Phytoplankton bloom off Vancouver Island. Credit: NASA.
Phytoplankton are free-floating, oceanic plants life as opposed to zooplankton, the free-floating animal life, which feed on phytoplankton. Most of the organisms are microscopic, such as diatoms and dinoflagellates.
Phytoplankton are the 'producers' of the ocean because they make food for themselves by transforming energy from the Sun via photosynthesis. This energy is carried up the food chain as one animal eats the next. Since phytoplankton support all other animals in the food chain, their population size and the timing of their population increases or 'blooms' are important.