Common duckweed (Lemna minor).
There's also a few watermeal plants in one photo; the seed-like specs that are clustered together in upper part of the sunlit ring finger.

From my limited experience, the watermeal will assume dominance, depending on temperatures and the nutrient levels in the water. Adding to Kelly's comments: watermeal does not have a root dangling from it's leaves. Duckweed does.

Watermeal is supposed to be a superfood, too. I might try using it as a mulch. Or in the compost heap.

Have a look at these posts:
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=485983#Post485983

http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=486149#Post486149