Plankton are relatively small organisms whose movements are dominated by currents, though many can swim. They come in all shapes and sizes, the smallest of which are the bacteria, which are too small to be seen without a microscope. Phytoplankton include all kinds of microscopic plants, like diatoms and dinoflagellates. These tiny plants are critical primary producers that form the base of the ocean food chain. Zooplankton (animal plankton) graze on phytoplankton, or may eat other zooplankton. They are an important food source for many other marine animals, including fish, seals, and whales.
Studying plankton helps scientists understand many other things about the ocean, such as changes in fish stocks, pollution, and climate. Most phytoplankton are harmless, however, a few species can produce toxic chemicals. Shellfish like clams, mussels, and oysters are filter feeders that can consume these harmful plankton, causing the poisons to build up in their flesh. If other animals—including humans—eat these shellfish, they can get sick. It is important to monitor harmful algal blooms and understand the organisms that cause them.