The seas are alive with tiny single-celled organisms called phytoplankton. These microscopic creatures account for the majority of the biomass in the oceans, and have a greater effect on our planet's climate than any other living thing - including all the world's forests. It takes a microscope to see a single phytoplankton, but the signs of untold billions can be detected from space. Such blooms may be indicative of pollution (extra nutrients, as far as the plankton are concerned) or a change in local ocean climate. Either way, they are a key warning sign of environmental change, and must be monitored constantly.
Photo: JRS/Ispra/ESA 2001