Why does phosphate act as an environmental contaminant?

Too much phosphorus in the water causes algae to grow faster than ecosystems can handle. Large growths of algae are called algal blooms and they can severely reduce or eliminate oxygen in the water, leading to illnesses in fish and the death of large numbers of fish.

Explanation

Futhermore, with a loss of fish the whole ecosystem will be upset and will be in need of serious recovery.