Unseen by the naked eye, a community of predominantly microscopic organisms fuels the ecosystem we call the Chesapeake Bay. Plankton include free-floating microscopic plants (phytoplankton) and free-floating or weakly swimming animals (zooplankton) that range in size.
These tiny plants and animals drift in the water, largely at the mercy of the currents and tides. Some of the organisms move up and down in the water column to take advantage of light. Others will drop below the pycnocline, a layer where the increase in salinity is more pronounced, to avoid being washed out to the ocean.
Phytoplankton are tiny, single-celled plants. Although there are many species of phytoplankton, the major types in the Bay are diatoms and dinoflagellates.
Like other terrestrial plants, phytoplankton are the primary food producers in aquatic systems. Through a process known as photosynthesis, phytoplankton convert energy from the sun into organic compounds that they use as food. A byproduct of photosynthesis is the production of much-needed oxygen. The largest concentrations of phytoplankton occur near the water's surface, where light is readily available. Salinity also affects where phytoplankton are found, with the largest number of species preferring the saltier waters near the mouth of the Bay.
The amount of nutrients in the water is a major determinant in the abundance of these plants. The largest concentrations of phytoplankton in the Bay occur during the spring when nutrients are replenished by freshwater runoff from the watershed. These high concentra-tions produce the character-istic brown-green color of estuarine and nearshore waters. When dinoflagellates dominate the water, a red-tinted bloom, called a mahogany tide, may be is produced. Mahogany tides typically occur on warm, calm days, often after rain.
Diatoms, which are present throughout much of the year, may account for 50 percent of total algal production.
Changes in chemical conditions, such as exccessive amounts of nutrients, can cause rapid increases in the numbers of algae. These algal blooms can have serious consequences. They block light from reaching SAV beds.
Even after they die, they cause problems. Deposition and subsequent decomposition of large masses of plankton in the mainstem of the Bay can deplete dissolved oxygen, suffocating other estuarine animals.
Phytoplankton are the major food source for microscopic animals called zooplankton. Dominating the zooplankton are the copepods, a group of tiny crustaceans, about one millimeter long. Zooplankton are distributed according to salinity levels.
Distribution patterns are also related to those of their main food source, the phytoplankton. Zooplankton also feed on other particulate plant matter and bacteria.
The tiny larvae of invertebrates and fish are also considered zooplankton. Although this planktonic stage is only temporary, the larvae are a significant part of the community. These larvae are consumed by larger animals, and may, as they grow, consume copepods.
Another group of zooplankton found in the Bay are the protozoa. These single-celled animals feed on detritus and bacteria. They, in turn, become food for larvae, copepods and larger protozoa.
Bacteria have an important function in the Bay. They are essentially the decomposers. Their primary function is to break down dead matter, particularly plants. Through this process, nutrients in dead plant and animal matter again become available for growing plants. Bacteria are eaten by zooplankton and other filter- feeding animals in the Bay.
Bacteria can be residents of the Bay or introduced through various pathways, including human sewage and runoff from the land. Coliform bacteria are normal resident bacteria found in the intestines of mammals. The presence of coliform in a body of water indicates that human or other animal wastes are present. Coliform bacteria are also an indicator that disease-producing pathogens may be present in the water.
Clearly visible to the unaided eye, jellyfishes and comb jellies are the largest zooplankton. Some of these gelatinous creatures swim, though they are still at the mercy of the water currents. Jellyfishes have tentacles with stinging cells used to stun prey.
The most famous jellyfish in the Chesapeake is the sea nettle. Sea nettles feed voraciously on zooplankton, including fish larvae, comb jellies and even small fish. Because of the large volume of water in their bodies, few animals except sea turtles prey on sea nettles.
Comb jellies, lacking the stinging cells of nettles, capture prey with adhesive cells. They, too, consume vast quantities of small copepods and zooplankton, especially oyster larvae.
One gallon of Bay water can contain more than 500,000 zooplankton and one drop may contain thousands of individual phytoplankton!
Here, the cycle of Chesapeake Bay life begins. Phytoplankton, like algae, are consumed by zooplankton such as copepods. The copepods are in turn eaten by larval and juvenile fish which are then consumed by larger fish and wildlife. It is this intricate food web that makes it possible for you and me to enjoy the succulent and abundant food the Bay produces.
Kathryn Reshetiloff is with the U.S. Fish & Wildlife Service's Chesapeake Bay Field Office in Annapolis.