Plankton

Plankton are free-floating, generally microscopic plants, animals and bacteria that are part of the lower food web. Plankton generally have limited or no swimming ability; instead, they are transported by tides and currents. The name plankton, like the word planet, is derived from a Greek word that means "wanderer."

What Types of Plankton Live in the Chesapeake Bay?

Plankton can be divided into three major classes: phytoplankton, or algae; zooplankton; and bacteria and viruses.

Phytoplankton

Phytoplankton, or algae, are tiny, single-celled plants. Phytoplankton are the primary producers of food and oxygen in the Chesapeake Bay — the base of the food web. Like plants on land, phytoplankton need sunlight to live and grow, so the largest concentrations of phytoplankton are found near the surface of the water.

Major groups of phytoplankton in the Chesapeake Bay include:

• Diatoms (Bacillariophyta)
• Golden brown algae (Chrysophyta)
• Green algae (Clorophyta)
• Blue-green algae (Cyanophyta)
• Dinoflagellates (Pyrrophycophyta)
• Cryptomonads (Cryptophyta)
• Microflagellates (Prasinophyta, Euglenophycota, Protozoa)

Excess phytoplankton due to nutrient pollution in the water is one of the primary reasons the Chesapeake Bay is unhealthy. When water temperatures rise in the presence of too many nutrients, phytoplankton multiply rapidly and form algae blooms. These algae blooms typically begin to occur each spring, as rain storms and melting snow wash excess nutrients into the Bay.

More phytoplankton may mean more food for animals at higher trophic levels, but too much phytoplankton can harm the Bay when fish and shellfish are not able to eat all of it. The uneaten phytoplankton die and sink to the bottom, where they are decomposed by bacteria in a process that depletes bottom waters of oxygen. Algae blooms also harm the Bay by blocking sunlight from reaching underwater bay grasses growing at the bottom of the Bay's shallows.

Zooplankton

Zooplankton are planktonic animals that range in size from single-celled protozoa to tiny fish larvae to larger jellyfish. One gallon of water can contain more than a half-million zooplankton.

The zooplankton community is composed of both primary consumers (which eat phytoplankton) and secondary consumers (which feed on other zooplankton). Nearly all fish depend on zooplankton for food during their larval phases, and some fish continue to eat zooplankton their entire lives. One herring may consume thousands of copepods — the most abundant type of zooplankton found in the Bay — in a single day.

• The smallest zooplankton are able to recycle nutrients found in the water column. Because of this, they are often closely tied to nutrient pollution measurements.
• Larger zooplankton are important food for forage fish and fish larvae. They also link the primary producers (phytoplankton) with larger or higher trophic level animals.
• Zooplankton also feed on bacteria and particulate plant matter.
• Tiny larvae of fish and invertebrates, which feed on copepods, are also considered zooplankton. Although this planktonic stage is only temporary, larvae are a significant part of the zooplankton community because they are a food source for larger animals.

Zooplankton are distributed according to salinity and the availability of phytoplankton, their main food source. Like phytoplankton, zooplankton make excellent indicators of environmental conditions within the Chesapeake Bay because they are sensitive to changes in its health. Scientists can get a good picture of current Bay conditions by looking at the amount and diversity of different species of zooplankton.

Bacteria

Bacteria play an important function in the Chesapeake Bay:

• Bacteria are the Bay's decomposers, breaking down dead matter. Through this process, nutrients in dead plant and animal matter again become available for growing plants.
• Bacteria are food for zooplankton and other filter-feeding organisms in the Bay.

Bacteria can be residents of the Bay or be introduced through various pathways, including human sewage and polluted runoff from the land.

How Are Plankton Important?

• Plankton communities form the base of the Chesapeake Bay food web, acting as food for fish, shellfish and other upper trophic level organisms. All fish and shellfish depend on plankton for food during their larval phases, and some species continue to consume plankton their entire lives.
• Plankton are often used as indicators of environmental and aquatic health because of their short life span and high sensitivity to environmental change.