Phytoplankton are plants, zooplankton are animals. The phosphates and nitrates that are often found in sewage act as fertilizer.......add in abundant sunlight and you get an algae bloom. The blooms typically aren't as problematic as the depletion of oxygen that results when the blooms die off and decay.

Many scientists believe that these growth cycles of algae/plankton are what trigger Ice ages here on Earth.

Definitely some interesting reading on this subject..........