Too transparent water: a defect of corbiculae, invasive clams that overly filter the Loire and affect nuclear power plants

Has the Loire water become too transparent? Appeared in France about fifty years ago, the corbiculae, also known as the Asian clam, colonized the Royal River to subtly modify its ecosystem. And there is already nothing to do.

A short arm of the Loire rises, guarded by its rubber warriors, before Damien Bouzens pours his hand into the bottom of the river, near Vouvre (Indre-et-Loire), between sand banks.

Project managers from the Indre-et-Loire Fishing Federation take out a handful of gravel. Very quickly, he removes the sand to keep only five or six small creamy yellow shells.

,It is a small biennial mollusk that arrived in the Loire Basin in the late 1970s. (…) Since the 2000s, it has been found in the main course of the Loire and its tributaries.“Like Indre, Cher or Creuse, he says.

impact on nuclear power plants

Intelligent, the Asian clam has little impact on human activities. However, it can interfere with the operation of nuclear power plants, colonizing and disrupting their cooling circuits.

Originally from Southeast Asia, where it is eaten as a seafood, Corbicula fluminia has colonized many rivers in France, Europe, and North America without making waves.

The Loire offers them a particularly welcoming habitat, according to David Brunet, in charge of the aquatic environment project at the Loire-Brittany water agency. Europe’s last great wild river, not much developed by man, still has an outflow”quite natural“And a sandy bed with gravel to bury quietly.

A favorable environment, especially since the hermaphrodite shell reproduces at a high rate: 70,000 larvae per year. The bivalves in the Royal River are so happy that the density reaches 50 to 300 individuals per square metre. Enough to attack the phytoplankton on which it feeds…

an impossible struggle

,Corbicule removes phytoplankton, suspended solids, which promotes clarity and allows for more efficient photosynthesis. The sun’s rays reach the bottom of the Loire“, explains David Brunet.

This change in water quality is not without consequences. Water clarification, which has also been an effect of investment in sewage treatment plants over several decades, has been beneficial to the development of aquatic vegetation. This herbariadid not exist in the 1980s”, assures Damien Buzzens.

On the positive side for the fisherman, these aquatic grass beds form a habitat for many species of fish. But it is the enormous filtration capacity of Asian molluscs that “raises questions today,

,This corbicule (…) is brought in to filter the water so much that the availability of phytoplankton, which form the basis of the food chain, is too limited for the growth of some species of small fish.“, he explains.

impact on the food chain

However, these cockroaches, spiralins and other chub also fall prey to predators such as pike.

,It disrupts the food chain. Essentially, if there is less phytoplankton, there will be fewer species that will end up in the bed of the Loire.“Sorry, David Brunet.

,It is an invasive species and all invasive species will disrupt the ecosystem, so there is no positive effect“, decides the technician.

Still, the fight against smaller clams, which have no effective predator, strictly speaking, on the Loire.”completely impossible“, according to experts.

So the ecosystem has to find a balance. This is already the case with Corbicule, whose population is declining. Global warming, and the threats it poses, may have had its effect, but the mollusk appears to be a victim of its adaptation: it probably lacks the phytoplankton to feed at all.