How can phytoplankton result in a public health risk?

Biomagnification may be the word you're looking for. This process involves phytoplankton being infected with a form of pollution, then ingested by a predator. This predator is then eaten, and the pollution collected from all of the plankton it has eaten is transferred to the organism that consumed it. This process continues until the animal that humans eat- most likely fish, in this case- is FILLED with toxins from all of the animals it has eaten that were all laced with pollution originally from the plankton.