How does an overpopulation of phytoplankton affect populations in an ecosystem

Overpopulation of phytoplankton results in a formation of algal bloom which is harmful because these algal blooms produce highly toxic and harmful compounds that affect the population such as birds, fish, mammals, and also people.

Excessive growth of phytoplankton affect the concentration of carbon dioxide, deplete the oxygen present in water and also results in unavailability of food that affects the marine organisms,