Lemnoideae is a subfamily of flowering aquatic plants, known as duckweed, water lentils, or water lenses. They float on or just beneath the surface of still or slow-moving bodies of fresh water and wetlands. Also known as bayroot, they arose from within the arum or aroid family (Araceae), so often are classified as the subfamily Lemnoideae within the family Araceae. Other classifications, particularly those created prior to the end of the twentieth century, place them as a separate family, Lemnaceae.
I shot this on Chicester canal, I was surprised the ducks could move at all as there was so much weed.