Plankton are free-floating, mostly microscopic plants, animals and bacteria. They generally cannot swim; instead, plankton are transported by tides and currents. The name plankton, like the word planet, comes from a Greek term meaning "wanderer."

Why are plankton important?

Plankton form the base of the Chesapeake Bay food web. All fish and shellfish depend on plankton for food during the early part of their lives, and some consume plankton their entire lives.

Plankton are often used as indicators of the Bay’s health because they have a short life span and they are very sensitive to environmental changes.

What types of plankton live in the Chesapeake Bay?

Plankton can be divided into three major classes: phytoplankton (algae), zooplankton and bacteria.


Phytoplankton (or algae) are tiny, single-celled plants. One drop of water from the Bay may contain thousands of phytoplankton.

Phytoplankton are the primary producers of food and oxygen in the Bay, forming the base of the food web. Like all plants, phytoplankton go through photosynthesis, so they need sunlight to live and grow. Therefore, the greatest number of phytoplankton are found near the water’s surface.

There are many species of phytoplankton, but diatoms and dinoflagellates are the major types in the Bay. When dinoflagellates dominate the water, they sometimes produce a red-tinted bloom called a mahogany tide.

The largest concentrations of phytoplankton in the Bay occur in spring, when heavy rain and melting snow wash excess nutrients into the Bay and its rivers. Nutrient pollution can cause phytoplankton populations to grow rapidly, forming algae blooms that can lead to low oxygen levels in the Bay’s bottom waters.

Scientists monitor phytoplankton populations because they are an effective way to measure pollution reduction efforts. Phytoplankton respond quickly to changes in nutrient levels, which gives researchers an excellent indication of the Bay’s health.


Zooplankton are free-floating animals. They are the most plentiful animals in the Bay and its rivers. One gallon of water can contain more than a half-million zooplankton.

Zooplankton form a link between the phytoplankton community and larger species at higher levels in the food web. The zooplankton community includes both primary consumers (which eat phytoplankton) and secondary consumers (which feed on other zooplankton).

Zooplankton are mostly microscopic, but can range in size from single-celled protozoa to large jellyfish.

  • Copepods—tiny crustaceans about one millimeter long—dominate the Bay’s zooplankton community.
  • Tiny fish and shellfish larvae are also considered zooplankton. Even though this planktonic stage is only temporary, larvae are a significant part of the zooplankton community.
  • Protozoa are single-celled animals that feed on detritus and bacteria. They, in turn, become food for larvae, copepods and other zooplankton.
  • Jellyfish and comb jellies are the largest zooplankton. Although these gelatinous creatures can swim, they are mostly at the mercy of tides and currents. Jellyfish and comb jellies feed on larvae and copepods.

All fish depend on zooplankton for food during their larval phase. Some species, such as shad, herring and anchovies, eat zooplankton their entire lives. One herring may consume thousands of copepods in a single day.

Like phytoplankton, zooplankton populations are an excellent indicator of the Bay’s health because they are sensitive to environmental changes. Scientists can get a good picture of current Bay conditions by looking at the amount and diversity of zooplankton.


Bacteria play an important role in the Bay. They are the decomposers, breaking down dead plants and other matter. Through this process, nutrients in dead plant and animal matter once again become available for growing plants. Zooplankton and other filter-feeding animals eat bacteria.

Some bacteria are permanent Bay residents. Others, such as coliform bacteria, are introduced through various pathways, including human sewage and polluted runoff.