Plankton, explained

What is plankton?

Plankton are a collection of tiny organisms that live at and beneath the surface of lakes, rivers, ponds, and oceans across the planet. They’re named for the Greek word planktos, meaning to drift or float. Plankton don’t swim on their own—they’re carried by tides, currents, and other forces, which determine where they go.

Plankton are an important food source for many large and small ocean creatures. They also play a vital role in absorbing carbon dioxide and producing oxygen in the ocean. 

Different types of plankton

The two most prominent types of plankton in the ocean are zooplankton, which are tiny animal organisms, and phytoplankton, which are plantlike. (Other forms of plankton include bacterial and fungal. Plankton are also not strictly aquatic; there is a wide assortment of aeroplankton that float through the atmosphere, just as their better-known counterpart float in the ocean.)

Many phytoplankton are microscopic, and they range from single-celled algae to bacteria to protists, which aren’t plants or animals. One thing they all have in common: They require sunlight. Phytoplankton float at or near the surface of the ocean, where they can use the sunlight and their chlorophyll to create energy. In the process, they take in carbon dioxide and release oxygen. 

Scientists estimate that at least 50 percent of the oxygen production on Earth comes from the ocean, a large majority of which is from phytoplankton.

In addition to being a crucial source of oxygen, phytoplankton form the base of aquatic food webs. Small fish, crustaceans, and zooplankton feed on phytoplankton. Those creatures are then eaten by larger fish and corals, which, in turn become food for top ocean predators like sharks.

Zooplankton live almost anywhere in the ocean but tend to be concentrated in the upper part—the same area as phytoplankton. Zooplankton range from microscopic animals like rotifers to krill (small crustaceans that are a critical food source for baleen whales and other creatures) to jellyfish. Although zooplankton are typically drifters not swimmers, some can swim quite well vertically, spending their time migrating from the sea surface to a half-mile below and back again each day. The larval form of many fish and crustaceans, like crabs and shrimp, are considered zooplankton, before they grow into their adult forms.

Threats to plankton

Ocean acidification, caused by excess carbon dioxide dissolving in seawater, poses a significant threat to phytoplankton. A more acidic ocean will cause some types to grow slower, some to grow faster, and the balance among them to change, which could have big ripple effects for the higher levels of the food web. Warming water because of climate change can have the same effects. 

Plankton are also threatened by microplastics—billions of tiny bits of plastic, less than five millimeters in size, which have spread into virtually every part of the ocean, from the deepest sea trenches to the water’s surface, where they get caught in algae. Microplastics can then block phytoplankton from receiving enough sunlight to survive. (Additionally, organisms that feed on phytoplankton also ingest large quantities of the microplastics.)

Where plankton suffers, so does the entire ocean food chain. 

Threats from plankton

When environmenmental conditions change, often triggered by too much of a nutrient—such as nitrogen or phosphorus from fertilizer runoff—populations of some types of phytoplankton can grow rapidly, in what’s called a “bloom.” Some blooms can be harmful, depleting oxygen in the water, blocking sunlight, and secreting toxins.

Harmful algae blooms, some of which are also known as red tides for their rust color, can decimate wildlife populations. A particularly brutal recurring red tide in Florida’s Gulf Coast has killed countless creatures from dozens of different species, including manatees, whale sharks, and critically endangered Kemp’s ridley sea turtles.