you an use the green algae as mulch, it seems to me an easy sort of mulch, it is easier to pull algae off a pond than pull up or cut plants. Ken Fern has an Asian sort of algae, it is one tha picks up a lot of nitrogen because it is acompanied by nirtrogen fixing bacteria or some such, azolla its called, that goes a pretty red in some seasons. It is a quickley replenishing easy source of mulch. There are people who consider it a pest but it seems it is not too much of one. I should check all that out before writing.
Cattails are good if your water has pollutants in it they and the bacteria at their roots clean up pollutants.
I wonder if they will serve in my torrent rapid mountain stream that disappears in the summer mounths to block it a bit so that it flows slower and gets wider, i am hoping they will serve as a damn as well as a water cleaner. trouble is get one nice effect lose another. The torrent rushing over stones is pretty.
rose macaskie.