It is a fragile, precious environment, saved several times. But for how much longer? Between land and sea, the Etang de Thau is closely monitored by scientists. Because, in addition to the ecological uniqueness of this lagoon separated from the sea by islands of sand, the activity of many shellfish and oyster farmers depends entirely on its ecological balance.
Proof that we can reverse past mistakes and restore an environment, the Etang de Thau now lives under a new threat: the rise in the temperature of its waters due to global climate change.
The “malaïgue”, this bad water
In the 1970s, the Hérault basin was in agony. During the previous two decades, agriculture intensified and the number of inhabitants increased without sewage treatment plants keeping pace.
Unlike the sea where nutrients from rivers are dispersed, everything that arrives in the Etang de Thau, a relatively closed environment, accumulates. Result: very high phosphate and nitrogen inputs causing eutrophication, ie the development of macroalgae, then phytoplankton, to the detriment of seagrass beds.
And this is how “malaïgue” occurs, bad water in Occitan, which decimates oysters and mussels by asphyxiation because the algae consume all the oxygen present in the water. “Mortality reached up to 80% of shellfish production.
IN DECEMBER 2018, THE WATERS OF THE SETE BASIN AND CANALS TURNED EMERALD GREEN.
There was then a major mobilization for the purification of water in the catchment area,” explains Valérie Derolez. As for phosphate in detergents, designated as the main cause of the colonization of rivers and lakes by algae in 1990, it was gradually abandoned and then simply banned. “In fifty years, all phosphate discharges have been divided by 10 into the pond. The quality of the water has gradually improved,” continues the Ifremer researcher at the Marbec Marine ecology laboratory in Sète.
Even the Zostera marina seagrass eventually recovered. “The return of sea grasses in the 2000s further increases the resilience of the system: more sea grasses reduce the risk of asphyxiation”, continues the researcher.
A disturbing colonization
“As soon as the inputs stop, the trajectory is virtuous,” rejoices Rutger de Wit, researcher at the CNRS and the Marbec laboratory, specializing in ecological restoration. Before recalling a much less encouraging reality: “From now on, we must also take into account climate change. If the water temperature rises, this can be the new starting point of a crisis. This was the case in 2003, 2006 and 2018, despite all the efforts we had made. In December 2018, the waters of the Sète basin and canals turned emerald green in a few days.
In question, high heat in the fall and rains that loaded the rivers around the lagoon with nutrients. The combination of the two phenomena contributed to the colonization of the environment by a new microalgae, Picochlorum. Alas, the shellfish could not feed on it and the oyster farmers and shellfish farmers could only observe, powerless, their decline, for lack of food for their production. The researchers are worried, they know that such episodes will recur in the future because a single degree increase in water temperature triples the risk of eutrophication.
So what to do about rising temperatures? One way is to replace the oysters farmed today in France, which are Japanese cupped oysters, with a local spat that is more rustic and more resistant to heat. Mussels have a lower tolerance to rising temperatures. “At 27°C, they die. We are already seeing a gradual abandonment of shellfish farming in the basin, which will be amplified by climate change,” observes Valérie Derolez. Shellfish farmers may decide to cross the coastal strip and reach the sea.
The speckled seahorse or Hippocampus guttulatus. © Getty Images/iStockphoto
If the Etang de Thau is famous for its production of shellfish, another animal species is the pride of the area, the speckled seahorse or Hippocampus guttulatus, of which it is home to a large population. In 2005, a team from the Montpellier Institute of Evolutionary Sciences (Isem) decided to find out more about these strange fish. She therefore launched a research project on the Hippocampus guttulatus of the French coasts, more precisely on its genetic material.
Its results, published in 2019, are unexpected. The researchers have in fact discovered that within the species Hippocampus guttulatus, there are four distinct genetic lineages: in the Atlantic Ocean, there is one lineage in the north and another in the south of the Basque Country and, on the side of the Mediterranean Sea, there are also two lineages, one marine and the other lagoon.
Geographical isolation, therefore the impossibility of reproducing with other seahorses, but also adaptation to a specific environment, which is particularly the case for the seahorses of the Thau pond, have consequences on their chromosomes. Gradually, genetically distinct lines were formed. A rare event is happening in the living world: speciation. Each of these lineages will indeed become a separate species from the others. One more reason to protect the unique seahorses of the Hérault lagoon.