The Food Web
The Food Web
This is a simplified representation
of the food web showing the main pathways. Food (energy) moves in the direction
of the arrows. The driving force is sunlight. Depictions of the various
organisms are not to scale. The food web is a simplified way of understanding
the process by which organisms in higher trophic levels gain energy by consuming
organisms at lower trophic levels. All energy in an ecosystem originates with
the sun. The solar energy is transformed by green plants through a process of
photosynthesis into stored chemical energy. This is consumed by plant-eating
animals, which are in turn consumed as food. Humans are part of the food web.
The concept of the food web explains how some persistent contaminants accumulate
in an ecosystem and become biologically magnified.
Bioaccumulation And Biomagnification
The nutrients necessary for plant growth (e.g., nitrogen and phosphorus) are found at very low concentrations in most natural waters. In order to obtain sufficient quantities for growth, phytoplankton must collect these chemical elements from a relatively large volume of water.
In the process of collecting nutrients, they also collect certain human-made chemicals, such as some persistent pesticides. These may be present in the water at concentrations so low that they cannot be measured even by very sensitive instruments. The chemicals, however, biologically accumulate (bioaccumulate) in the organism and become concentrated at levels that are much higher in the living cells than in the open water. This is especially true for persistent chemicals - substances that do not break down readily in the environment -like DDT and PCBs that are stored in fatty tissues.
The small fish and zooplankton eat vast quantities of phytoplankton. In doing so, any toxic chemicals accumulated by the phytoplankton are further concentrated in the bodies of the animals that eat them. This is repeated at each step in the food chain. This process of increasing concentration through the food chain is known as biomagnification.
The top predators at the end of along food chain, such as lake trout, large salmon and fish-eating gulls, may accumulate concentrations of a toxic chemical high enough to cause serious deformities or death even though the concentration of the chemical in the open water is extremely low. The concentration of some chemicals in the fatty tissues of top predators can be millions of times higher than the concentration in the open water.
The eggs of aquatic birds often have some of the highest concentrations of toxic chemicals, because they are at the end of along aquatic food chain, and because egg yolk is rich in fatty material. Thus, the first harmful effects of a toxic chemical in a lake often appear as dead or malformed chicks. Scientists monitor colonies of gulls and other water birds because these effects can serve as early warning signs of a growing toxic chemical problem. They also collect gull eggs for chemical analysis because toxic chemicals will be detectable in them long before they reach measurable levels in the open water.
Research of this kind is important to humans as well, because they are consumers in the Great Lakes food chain. Humans are at the top of many food chains, but do not receive as high an exposure as, for example, herring gulls. This is because humans have a varied diet that consists of items from all levels of the food chain, whereas the herring gull depends upon fish as its sole food source. Nevertheless, the concerns about long-term effects of low-level exposures in humans, as well as impacts on people who do eat a lot of contaminated fish and wildlife, highlight the importance of taking heed of the well-documented adverse effects already seen in the ecosystem.
source: The Great Lakes: An Environmental Atlas and Resource Book Jointly produced by Government of Canada and the United States Environmental Protection Agency. 3rd ed. 1995 - For copies contact: Great Lakes National Program Office, USEPA, 77 W. Jackson Blvd., Chicago, IL 60604 USA