Lake biodiversity supported by vegetated floating islands


By creating wide variations in water levels, human activity is having a significant impact on, and destroying the habitats of, some lake-based species. As part of the UROS project, the AFB-Irstea research and development hub in Aix-en-Provence and Ecocean are developing and testing vegetated floating islands that help stimulate biodiversity in lakes.


Ecosystems affected by tidal range

Lake ecosystems are particularly important to society because of the water resource they represent as well as for their rich biodiversity. At the heart of these ecosystems, shore zones are transitional areas between land and water in which are concentrated a large variety of habitats and species. Using lake water to meet hydroelectric, irrigation and drinking water requirements can create changes in water levels (tidal range) of dozens of meters, separating banks from shore zones and depleting habitat diversity. Often, these shore zones are then left with no vegetation. The depletion implies a loss of breeding habitats for some fish species along with the destruction of egg clutches, the loss of refuge areas and nurseries. Macroinvertebrates such as crustaceans, insect larvae, molluscs and worms are also affected by the tidal range. In France, where over 70% of lakes larger than 50 hectares are artificial, it is crucial to find solutions to this problem.

Restoring the functionality of shore zones

The French Biodiversity Agency (AFB), Irstea Aix-en-Provence Center and Ecocean have joined forces as part of the UROS Research and Development project to explore the potential relevance of artificial floating islands in restoring biodiversity in lakes. By moving up and down with the water, these rafts can recreate habitats that closely mimic those naturally present in lake shore zones while ensuring they remain permanently accessible to biodiversity.

Measuring 10 × 7 metres (70 square metres), the structure consists of an external rectangle of pontoons to ensure buoyancy, to which are fixed underwater wire mesh cages, patented and already in use at sea by Ecocean. "The cages have a dual purpose," explains Samuel Westrelin, researcher at Irstea's Aix-en-Provence center. "Some are empty and act as a shelter to protect young fish from predators, while others are filled with substrate for aquatic plants and animals to colonize, so that life can establish itself."

These artificial islands are thus capable of mimicking a variety of shore zones, stretching from the banks to underwater areas (1.5 m deep), and can hopefully provide habitats that are functional for fish and aquatic macroinvertebrates. This is the hypothesis being tested as part of the project. They could also be colonized by a wide variety of other animals such as birds or amphibians.

From theory to practice

In order to test the feasibility and relevance of the project, research teams launched 3 of the structures in September 2018 in the Serre-Ponçon reservoir, between the Alpes-de-Haute-Provence and the Hautes-Alpes departments, under guidance from the Serre-Ponçon Joint Planning and Development Association (SMADESEP), the National Alpine Botanical Conservatory (CBNA) and the Hautes-Alpes Fishing Federation. For at least 3 years, the islands will be continuously available on the lake, which has a tidal range of around 30 meters (a record -48 meters was achieved this spring). Colonization of the islands by aquatic macrofauna and plants will be closely monitored in order to optimize the functionality of the structures and evaluate their efficacy. "Environmental parameters, including oxygen, temperature and luminosity, will be measured. Additionally, the islands will be regularly monitored for colonization by fish and the growth of macroinvertebrate fauna will be quantified," specifies Samuel Westrelin. "Irstea will provide its knowledge in monitoring and scientific project validation and has also actively participated in creating the operational specifications of the islands," he adds.

"If they are effective, the islands could be used to improve the ecological status of many lakes, without affecting their use, as well as to support measures implemented to achieve good ecological potential in lakes (European Water Framework Directive) nationally and across Europe. If the results are positive, Ecocean could develop the product commercially. There is already demand for the product as the problem is widespread in France, where 70% of all lakes are artificial," concludes Samuel Westrelin. A project to watch closely!

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