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46.2E: Biological Magnification

  • Page ID
    14229
  • When toxic substances are introduced into the environment, organisms at the highest trophic levels suffer the most damage.

    LEARNING OBJECTIVES

    Describe the consequences of biomagnification between trophic levels

    KEY TAKEAWAYS

    Key Points

    • Biomagnification increases the concentration of toxic substances in organisms at higher trophic levels.
    • DDT is an example of a substance that biomagnifies; birds accumulate sufficient amounts of DDT from eating fish to cause adverse effects on bird populations.
    • The presence of polychlorinated biphenyls (PCB) in phytoplankton causes increased PCB concentrations in walleyes and birds.
    • Heavy metals, such as mercury and cadmium, found in certain types of seafood can also biomagnify.

    Key Terms

    • biomagnification: the process, in an ecosystem, in which a higher concentration of a substance in an organism is obtained higher up the food chain
    • dichlorodiphenyltrichloroethane: a chlorinated hydrocarbon which is mainly used as an insecticide (DDT)
    • apex consumer: consumers with few to no predators of their own, residing at the top of their food chain

    Consequences of Food Webs: Biological Magnification

    One of the most important environmental consequences of ecosystem dynamics is biomagnification: the increasing concentration of persistent, toxic substances in organisms at each trophic level, from the primary producers to the apex consumers. Many substances have been shown to bioaccumulate, including classical studies with the pesticide dichlorodiphenyltrichloroethane (DDT), which was published in the 1960s bestseller, Silent Spring, by Rachel Carson. DDT was a commonly-used pesticide before its dangers became known. In some aquatic ecosystems, organisms from each trophic level consumed many organisms of the lower level, which caused DDT to increase in birds (apex consumers) that ate fish. Thus, the birds accumulated sufficient amounts of DDT to cause fragility in their eggshells. This effect increased egg breakage during nesting, which was shown to have adverse effects on these bird populations. The use of DDT was banned in the United States in the 1970s.

    Other substances that biomagnify are polychlorinated biphenyls (PCBs), which were used in coolant liquids in the United States until their use was banned in 1979, and heavy metals, such as mercury, lead, and cadmium. These substances were best studied in aquatic ecosystems where fish species at different trophic levels accumulate toxic substances brought through the ecosystem by the primary producers. In a study performed by the National Oceanic and Atmospheric Administration (NOAA) in the Saginaw Bay of Lake Huron, PCB concentrations increased from the ecosystem’s primary producers (phytoplankton) through the different trophic levels of fish species. The apex consumer (walleye) had more than four times the amount of PCBs compared to phytoplankton. Also, based on results from other studies, birds that eat these fish may have PCB levels at least one order of magnitude higher than those found in the lake fish.

    image

    PCB concentration in Lake Huron: This chart shows the PCB concentrations found at the various trophic levels in the Saginaw Bay ecosystem of Lake Huron. Numbers on the x-axis reflect enrichment with heavy isotopes of nitrogen (15N), which is a marker for increasing trophic levels. Notice that the fish in the higher trophic levels accumulate more PCBs than those in lower trophic levels.

    Other concerns have been raised by the accumulation of heavy metals, such as mercury and cadmium, in certain types of seafood. The United States Environmental Protection Agency (EPA) recommends that pregnant women and young children should not consume any swordfish, shark, king mackerel, or tilefish because of their high mercury content. These individuals are advised to eat fish low in mercury: salmon, sardines, tilapia, shrimp, pollock, and catfish. Biomagnification is a good example of how ecosystem dynamics can affect our everyday lives, even influencing the food we eat.

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