French floating-solar experiment found fewer aquatic species as panel coverage rose, while one small crustacean group thrived
In Moulis, nestled within the French Pyrenees, there is an arrangement of small experimental ponds that are partially covered with arrays of solar panels on the water’s surface.
However, the reality beneath the water tells another tale. As scientists increased the percentage of surface area covered by the panels, there was a change in the life forms under the water, although not always in the direction anticipated by scientists.
A solar experiment unlike any other
It is a joint effort of scientists, working out of the Centre de Recherche sur la Biodiversité et l’Environnement (CRBE). They have been conducting the experiment for five years at Moulis using a controlled ecological approach to the issue. And the key instrument used in their experiments is the mesocosm, which is an artificially built mini-lake to model real floating photovoltaic (FPV) installations.
However, diversity, which refers to community evenness, not simply the number of species, decreased at intermediate FPV covers when the level of nutrients was high.
What makes mesocosms unique is that scientists can control the experiment, altering only one variable while keeping all others constant. Four levels of coverage (0%, 25%, 45%, and 65%) and various levels of nutrients simulating the diversity of lake environments across Europe, ranging from pristine mountain lakes to high nutrient-content lowland reservoirs, have been tested.
The ambition of the research cannot be underestimated. Five-year-long observation included monitoring of hydrophysical, biological parameters, biodiversity of different taxa, and ecological processes, such as photosynthesis and decomposition. Moreover, FPVs are spreading rapidly all over the world, but science still lags behind this development.
What the panels do to the water itself
Physical effects were very evident. An increase in the coverage of FPV resulted in a reduction in water temperature and evaporation regardless of the nutrient content of the water. The panels absorb solar energy, cast shadows and limit wind mixing of water; thus, they create an entirely new regime of illumination and temperature in the lake.
There were metabolic effects as well. Gross primary productivity, which is the process of photosynthesis made by algae and aquatic plants, and ecosystem respiration decreased with the increase of the panel coverage. The biological machine slowed down its work.
However, one crucial detail should be considered. Carbon dioxide dynamics and rate of decomposition were determined by nutrient content and not by the presence of panels. This is quite important when modelling the carbon footprint of FPV-covered lakes.
The crustaceans that thrived in the dark
This is where the story takes an unexpected twist. With respect to macroinvertebrates—insects, worms, and crustaceans constituting an important link in the freshwater food web—there was a decline in richness and diversity along with an increase in panel cover. However, one particular group stood apart from the trend.
The mesocosms were dominated by the Asellidae family, small crustaceans similar to pill bugs, at the level of 65% of panel cover. While other species disappeared, Asellidae flourished. The reasons behind this phenomenon are clear: these animals are adapted to conditions of low light and low oxygen levels associated with stressed shade water bodies, according to the Association for the Sciences of Limnology and Oceanography.
This is a simplified ecosystem in which one resilient group has managed to replace all others. In contrast, the control mesocosms without panels were dominated by the Chironomidae family, midges, in a more diverse and balanced environment.
Zooplankton: A more complicated story
However, not all groups reacted in the same fashion. The zooplankton group gave a little bit of a complex picture.
Here, the maximum was reached at 45% FPV cover. This means that there may be an optimum for certain species at mid-range FPV covers due to decreased predation and/or changes in the composition of algae that favor certain species of grazers. On the other hand, the diversity of the species was not affected by the presence of FPV panels. However, diversity, which refers to community evenness, not simply the number of species, decreased at intermediate FPV covers when the level of nutrients was high. In addition, the dominant taxa changed–Ceriodaphnia setosa, Simocephalus vetulus, and Polyarthra varied depending on coverage-nutrient combinations.
The conclusion one can make from the results of this experiment is that the influence of FPV is rather variable.
Why the trophic status of the lake changes everything
“Trophic status” is simply an indication of how many nutrients are in the lake. There is a big difference between nutrient-poor (oligotrophic) lakes and nutrient-rich (eutrophic) lakes, and this research indicated that FPV had either more or less effect depending on which one it was.
Richness and diversity of macroinvertebrates dropped most dramatically in the nutrient-rich mesocosms, indicating that floating solar power could potentially cause the biggest environmental impact on lakes that are already under pressure from fertilizers or pollutants. The one-size-fits-all approach does not consider the simple ecological fact that environment will play an important role in the outcome, but it can still be measured to be understood, according to CNRS Images.
Balancing clean energy with living water
None of this means floating solar should be abandoned. It generates electricity without consuming land and can reduce water evaporation in drought-prone regions. The wrong takeaway from this research would be to treat it as a verdict against the technology itself.
What the data do suggest is that lower coverage levels—around 25% or less—appear to impose far smaller ecological penalties, particularly in nutrient-poor lakes. That’s a starting point for more ecologically informed design standards. Long-term monitoring of commercial FPV installations, mitigation designs like gaps in panel arrays, and the responses of fish and amphibians remain open questions. The mesocosm study offers a foundation, not a final answer.
But it does leave a tension worth sitting with. As floating solar scales up to meet climate targets, the underwater world it casts in shadow—the midges, the crustaceans, the drifting zooplankton—deserves a seat at the planning table. The energy transition and the biodiversity crisis are happening simultaneously. How we navigate one shouldn’t quietly worsen the other.
Kelly is an experienced writer with 15 years of experience exploring the big stories that shape our world, from tech breakthroughs and space exploration to climate, energy, and the fascinating quirks of science. She has a talent for turning complex ideas into sharp, memorable insights that stay with readers long after they’ve finished reading.
