The Seine, long considered as a polluted river, has a remarkable transformation. Formerly synonymous with troubled and unsanitary waters, it is today the subject of large depollution efforts, in particular for the Paris Olympic Games 2024. These initiatives seem to bear fruit: a recent study by the Spygen research laboratory And the Office of Ecological Engineering (OGE) revealed the presence of three species of freshwater mussels threatened with disappearance in the heart of the capital.
These molluscs, natural indicators of water quality, were so far considered almost disappeared from the Seine basin. Their rediscovery raises essential questions on the ecological evolution of the river and the mechanisms which allow certain species to reappear in strongly anthropized urban environments.
An unexpected discovery at the heart of the capital
To identify the presence of these molds, scientists have used environmental DNA (EDNA). This technique consists in collecting water samples and looking for genetic traces left by living organisms. Each living being naturally loses cells in its environment, containing its DNA. The researchers removed and filtered 20 liters of water in eight strategic points in the Seine, especially near the island of the City and the Île Saint-Louis. Once filtered, the water was analyzed in the laboratory, where its DNA was extracted and amplified. Scientists compared it to reference databases to identify the present species. This process, used in ecology to detect rare or discreet species, revealed the presence of 23 species of freshwater mussels, including the three endangered species.
The analysis revealed an unexpected fact. Some of the species detected had not been identified in the Seine for decades. In particular, the rivers mullette was considered as disappeared from the Paris basin. “” We had historical data in the form of fossil shells, but no proof of a living population “Explains Vincent Prayed at Guardian. The magnitude of this rediscovery remains to be evaluated. Unfortunately, the technique of environmental DNA does not allow precise quantifying the number of individuals present or determining their exact location. To confirm the presence of a viable population, new field studies will have to be carried out.
An indicator of the return of a healthier ecosystem
Freshwater mussels are recognized as particularly reliable bio-indicators of the quality of aquatic environments. They are sensitive to the physico-chemical variations of water, in particular to the levels of nitrates, phosphates and heavy metals from agricultural and industrial discharges. Their complex life cycle, which implies a larval phase requiring the presence of specific host fish, makes them even more vulnerable to environmental disturbances. Their reappearance therefore suggests a reduction in organic and chemical pollution in the Seine. In parallel, the presence of 36 species of fish, against only a handful in the 1960s, testifies to a general improvement in the ecological conditions of the river. Some fish such as the bouvière (Rhodeus ammarus), which depends on the molds for its reproduction, could also benefit from this restoration of aquatic housing.
The progress observed in the Seine is resulting from several decades of efforts in terms of sanitation. Since the 1970s, the establishment of modern treatment plants has made it possible to considerably reduce the rejection of untreated wastewater. More recently, initiatives such as the water quality plan of the city of Paris and the upgrading of sanitation infrastructure upstream of the basin have further strengthened this dynamic. The work undertaken for the Paris 2024 Olympic Games, such as the modernization of treatment plants and the construction of the Austerlitz basin to limit wastewater discharges, undoubtedly contributed to the improvement of the quality of the water. However, scientists remain cautious about the exact causes of this ecological improvement. Vincent Prié stresses that the presence of these molds could also be linked to natural phenomena. Namely climatic variations or discreet migration of relicual populations.
The essential ecological role of freshwater mussels
The freshwater mussels play a crucial role in the regulation of aquatic ecosystems thanks to their filtration capacity. Each individual can treat up to 40 liters of water per day, thus eliminating suspended particles, bacteria, microalgae and even certain chemical contaminants. This exhausting function contributes to improving transparency and water quality. It promotes the photosynthesis of aquatic plants and the overall balance of the environment. Studies, such as those published in Science of the Total Environmenthave shown that mold populations collectively filter millions of liters of water in rivers and lakes. This reduces de facto Eutrophication and limits harmful algal proliferations. They also trap heavy metals and organic pollutants, incorporating them into their tissues or sedimenting them into the substrate, thus contributing to the natural bioremediation of watercourses.
Their life cycle is based on a complex interaction with Host Fish, essential for their reproduction. Microscopic larvae, called glochidia, must imperatively cling to the branches or fins of certain fish to develop. This mutualist relationship promotes the dispersion of molds through the watershed. A guarantee of their expansion and their ecological resilience. However, the collapse of populations of Host Fish, often linked to the degradation of habitats and pollution, compromises their reproduction. A study shows that the disappearance of certain species of mussels coincides with that of specific fish, which causes cascade disturbances in the ecosystem.
Perspectives and uncertainties for the future
Admittedly the reappearance of these freshwater mussels in the Seine is an encouraging signal. Nevertheless, it raises several questions about the mechanisms that have enabled their survival and their expansion in an urban environment. One of the factors envisaged is the increase in artificial light in Paris. It could stimulate the proliferation of phytoplankton, the main food of these molluscs. However, this hypothesis requires being deepened. Indeed, an excess of phytoplankton can also cause ecological imbalances, such as oxygen depletion of water.
In addition, researchers must determine whether these populations are really expanding. Or perhaps they already existed in the state of relics in unknown areas of the Seine basin. More precise statements, via dives and sedimentary analyzes, will be necessary to establish a reliable cartography of these species.
Climate change could also influence these dynamics. Higher temperatures and changes in hydrological diets may have favored conditions more conducive to these species. But they could also expose them to new risks. We can cite more frequent droughts or increased proliferation of invasive species. The challenge is therefore to follow these long -term populations to better understand their evolution and identify the best conservation strategies. Be that as it may, this discovery constitutes an additional argument to continue the restoration efforts of the Seine. It remains essential to further improve the quality of its water and to protect aquatic habitats. Sustainably restoring these ecosystems will not only benefit freshwater mussels. But also to all the biodiversity of the river and to the ecological services it renders to the inhabitants of the capital.
With an unwavering passion for local news, Christopher leads our editorial team with integrity and dedication. With over 20 years’ experience, he is the backbone of Wouldsayso, ensuring that we stay true to our mission to inform.
