Harmful algal blooms (HABs) occur when certain kinds of algae grow very quickly, forming patches or “blooms”, in the water. Blooms are more likely when water is warm, slow-moving and full of nutrients, such as nitrogen or phosphorous. These blooms can emit powerful toxins which endanger human and animal health.
Image from DAotM https://www.un-spider.org/links-and-resources/data-sources/daotm-HABs