Plankton

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

Plankton can be divided into three major classes:

• Phytoplankton, or algae, are planktonic plants.
• Zooplankton are planktonic animals, such as fish larvae and jellyfish.
• Bacteria and viruses.

Why are plankton important?

• Plankton communities serve as the base of the Bay food web, supporting 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.

Phytoplankton

Phytoplankton are tiny, single-celled plants. They are the primary producers of food and oxygen in the Bay food web. Like plants on land, phytoplankton need light to go through photosynthesis. Because of this, the largest concentrations of phytoplankton are found near the surface of the water.

Major groups of phytoplankton in the 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 over-enrichment is one of the primary causes of poor water quality in the Bay. Phytoplankton multiply rapidly when water temperatures rise in the presence of excess nutrients. These “algae blooms” typically begin to occur each spring, as rain storms and melting snow wash excess nutrients into the Bay.

While increased amounts of phytoplankton provide more food for organisms at higher trophic levels, too much phytoplankton or toxin-producing phytoplankton can harm the overall health of the Bay. When algae blooms occur, most of the phytoplankton are left uneaten by fish and shellfish, so they die and sink to the bottom. There, they are decomposed by bacteria in a process that depletes bottom waters of dissolved oxygen needed to sustain all aquatic life. Algae blooms also block sunlight from reaching underwater bay grasses.

Zooplankton

Zooplankton are planktonic animals that range in size from single-celled protozoa to larger jellyfish and comb jellies. 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.

• The smallest zooplankton are recyclers of water-column nutrients and are often closely tied to measures of nutrient over-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.
• Zooplankton also feed on bacteria and particulate plant matter.
• Copepods—tiny crustaceans about one millimeter long—are the most abundant type of zooplankton.
• 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.
• Nearly all fish depend on zooplankton for food during their larval phases, and some species continue to eat zooplankton their entire lives. One herring may consume thousands of copepods in a single day.

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 Bay because they are sensitive to changes in water quality. Scientists can get a good picture of current Bay conditions by looking at the amount, populations and the number of different species of zooplankton.

Bacteria

Bacteria play an important function in the Bay.

• They are the 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.