Intermittent rivers: alarm bells for biodiversity

What are the specificities of intermittent rivers?

Thibault Datry : Intermittent rivers are watercourses that experience periods of no flow or completely without water. They are home to species adapted to these water or dry cycles, such as certain larvae and eggs, capable of surviving dormant during dry periods, before coming back to life when water arrives. Other species evolve into adult terrestrial forms just before the river dries up, breed outside the water during the summer, and lay eggs in the fall when the water returns to the rivers.
These rivers contribute to the food chain because the invertebrates they support provide food for fish and support food chains, often all the way downstream. Furthermore, they play a key role in the carbon cycle, transporting and breaking down terrestrial organic matter along hydrographic networks. Finally, they mitigate flooding, recharge groundwater and can limit the expansion of invasive species. The reference maps show that a large proportion of rivers are naturally intermittent: 60% of the ~7,500,000 km of rivers mapped by the IGN (BD Topo), such as the Albarine (01), the Gardons (30) or Drôme (26).

Intermittent rivers, located at the boundary between terrestrial and aquatic ecosystems, are fascinating intermediate environments, or ecotones.

How does biodiversity evolve in these environments?

Thibault Datry : By comparing data from the United States, New Zealand, Australia and Europe on biodiversity in the context of natural intermittency, the evidence was there: the number of species which disappears with the increase in the drying time was constant, whatever the territory. Even in the Amazonian plain of Bolivia, I discovered species and quantitative responses identical to those measured elsewhere. The importance of intermittency and common underlying evolutionary processes in rivers was demonstrated.

Climate change amplifies periods of drought and floods, which affects biodiversity. Perennial rivers become intermittent. Others dry out less and less. The transitions between terrestrial and aquatic phases in these rivers are particularly interesting. They can generate peaks in CO emissions₂ over a few hours when the water returns or can carry contaminants accumulated in their bed downstream, endangering the surface and groundwater bodies then connected. Finally, these phase transitions can be moments of rich interactions between terrestrial and aquatic biodiversity. These processes and their impacts are currently being studied.

How original is your approach?

Thibault Datry : I have developed my research from local to global scale through valuable collaborations.
The DRYvER project models the impact of climate change on intermittent rivers and the services they provide. Through citizen science, citizens can report drying rivers through the DryRivers app, helping to map these rivers in Europe and improve ecohydrological models to better manage them.
The “1,000 Intermittent Rivers” network brings together 250 scientists from 28 countries. Currently, in Lyon in the EcoFlowS team, we are comparing 150 environmental DNA samples from water and dried sediments taken from perennial and non-perennial rivers in these countries.

What do you hope for in terms of impact?

Thibault Datry : My objective is to raise awareness among water stakeholders and citizens of the importance of protecting fresh water and in particular intermittent rivers. Often perceived as useless, used as landfills or to build water reservoirs, or even removed from legislation because “atypical” or seen as “non-watercourse”, they require adapted management tools, and above all a change of consideration.
I have made recommendations used in national water management plans. A manual is intended specifically for water managers (SMIRES project). As a French expert within ECOSTAT, I raise awareness among EU Member States about the connectivity, even temporary, of permanent and intermittent rivers. A grant from Make Our Planet Great Again in 2020 allowed me to oversee the creation of a new framework for assessing the ecological status of these rivers.
90% of my data publications and publications are open access, as is the Intermittent River Biodiversity Database (IRBAS).

Each watershed contains intermittent rivers. We must consider intermittent rivers like any other river, even when they have no water.

And tomorrow?

Thibault Datry : The Water Framework Directive requires assessing the ecological state of water bodies, which raises the challenge of defining indicators specific to intermittent rivers to help managers preserve and restore global freshwater biodiversity. New tools such as satellite images and environmental DNA allow us to improve our understanding of the ecohydrological responses of these environments. It is also urgent to work with society (citizens, managers, politicians) to change perceptions. Finally, understanding what is at stake in the modifications of intermittency in response to global change is particularly important to me.