That's a LOT of FA. If the manpower is available, I'd get to work raking it out. Chemically, I'd look into one of the non copper based algacides. (Green Clean Plus, etc.)

www.hoosierpondpros.com

[Linked Image from lh6.ggpht.com]
http://www.pondboss.com/subscribe.asp?c=4
3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).