Grand-Lieu Lake, located in the southern part of Loire-Atlantique, undergoes dramatic seasonal changes. In winter, the lake expands to cover around 6,300 hectares, but by summer, its surface area shrinks to about 2,200 hectares. Similarly, the lake’s depth decreases from a maximum of 3.5 meters in winter to just 1.5 meters in summer. This fluctuation makes the lake particularly sensitive to environmental changes, especially those related to sediment and nutrient accumulation. The lake’s vulnerability is largely due to siltation, which is driven by intensive farming in its catchment area. This region spans 850 square kilometers across 42 local communes and includes the Ognon and Boulogne rivers, which carry significant amounts of soil and nutrients into the lake, particularly during winter when farmland is often left exposed. The use of fertilizers containing nitrogen and phosphorus has worsened the problem, leading to eutrophication—a process where excess nutrients cause rapid algae growth. These algae deplete oxygen levels in the water, posing a threat to fish and other aquatic life. Grand-Lieu is a vital habitat for numerous bird species, including gray herons and white storks, as well as migratory birds that rely on the lake for feeding and resting. However, ongoing siltation could eventually turn the lake into a marsh or wet meadow, diminishing its ecological importance. Over centuries, the accumulation of organic material has formed layers of peat, which were historically used as fuel until the 1950s. To combat the effects of siltation, efforts have focused on modifying agricultural practices. These include replanting hedges and maintaining vegetative cover to reduce soil erosion. Additionally, the restoration of river channels, such as the Ognon and Boulogne, involves recreating natural meanders and buffer zones to trap sediments before they reach the lake. The Buzay canal, which links the lake to the Loire River, helps manage water levels and remove suspended particles during flood events. The lake now holds a natural reserve status, limiting boat traffic to a small number of licensed professional fishermen to minimize environmental disturbance. Despite these conservation efforts, the lake continues to undergo gradual changes, prompting ongoing discussions about the need to reconsider agricultural practices to better protect such unique ecosystems.