What Is a Phytoplankton Bloom?

Phytoplankton are single-celled organisms — diatoms, dinoflagellates, coccolithophores — that float in the sunlit upper ocean and photosynthesise like plants. When conditions align — nutrient-rich upwelling, ample sunlight, favourable temperatures — their populations can double daily, creating dense blooms covering thousands of square kilometres. These blooms produce roughly half of all oxygen on Earth and form the base of the marine food web. However, some species produce toxins (harmful algal blooms, or HABs) that kill fish, contaminate shellfish, and endanger human health. Satellite imagery from NASA regularly tracks bloom dynamics globally.