At the sight of the immense plastic continents which pollute the oceans, we are more moved by the fate of whales or turtles than that of the studs and barbels of the Garonne. It is wrong. They too are entitled, well upstream, to a regrettable food diversification through sediments seasoned with microplastics. And, for the first time, researchers went fishing for information to quantify this “reality”, from the foothills of the Pyrenees to Agen.

The study PlastiGar (for Plastique Garonne) occupied over the year 2019 a dozen ecologists and chemists from two Toulouse laboratories * of the CNRS. During four seasons, they threw their nets, wielded the landing net, to filter surface water, scraped in the mud to sieve the sediments, and caught fish to make them regurgitate their food bowl.

Their quest was for microplastics (mainly derived from the fragmentation of larger waste) ranging from 0.7 to 5 mm. She has just delivered her verdict. Unsurprisingly, this polluting waste floats or lies well in the Garonne, up to 25 per m2. “The values ​​observed are of the same order as in the other large rivers in Western Europe,” explains Julien Cucherousset, ecologist at the Evolution and Biological Diversity Laboratory (EDB-CNRS, Univ. Toulouse III). Urbanization plays an important role leading to an increase in concentrations ”. The peri-urban area and downstream of Toulouse are particularly affected. And the problem worsens in the summer when flows are reduced or during floods.

In terms of the colors of the tiny fragments, white (32.4%) and black (31.1%) dominant. “90% of their composition is that of polymers used more in packaging,” underlines Aline Reis de Carvalho, a doctoral student in chemistry during the project.

Up to four pieces of plastic in a fish’s stomach

Regarding the Garonne, “globally all the species studied had plastic,” notes Julien Cucherousse. But not all individuals. Scientists focused on the outskirts of Toulouse where, according to their samples, 2% of invertebrates (insects and molluscs) and 10% of fish are contaminated. This being an average. “In the Hers [du côté de Launaguet] the contamination of fish drops to 16% ”, specifies Aline Reis of Carvalho. Each “plasticized” fish ate between one and four microplastics.

Among invertebrates, the most contaminated individuals are the rather large predators, such as dragonfly larvae and crayfish. While in fish, it is the studs and barbel, which feed on the sediment, which ingest the most plastic. “It is rather a direct and accidental consumption”, notes Julien Coucherousset. Not necessarily transmission through the food chain.

These realities being posed, and while PlastiGar teams attempt to analyze even smaller microplastics, other studies must now address the consequences of this pollution and other impacts of human activities on ecosystems.

* The Evolution and Biological Diversity laboratories (EDB – CNRS, Toulouse III University – Paul Sabatier, IRD) and Molecular Interactions and Chemical and Photochemical Reactivity (IMRCP – CNRS, Toulouse III University – Paul Sabatier)