Plankton blooms are a common phenomenon in the ocean. These are normally part life cycle of the ecosytem of plants (plankton), plant eaters, fish, etc. These blooms occur when many more than the normal numbers occur. That produces the "bloom". An increase in the numbers CAN throw the system out of balance.
Examples of common harmful effects of "blooms" include:
1. the production of neurotoxins which cause mass mortalities in fish, seabirds, sea turtles, and marine mammals
2. human illness or death via consumption of seafood contaminated by toxic algae
3. mechanical damage to other organisms, such as disruption of gill tissues in fish, resulting in asphyxiation
4. oxygen depletion of the water column