Unless I got the coordinates wrong, it appears to be part of a protected wetland area called Marais de Balençon. I can't find much more information aside from hiking trails and this very short description on the merlimont website.
I might be off by a bit though, navigating maps on a phone is a pain for me.
I found an explanation on a French blog. These shapes are called "Platières à bécassines". Bécassine is the French for the wild bird "snipe". The idea is to cut the grass/vegetation on some patches of the swamp in June (when it's dry), so that when water comes back in the autumn it creates small ponds that snipes like. It's for bird/duck hunting purposes basically.