Common Duckweed and Watermeal (smaller plants)
Common duckweed is a free-floating aquatic perennial that forms a floating mat of minute individuals on the water surface. They have no true leaves or stems - instead they have a flat, green plant body called a thallus that is elliptically shaped and flat. They are extremely small, only about 1/16 to 1/8 inch in diameter.
Watermeal is very similar to common duckweed and belongs to the same family. Unlike its relative, duckweed, this plant completely lacks roots. The plant must be inspected closely to detect this difference. Watermeal may also resemble an algae bloom, but can be distinguished by simply touching the waters surface to see if tiny plants are stuck to your fingers (watermeal) or if the water has a slimy film (algae). It is commonly found growing along with duckweed. (Aquatic Plants and Algae of New Hampshire’s Lakes and Ponds)