Floating heart or fringed water lily
(Nymphoides peltata):

Floating heart is a rooted plant with long branched stems, shallow scallop-margined leaves and bright yellow flowers. It grows in dense mats; outcompetes and excludes native plants; creates stagnant, low oxygen areas below mats; and impedes recreation and navigation. Floating heart is a native of Europe and Asia and now inhabits North America. Mechanical and hand removal may be most effective. There is also evidence that it can be controlled using herbicides.