Duck weed, Lemna sp., concentrated at the edges of Langmuir circulation cells in Puno Bay, Lake Titicaca, Peru. When moderate winds blow over a lake, circulation cells are set in motion. Buoyant organisms, like duck weed, concentrate where the water currents downwell. Parallel rows of floating organisms or foam can sometimes cover a lake. In this photo, parallel rows are just beginning to form near a sunken ship.