Chesapeake Bay - Zooplankton are the most plentiful animals

Sea Soup: Zooplankton
State of the Bay / By Karl Blankenship - Bay Journal

The term "zooplankton" describes the community of floating, often microscopic, animals that inhabit aquatic environments. Zooplankton are the most plentiful animals in the Chesapeake Bay and its tributaries. Being near the base of the food chain, they serve as food for larger animals, such as fish. One gallon of water can contain more than 500,000 zooplankton, ranging in size from tiny, single-celled protozoa to large jellyfish such as sea nettles. The most common zooplankton are the Crustacea, which include such animals as crab and barnacle larvae.

Because the zooplankton are a critical component of the Chesapeake Bay ecosystem, they are monitored monthly at 32 stations throughout the Bay and its tributaries. Using this information, it is possible to track changes in the habitat quality of the Bay over time, test for the presence of exotic organisms such as the zebra mussel, and develop environmental indices that reflect the relative "health" of different areas of the Bay.

Zooplankton are proving to be good indicators of water quality conditions, habitat quality for living resources, and the effects of toxic contamination in the Bay.

An example of one zooplankton environmental index that is still under development is an index of the food available for larval striped bass. Several studies have indicated that sufficient numbers of zooplankton during the critical life stages of larval striped bass are vitally important to their growth and survival. Although larval striped bass are able to survive several days without food, low densities of zooplankton diminish the likelihood of larval survival in a hostile environment where more than 90 percent of larvae perish. While other factors are also important, larval striped bass are not likely to survive and produce a strong year class without sufficient amounts of food.

To develop the index, four distinct feeding categories were established, based on the density of zooplankton per liter of water. These categories, "optimal," "minimum," "below minimum," and "poor" indicate the status of food availability during critical larval periods of striped bass. To apply the index, the spring (April to June) density of zooplankton was calculated for monitoring stations located in striped bass nursery areas.

Measures such as the food availability index demonstrate the use of zooplankton data in providing important information about the health of the Chesapeake Bay ecosystem. This index also illustrates how zooplankton act as a critical link between water quality and living resources. Additional zooplankton environmental indicators are currently under development for use in assessing the health of the Bay.

State-of-the-Bay is excerpted from the soon-to-be published Bay Program report, "The State of the Chesapeake Bay," edited by the Maryland Department of the Environment, Chesapeake Bay and Watershed Management Administration.

Karl is the Editor of the Bay Journal.

Article from Bay Journal - March 1995

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