Giant salvinia (Salvinia molesta):

Giant salvinia is a free-floating rootless aquatic plant that grows in clusters and forms dense mats which shade and outcompete native plants. These mats also cause problems for fish by shading the water and creating low oxygen envioronments below them. Each node has a pair of floating leaves that are oval to ovate in shape. Plants also have a third leaf that is brown and hangs below the two floating green leaves. Below the leaves are free-floating root-like rhizomes. A native of Brazil, giant salvinia is now found throughout much of the southern US. Mechanical harvesting has been used to control salvinia; however, it will reestablish from any fragments that remain. Grass carp will eat salvinia, but they don't provide total control. Herbicides have been used with moderate success.