastline, but today they are shrinking like a stone... Since the 1960s, they have been destroyed mainly for firewood, coastal development needs, coastal property development and to create passages for boats. In 1980, only 0.45 km2 remained!

But why are they essential?

Thanks to their root system and dense foliage, mangroves provide an excellent habitat for many species of animals. The waters are calm, rich in organic matter and provide an ideal environment for a wide variety of aquatic animals, especially juvenile fish, which can grow up protected from predators. Many of these fish, crustaceans and molluscs are part of our diet, and their capture provides a significant income for local coastal communities.

But they have other properties too... Mangrove forests are like wooden reefs. Thanks to their structure, mangrove coasts are protected from cyclones and other meteorological events. The roots act as a bulwark, preventing waves from advancing and trapping sediment, while the compact foliage of these forests protects the coast from violent winds.

Mangroves are able to naturally filter large amounts of water from urban coastal areas. This natural filtration system helps to maintain the quality of the lagoon water and the ecosystems it supports, such as seagrass beds and coral reefs. Mangroves have the ability to purify these waters by absorbing nutrients and removing pathogens. Mangroves have been tested for domestic wastewater treatment in certain regions of East Africa and have proven to be highly reliable (Pumpsea project). They could be an effective and low-cost solution.

So what do you do?

Alarmed by the drastic decline in fish stocks, the Mauritian Ministry of Fisheries is promoting mangrove reforestation programmes. Between 1992 and 2004, a programme was implemented to protect and rehabilitate certain coastal areas with mangroves: 157,900 seedlings were planted. In 2013, the area covered by mangroves reached 1.45 km2 and is now close to 2.4 km2. Since 1998, legislation has been in place to protect mangroves and support these rehabilitation efforts. The Fisheries and Marine Resources Act 2007 states that no person has the right to cut, take, remove or damage mangroves. Unfortunately, mangroves are not yet safe as special permits can be issued to destroy mangrove areas.

The blue carbon of mangroves

At a time when climate change is at the forefront of global concerns, mangroves are an exceptional solution for offsetting our carbon emissions. Carbon sinks are reservoirs that absorb carbon dioxide (CO2) from the atmosphere. Global warming is caused by excessive concentrations of greenhouse gases in the atmosphere, including CO2. Forests play an important role in offsetting these high emissions by converting CO2 into organic matter through photosynthesis.

The older a mangrove is, the more carbon it stores, thanks to the dead wood that decomposes and the litter that accumulates at the base of the trees. Carbon remains sequestered for centuries, even millennia, resulting in carbon stocks and burial rates that are relevant to global climate change mitigation (Duarte et al., 2013; Macreadie et al., 2021). This organic carbon stored below the water surface is known as blue carbon. Seagrass meadows and salt marshes are also blue carbon sinks.

If we want to offset our atmospheric carbon emissions and thus limit the progression and impacts of climate change, it is essential to shift from deforestation to reforestation!