Phytoplankton are microscopic organisms, mostly plant-like, that float near the surface of oceans, lakes, and rivers. Like land plants, they use sunlight to make their own food through photosynthesis, producing oxygen as they grow. Though each individual is far too small to see without a microscope, phytoplankton exist in such enormous numbers that they produce a large share of the oxygen in Earth's atmosphere.

These tiny organisms form the foundation of ocean food webs. Small creatures eat phytoplankton, larger fish eat those creatures, and so on up the chain to whales and sharks. Without phytoplankton, ocean life as we know it couldn't exist.

Unlike fish that swim wherever they want, phytoplankton drift with currents, going wherever the water takes them. When conditions are right, with plenty of sunlight and nutrients, phytoplankton multiply so rapidly they can turn the water green or brown in massive blooms visible from space. Scientists study these blooms carefully because phytoplankton populations affect everything from fish populations to Earth's climate.