Blue carbon. Why coastal salt marshes?

These wetland areas are generally created by the accumulation of silt in areas with low currents. The silt deposits do happen quickly, which means that in many cases, these marshes are regaining ground from the sea, although the sea level is rising due to global warming.  

Vegetation adapted to salty soils and sea spray is developing there. These are so-called halophilic plant species (adapted to salt) such as the Obione false-purslane (Halimione portulacoides, Aelen 1938) or the Salicornia (Salicornia sp.) which are specific to this environment. 

Due to the high accumulation of silt, oxygen does not penetrate deep into the silt and plant debris is very little degraded. Thus, ecosystems retain the carbon taken up by plants through the process of photosynthesis.  

The work carried out on salt marshes in the Pertuis Charentais (Charente Maritime and Vendée) has shown that these environments have the potential to process up to 8.8 TCO2eq/ha/year, which is the equivalent of the annual carbon footprint of a French person. These environments are therefore classified as carbon sinks, also called "blue carbon ecosystems" referring to the carbon retained by vegetated marine ecosystems. 

Salt marshes with high carbon absorption potential can also be found inshore of the coastal strip. However, their carbon capture (photosynthesis) and absorption capacities depend heavily on management methods and their state of health. If they are in poor condition, they can turn into a carbon source, i.e. become carbon emitters into the atmosphere. 

The Vendée marshes with its 800 ha in Talmont Saint Hilaire Jard, 1400 ha at Salines des Sables d'Olonne and 400 ha in Saint-Gilles Croix de Vie, offer a potential for "blue carbon" ecosystems in this territory that needs to be studied to really learn if they are sinks or sources of CO2. And so initiatives are being followed by the Carbone bleu des marais association on the Vendée territory. 

Let's keep in mind that in addition to being "blue carbon" ecosystems, coastal salt marshes have many other virtues and can provide a climate regulation service. They rise at the same time as sea level and dissipate storm waves, while preserving biodiversity and improving the quality of water and landscapes. They are an integral part of our natural and cultural heritage and are real "buffer" zones for pollution and the temporary storage of excess water. They thus protect our coasts from flooding and submersion. Coastal salt marshes thus offer many services, among which blue carbon is important to take into account to help us mitigate climate change. 

For more information:

Carbone bleu des marais 

Carbone bleu - Université de La Rochelle

La Rochelle, territoire zéro carbone

« Hé... La mer monte ! » de G. Bouzard, E. Chaumillon et M. Duméry