PlastiGar: microplastic pollution and contamination of organisms in the Garonne River

Researchers from the SMODD team at the Softmat laboratory, in collaboration with their partners at the EDB laboratory, have quantified microplastic pollution in the waters and sediments of the Garonne for the first time, in order to better understand the contamination of invertebrates and fish. This work, carried out as part of the PlastiGar research project funded by the Agence de l’Eau Adour-Garonne and the Occitanie Region, has recently been published and provides a better understanding of the extent of this pollution in freshwater ecosystems by identifying seasonal and geographical variations, as well as the contamination of aquatic organisms.

80% of plastics found in the oceans come from rivers. Rivers and streams act as conveyors of this pollution from the area where plastics enter nature to the oceans. During this sometimes lengthy transport phase, plastics age, oxidize, degrade and fragment into small pieces. These are known as microplastics, and are less than 5mm in size. Understanding the dynamics of this pollution in continental waters and the potential contamination of freshwater organisms is a crucial challenge.

First, the research team set out to characterise microplastic pollution in the waters of the Garonne river and its tributaries, using nets to filter surface water at 14 sites between the Pyrenees and the city of Agen, via the Toulouse area. In the samples collected in 2019, the scientists isolated almost 1,900 microplastic particles, ranging in size from 0.7 to 5mm, and made up of 3 main polymers of various colors. Microplastic pollution was extremely variable, ranging from zero pollution in some cases to over 3 microplastics per m3 of water in highly urbanised areas of Toulouse. There were also marked seasonal differences, with concentrations rising sharply as river flow decreased in summer.

Based on this observation, the scientists decided to focus their work on 6 sites located directly upstream, downstream and within the Toulouse conurbation, in order to gain a better understanding of whether these microplastics were contaminating the various species of invertebrates and fish in the Garonne river. The results showed that 2% of invertebrates and 10% of fish had ingested microplastics. In the contaminated fish, there were between 1 and 4 microplastics per individual. There were differences in colour and polymers between the microplastics ingested and those present in the sediment and water, indicating non-random consumption.

Using an innovative approach, scientists were able to better understand the origin of this contamination. Invertebrates, predators and large individuals, such as dragonfly larvae and crayfish, showed the highest levels of contamination, certainly indicating direct consumption of microplastics. Among fish, predators were not the most contaminated, but rather those, such as gudgeon or barbel, that feed in the sediments. For the species studied, these results indicate a mainly direct and accidental ingestion of microplastics composed of the densest polymers when they feed on the river bed.

As in other rivers in France and Europe, microplastic pollution and contamination of organisms are therefore omnipresent in the Garonne catchment. These phenomena are particularly acute around urban areas.
The ubiquitous presence of microplastics in ecosystems and the contamination of organisms raise new challenges for scientists, who will be working to gain a better understanding of how microplastics travel between water, sediment and organisms. It is also necessary to understand the ecological consequences of contamination on organisms and ecosystems, and how this pollution interacts with other forms of human pressure on aquatic biodiversity.

To find out more, please read the press release