Welcome to the PB forum. Watermeal and duckweed are tough to control. There is a lot here on the subject including some temporary success by netting or dragging it out. One problem is these plants can double in volume every 36-48 hours. Read the links below on a recent discussion.

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

http://www.pondboss.com/ubb/ultimatebb.php?ubb=get_topic;f=14;t=000260;p=1

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