Lesser duckweed (Lemna minor). Found in ponds, lakes, and stagnant waters throughout North America below 58 deg. N..lat. Also in Europe (After Britton & Brown)
Zantedeschia elliotianayellow calla lily. Information of Plantzafrica website leads me to believe that this is species Z. elliotiana, although I have been know to be wrong about these things. Photographed in a private garden in Berkeley, CA