I agree with Erik on this one. With over 25 year water treatment experience, I would not ever see alum being used for biological control. While it is true that some of ortho-phosphate may be tied up and removed as a solid by the use of alum, the mechanism of disociation creates AlOH precipitate that in turn, ties up solids though flocculation. This reduces the alkalinity, reduces suspended solids, and "clears up" turbidity. The water will likely still have sufficient phophorus to sustain biological activity, including filamentous algae, particularly since sunlight penetration will be enhanced. Keep in mind that the phosphorus precipitated will soon be redissolved into solution through biological processes in the water. If you want to see a prolific FA growth, look at an ditch receiving a constant well stream sometime. Even though the water is nutrient poor, it supports a healthy FA growth due to the clarity of the water. My belief is that FA growth is more directly dependent on water clarity than any other factor.
