Food Web

The most important relationship among the species living in the Chesapeake Bay is their dependence upon one another as food. While plants and some bacteria can produce their own food through photosynthesis, animals must eat organic matter to acquire carbon: the basic element of proteins, carbohydrates, lipids, nucleic acids and other "building blocks of life" that make up the bodies of living organisms. Each organism in a food web supplies the fuel needed to sustain other life forms.

How Does a Food Chain Form?

As one organism eats another, a food chain is formed. Each step along a food chain is known as a trophic level or feeding level. Every organism can be categorized by its trophic level.

• The most basic trophic level is made up of producers: plants such as underwater bay grasses and free-floating algae that make their own food through photosynthesis. Producers are the basis of all food and influence the production of all other organisms.
• Organisms that eat plants, algae or other animals are called consumers.
• Decomposers digest the bodies of dead plants and animals. Decomposers appear throughout the food web, breaking organic matter back down into nutrients for the producers to use once again.

A Food Chain Example

This example of a simple food chain shows how organic compounds originally produced by a plant pass through successively higher trophic levels.

• The food chain starts with phytoplankton converting sunlight and nutrients into living tissue.
• Phytoplankton, in turn, are eaten by copepods, which are members of the microscopic animal community called zooplankton.
• Copepods are then eaten by bay anchovies, which are eaten by large fish such as bluefish and striped bass.
• These large fish can then be harvested and eaten by humans.

However, food production and consumption in the Bay are rarely as simple or direct as this example. Seldom does one organism feed exclusively on another. Usually, several food chains are interwoven to form a food web. This complex network of feeding continuously cycles organic matter back into the ecosystem.

How Are Food Webs Important?

Each organism within a food web is connected to and depends on others for food. Filter feeders such as oysters, clams and menhaden must have enough plankton available to sustain themselves. Striped bass and bluefish, part of a higher trophic level, rely on menhaden and bay anchovies as their primary food source.

A healthy ecosystem is one with a balanced food web — not too much production or consumption of any one of the producers or consumers.

• An ecosystem must be enormously productive to support substantial populations of species at the highest trophic levels. For example, for every pound of commercial fish taken from the Bay, almost 8,000 pounds of underlying producers and consumers had to be produced.
• However, an overabundance of algae can be harmful, reducing oxygen in the water and blocking sunlight from reaching underwater bay grasses.

How Do Chemical Contaminants Move Through Food Webs?

Food webs can also be a pathway for harmful chemical contaminants. Mercury and PCBs can pass to higher trophic levels in a process called bioaccumulation.

• Small bottom-dwelling organisms take up contaminants that are in bottom sediments while feeding or through skin contact.
• Larger fish accumulate toxins in their tissues when they eat contaminated smaller organisms.
• In turn, birds, other wildlife and even humans may eat contaminated fish, allowing the contaminants to continue to move through the food web.

The severe decline of bald eagles and osprey in the 1950s through the 1970s was the result of bioaccumulation of DDT, a pesticide used to control insects and agricultural pests. DDT caused eagles to lay extremely thin-shelled eggs that would break in the nest.