[LUM#18] Thau, mirror of the future
With its 6,900 hectares of salt water, the Thau basin is the largest coastal lagoon in Occitanie and a major center for shellfish farming. This activity is now under close scrutiny due to the effects of climate change, which has already impacted the fragile balance of this small inland sea.
The summer of 2018 will long remain etched in the memory of shellfish farmers in the Thau lagoon. That year, unusually high rainfall in winter and spring, followed by scorching temperatures and a lack of wind, caused what is known as a "malaïgue," or bad water in Occitan. Deprived of oxygen for several days, the lagoon suffocated, and so did the shellfish. "100% of the mussels farmed in the basin died, along with 30% to 40% of the oysters, representing a loss of €6 million , " explains Franck Lagarde.
An exceptionally intense phenomenon that highlights the fragility of this unique ecosystem. "The Thau lagoon, this small inland sea, is a veritable open-air laboratory in which we can glimpse the future," explains the researcher from the Marbec* laboratory. This is because the average annual water temperature in the lagoon has already risen by 1.6 degrees in the space of 20 years.
Fragile balance
With warming waters coupled with more intense rainfall events in the Mediterranean basin, the fragile balance of the lagoon is under threat, and an entire economic sector is being called into question. Because there will be other malaïgues. " The risk of anoxia increases threefold for every degree of warming in the lagoon's water, as shown by the work of my colleague Valérie Derolez," explains Franck Lagarde. Faced with these prospects, professionals, researchers, and politicians are joining forces to safeguard the lagoon and its uses. "The good news is that all stakeholders in the basin are working together to adapt new farming practices and mitigate the effects of climate change."
For the specialist, the resilience of the pond also depends on continuing efforts to restore the ecosystem and biodiversity. This starts with eelgrass beds, marine grasslands that provide oxygen and thus limit the risk of malaya. "We must also encourage the return of species that were historically present in the lagoon but have since disappeared, such as flat oysters and European clams, which oxygenate the sediments and promote flow and biodiversity," adds Franck Lagarde. These ecosystem restoration approaches are nature-based solutions that will strengthen the ecosystem in the face of the more intense stresses we will encounter in the future.
“Will there be a limit?”
Efforts are also needed from professionals in the shellfish industry, who can also contribute to this rehabilitation, "for example by further improving cleanliness under the tables to limit the presence of plastic waste, pieces of nets, or piles of oyster bags and polyethylene ropes. Each plot must be clean to allow the return of this precious biodiversity."Individual and collective efforts that promote the resilience of the lagoon, "as evidenced by the absence of malaïgue in 2022 despite hydroclimatic conditions considered particularly sensitive."
Must-see:
- An animation by Valérie Dérolez illustrating the restoration process of the Thau lagoon, entitled "50 years of observation to understand the restoration dynamics and resilience of the Thau lagoon."
Listen to:
- In A l’UM la science, Caroline Montagnani, researcher at Ifremer and director of the IHPE** unit, talks to us about vaccinating oysters against ostreid herpesvirus.
*Marbec (UM, IRD, CNRS, Ifremer)
**IHPE (CNRS, Ifremer, UPVD, UM)
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