Nutrient overloading and warming temperatures give rise to algal blooms which eventually die and decompose, depleting oxygen from the surrounding water. Many HAB species produce toxins which are harmful to both aquatic organisms and humans and can poison important water supplies. Following HAB-death, excess organic matter remains in aquatic ecosystems where its decomposition leads to oxygen depletion.

Map depicting the locations of HAB events from 1980 – 2021. Source: IOC-UNESCO

Issue diagram of various harmful effects of HAB events. Source: NOAA