Florida - Algal Bloom Dashboard, page-29

All algae dies. It is a lifecycle/lifetime related thing. Population growth and persistence will depend on nutrient availability. If you remove the phosphate from the water body the living algae will die off as per normal for algae. But they will not reproduce wildly which is what happens when they have plenty of nutrients available and conducive conditions. Yes nitrates are necessary too, but either one will probably act as a limiting factor if it is removed. Subtract the phosphate and the algae bloom will dissipate until more phosphate is added. It is like sheep on a paddock that dries out to bare sand.