Duckweed, Lemna minor L. is a common pond covering throughout the world.  The plant has a long traditional use as medicine.  It is also edible.  I have cooked it and blended it with wild leeks, stinging nettles and watercress...It does nothing to improve an already excellent soup. (More)
Early spring jump start of duckweed growth, by summer the pond will be completely covered.

Medicine:  Star duckweed, L. trisulca  L. was used en masse as a poultice to treat arthritis by Iroquois.