Dominique Chevillon, Vice President of the Board of Directors of the Marine Natural Park of the Gironde Estuary and the Sea of Pertuis, sheds light on the little-known role of coastal marshes
“The large marshes represent an essential source of primary biological production in the coastal area of Pertuis Charentais. They alone produce 1,500 grams of dry living matter per square meter per year. Such productivity is only found in the richest sectors of the tropical forests!
The essential role of diatoms
The extraordinary biological productivity of these areas unjustly described as “muddy” is due to the presence of microalgae that live in contact with the bottom: microphytobenthos (MPB). In Pertuis Charentais, the MPB is composed almost exclusively of benthic diatoms, a group of motile, brown microalgae that live in and on muddy sediments.
When the huge mudflats are uncovered by the tide (emergence), these diatoms form a biofilm on the surface of the mud. A small part of this considerable organic matter is consumed on site, the rest is exported out of the marsh, after being resuspended and transported by tidal currents. It is then used by many species of particulate filtering.
It is thanks to microphytobenthos (MPB) that the pertuis charentais represent the first oyster production site in Europe and the first mussel production site in France. team work Pierre-Richard (LIENSs, UMR 6250 CNRS-University of La Rochelle) have shown that most of the nutritional resources of these farmed molluscs come from MPB.
These huge marshes are frequented by thousands of birds at low tide. And many species of fish, such as sea bass, come here to feed at high tide.
A fragile environment that must be protected
Far from being expanses of barren mud, the marshes are an intense part of life in the Charente Strait. Interfaces between land and sea, these complex environments are also highly vulnerable. Its correct operation can be affected by the presence of chemical products, such as certain herbicides used in agriculture.
Benthic diatoms may also suffer from particulate pollution linked to the excavation of the Grand Port of La Rochelle, which suspends limestone particles. Therefore, we must collectively remain very vigilant to sustainably preserve these rich ecosystems! »
by Alexandrine Civard-Racinais
Read to expand this article
“The sea of Perthuis Charentais, nutritious and fragile”Pierre le Gall, Re Nature Environnement.