The Lac de Grandlieu is a lake located in Loire-Atlantique to the south-west of Nantes, approximately 10 kilometres south of the river Loire. Nearly all of the lake lies within the municipality of Saint-Philbert-de-Grand-Lieu.
In winter, it becomes the largest natural lowland lake in France.
The lake’s plant and animal life are remarkable and, as a result, a large part of it (2,694.6 hectares) is classified as a national nature reserve, while the westerly end (655.79 hectares) has regional nature reserve status. Some 270 species of birds, 19 species of reptiles and amphibians, 30 species of fish and 50 or so mammals inhabit the site. The environment is particularly well suited to the European eel. There are also 250 varieties of plants and 220 types of algae growing in and around the lake. Eight of these plants are internationally protected. Most of the lake is covered by water plants. In 1995, it was classified as a Wetland of International Importance by the Ramsar Convention.
While very large in size, Grandlieu Lake is extremely shallow (1.6 metres deep on average in summer and around 4 metres deep in winter), which explains its special ecological characteristics. It occupies a very shallow basin with a very shallow gradient at its edges, and as a result the lake’s contours are constantly changing. The lake can double in size during the year, going from approximately 35 square kilometres to 65 square kilometres in winter.
About 25 square kilometres of it, mostly to the eastern side, are formed from a core of livelier waters. The rest is composed of floating forests or ‘levis’, marshes, and flood meadows which are inundated with water in certain seasons.
The area of open water is at its maximum (about 2,200 hectares) in winter when the water plants disappear. This area has increased by nearly 13 hectares over the last ten years, reducing the area of reed-beds accordingly. A few commercial fishermen are licensed to fish in this open water. At the lake’s centre, the open water is scattered with floating pondweed.