Floating-leaved marsh is dominated by aquatic macrophytes with leaves that rest on and cover at least 50% of the water's surface or exceed cover values for submersed and emergent macrophytes. These marshes occur on large and small lakes, especially those with irregular shorelines and protected shallow bays, shallow lakes and ponds occupying steep-sided kettle depressions, and the backwaters of large and medium-sized rivers. Water depths of about 7-8 feet favor stands of floating-leaved aquatic macrophytes and are generally greater than stands of emergent vegetation, though there can be spatial overlap.
Leaves of the floating-leaved species vary in size and shape, but in many species are round, oval, or heart shaped. Pond lilies (Nymphaea odorata, Nuphar variegata) have large leaves, and when dominant, can cover virtually the entire surface of the water in the area they occupy. Watershield (Brasenia schreberi) is also successful in doing this in many acidic, shallow marshes. Under such conditions, the heavy shading can inhibit the development of beds of submergent or emergent plants. Some macrophytes, like long-leaf pondweed (Potamogeton nodosus) and floating-leaf bur-reed (Sparganium fluctuans), have floating leaves that are narrow or strap-shaped and there is an intermediate stage in the life cycle of wild rice (Zizania spp.) when the narrow, strap-shaped leaves are flexible and float on the surface.
Floating-leaved marsh may occur with other wetland and aquatic communities (especially other marshes and sedge meadows) in poorly drained glacial landforms such as till plains and pitted outwash. It also occurs in lagoons protected by sand spits along the Great Lakes shores, especially on Lake Superior. In unglaciated southwestern Wisconsin, the community occurs mostly within the floodplains of larger rivers as well as in impoundments.
