Alum as a device for algae control?

The local lakes in my area are in serious trouble.  High levels of phosphates have caused a brown algae bloom that is crowding out the higher order plants and causing other problems.  One of the plans to help clean up a small lake involves the addition of alum to the water.

The person who mentioned that to me didn't know how the alum would prevent algae growth, so I thought someone on the list might be able to explain it.  I presume that the aluminum would bond with the phospohates making them unavailable to algae, but I honestly have no real idea.  And if alum prevents algae in lakes, could we use it in our tanks as well?

-NL in cold, rainy Seattle where my family is thinking of having a fire to keep warm on the Fourth (100% chance of rain).  =(