Though they are microscopic in size, organisms called plankton play a big role in marine ecosystems. They provide the base for the entire marine food web. The word plankton comes from the Greek word planktos, which means “drifter.” Their name fits, because plankton do not swim on their own or stay in one place like coral. They drift about in the water, allowing tides, currents, and other factors determine where they go.

There are two main types of plankton: phytoplankton, which are plants, and zooplankton, which are animals. Zooplankton and other small marine creatures eat phytoplankton and then become food for fish, crustaceans, and other larger species.

Phytoplankton make their energy through photosynthesis, the process of using chlorophyll and sunlight to create energy. Like other plants, phytoplankton take in carbon dioxide and release oxygen. Phytoplankton account for about half of the photosynthesis on the planet, making them one of the world’s most important producers of oxygen. Phytoplankton rely on nutrients found in their surroundings, such as phosphate, nitrate, and calcium, to thrive.

In addition to phytoplankton and zooplankton, two even smaller kinds of plankton can be found floating in the sea. Bacterioplankton are bacteria and virioplankton are viruses.

Plankton can be found in saltwater and freshwater. One way to tell if a body of water has a large plankton population is to look at its clarity. Very clear water usually has less plankton than water that is more green or brown in color.

While plankton populations are needed for thriving marine ecosystems, too many plankton in one area can create a serious environmental problem. When a plankton population suddenly swells, it is called a “bloom.” When this happens with certain types of phytoplankton that release dangerous toxins, the region may experience a red tide or other serious algal bloom. These temporary conditions can cause high fish mortality and other damage to the marine ecosystem. Contaminated fish that are caught and served to people may also cause illness and even death.

Because the aquatic food chain depends so heavily on plankton, the survival of these tiny plants and animals is essential for healthy marine ecosystems. Climate change and rising sea temperatures pose serious risks to plankton populations.

National Geographic Explorer Gabrielle Corradino is a U.S. Department of the Interior Climate Science fellow whose research interests include the role of plankton in the marine food web. The more that scientists like Corradino understand how to protect these critical marine species, the more likely it is that their research will help creatures further up the food chain survive threats such as climate change.