There was a thread going a few years back about shading a pond with opaque material to block some of the heat gain in a southern pond. Our member anthropic participated. My interest at the time was to increase the heat gain in my cool water pond to increase productivity.

I don't recall whether we came to any conclusions about suspending material as you are suggesting. I tend to think that blocking the sunlight would limit beneficial algal production (plankton) in addition to blocking macrophytic growth.

It's possible your idea has not been tested. If you decide to give it a try, keep this thread up to date please. We'd all learn something valuable.