John I'm not a fan of cutting off light either via shades...I fear DO crashes and fish kill events just like you. I prefer addressing the phosphorous issue directly and stripping it from the water column as that's a more long term approach. Also, a siphon to remove bottom water a few times a year is helpful also...easy to build and they run for free. Removing 2-3 feet of that high nutrient anoxic water is a great insurance policy. Lastly, consider beneficial bacteria...Scott is using it now with clients and reporting some pretty impressive initial results removing muck/nutrients from ponds. I always considered it snake oil...looking like I was misinformed. These are all potential solutions for your planktonic algae blooms.

Many men go fishing all of their lives without knowing that it is not fish they are after. ~ Henry David Thoreau

[Linked Image from i1261.photobucket.com]