Plankton are tiny open-water plants, animals or bacteria. The name, like the word planet, is derived from a Greek root that means, "wanderer." These organisms range in size from microscopic bacteria and plants to larger animals, such as jellyfish. Plankton generally have limited or no swimming ability and are transported through the water by currents and tides. In the Chesapeake Bay, plankton communities serve as a base for the food chain that supports the commercial fisheries.

Plankton can be divided into three major size classes:

• phytoplankton–microscopic plants and bacteria
• zooplankton–microscopic animals
• macrozooplankton–larger fish eggs and larvae and pelagic invertebrates

Plankton are often used as indicators of environmental and aquatic health because of their high sensitivity to environmental change and short life span. Phytoplankton are useful indicators of high nutrient conditions due to their propensity to multiply rapidly in the right conditions. Zooplankton are useful indicators of future fisheries health because they are a food source for organisms at higher trophic levels, such as finfish. Currently, research is being conducted in the Chesapeake Bay concerning how plankton react to different environmental conditions.

Like land plants, phytoplankton fix carbon through photosynthesis, making it available for higher trophic levels. The major environmental factors influencing phytoplankton growth are temperature, light and nutrient availability. Phytoplankton growth is usually limited to the photic zone, or the depth to which sunlight penetrates the water. Other limitations to growth are nutrients such as nitrogen and phosphorous, which are prevalent in the Chesapeake Bay.

Phytoplankton can undergo rapid population growth or "algal blooms" when water temperatures rises in the presence of excess nutrients, which typically occurs each spring in the Chesapeake Bay. While increased phytoplankton populations provide more food to organisms at higher trophic levels, too much phytoplankton can harm the overall health of the Chesapeake Bay. During these blooms, most of the phytoplankton die and sink to the bottom, where they decompose. This process depletes the bottom waters of dissolved oxygen, which is necessary for the survival of other organisms, including fish and crabs.

Major groups of phytoplankton in the Chesapeake Bay include:

• diatoms ( phylum Bacillariophyta)
• golden-brown algae (Chrysophyta)
• green algae (Clorophyta)
• blue-green algae (Cyanophyta)
• dinoflagellates (Pyrrophycophyta)
• cryptomonads (Cryptophyta)
• microflaggelates (Prasinophyta, Euglenophycota, Protozoa)

Phytoplankton are being used as indicators of environmental conditions within the Bay because their populations are especially sensitive to changes in nutrient levels and other water quality conditions. A good picture of the current conditions in the Bay can be derived by looking at phytoplankton indicators such as chlorophyll, primary production rates, biomass and species composition.

Zooplankton

Zooplankton are planktonic animals that range in size from microscopic rotifers to macroscopic jellyfish. Their distribution within the Chesapeake Bay is governed by salinity, temperature and food availability. The smallest zooplankton can be characterized as recyclers of water-column nutrients and often are closely tied to measures of nutrient enrichment. Larger zooplankton are important food for forage fish species and larval stages of all fish. They also link the primary producers (phytoplankton) with larger or higher trophic-level organisms. The zooplankton community is composed of both primary consumers, which eat phytoplankton, and secondary consumers, which feed on other zooplankton.

Zooplankton can be classified into three size classes:

• Microzooplankton–(protozoans and rotifers) are usually less than 200 microcrons in size.
• Mesozooplankton–(including copepods and invertebrate larvae) are between 200 microns and 2 millimeters in size.
• Macrozooplankton–(including amphipods, shrimp, fish larvae and gelatinous zooplankton or jelly fish)are greater than 2 millimeters in size.

Zooplankton, like phytoplankton, make excellent indicators of environmental conditions within the Bay, because they are sensitive to changes in water quality. They respond to low dissolved oxygen, high nutrient levels, toxic contaminants, poor food quality or abundance and predation. A good picture of the current conditions in the Bay can be derived by looking at zooplankton indicators such as their biomass, abundance and species diversity.