Can someone tell me a method, and/or a chemical for removing Duck Weed?

My friend has a small but deep pond, probably 50 yards by 30 yards. It got this stuff on top of it, and hasn't been able to get rid of it. Only tried chemicals one time, and they were thinned out. Seems it gets on there not far into spring, and is a mat on the pond clear up thru late fall.

Sure do appreciate the help!

See these.

http://www.pondboss.com/ubb/ultimatebb.php?ubb=get_topic;f=14;t=000231;p=1

http://www.pondboss.com/cgi-bin/ubbcgi/ultimatebb.cgi?ubb=get_topic;f=14;t=000258;p=1

http://aquaplant.tamu.edu/database/floating_plants/common_duckweed_mgmt.htm