Bioaccumulation and biomagnification describe how contaminants build up in living systems, affecting ecosystem and human health. These concepts explain the movement and concentration of substances like heavy metals and synthetic chemicals within the environment. While both involve the accumulation of harmful materials, bioaccumulation occurs within a single organism, and biomagnification happens across an entire food web. Understanding this difference is fundamental to assessing the danger pollutants pose to wildlife and the safety of the food we consume.
Understanding Bioaccumulation
Bioaccumulation is the process where a single organism takes up a substance from its surrounding environment faster than it can excrete or break it down. The organism absorbs the chemical directly from the air, water, or soil, or by consuming contaminated food. For a substance to bioaccumulate significantly, it must have a long biological half-life, meaning the body takes a long time to eliminate the substance.
Many of these substances are classified as persistent organic pollutants (POPs), which are characterized by their resistance to environmental degradation. Since they are often lipophilic, or fat-soluble, the organism cannot easily dissolve and excrete them in water-based waste. Instead, the body stores these chemicals primarily in fatty tissues and organs, leading to an increase in the organism’s total body burden over its lifetime. This internal concentration can increase even if environmental levels of the contaminant remain relatively low.
The Role of Trophic Levels in Biomagnification
Biomagnification describes the process where the concentration of a contaminant increases exponentially as it moves up the food chain, or across successive trophic levels. This is a food-web-based phenomenon. Trophic levels are the feeding positions in an ecosystem, starting with primary producers like plants and algae, and moving up through primary consumers (herbivores), secondary consumers (carnivores), and so on, culminating in apex predators.
The mechanism of biomagnification occurs because energy transfer between trophic levels is highly inefficient, typically only about 10%, but the accumulated contaminant is transferred almost entirely. A predator must consume a large biomass of prey to meet its energy demands, and with each meal, it inherits the cumulative contaminant burden of every organism lower on the chain. For example, a larger fish that eats hundreds of those small fish will accumulate a far greater total concentration of the substance in its own body. This multiplication of the substance’s concentration at each step means organisms at the top of the food web, such as eagles or sharks, can end up with concentrations thousands of times higher than those found in the environment.
Historical and Current Examples of Contamination
The consequences of biomagnification were demonstrated by the use of the insecticide dichlorodiphenyltrichloroethane (DDT) in the mid-20th century. While initially successful at controlling insect populations, DDT and its stable metabolite, DDE, began to biomagnify through aquatic food webs. Fish-eating apex predators, such as Bald eagles, peregrine falcons, and brown pelicans, accumulated high concentrations of DDE.
The high DDE levels interfered with the birds’ calcium metabolism, causing them to lay eggs with thin shells. These fragile eggshells were often crushed by the parents during incubation, leading to reproductive failure and population declines of several bird species across North America. The eventual ban of DDT in the United States in 1972, spurred by evidence of this reproductive toxicity, allowed populations like the Bald Eagle to slowly recover, illustrating the direct link between biomagnified contaminants and species survival.
An example of this phenomenon involves mercury, particularly the toxic organic form, methylmercury. This substance is released into the environment from sources such as coal-fired power plants and converted into methylmercury by aquatic microorganisms. Methylmercury bioaccumulates in small aquatic organisms and then biomagnifies up the food chain, reaching its highest concentrations in large, long-lived predator fish like swordfish, shark, and albacore tuna.
Humans, as apex consumers, are exposed to this contaminant primarily through the consumption of contaminated seafood. Methylmercury is a neurotoxin, and exposure can cause neurological damage, especially to the developing nervous systems of fetuses and young children. Health advisories recommending limits on the consumption of certain fish species for pregnant women and children are a direct public health response to the persistent threat posed by the biomagnification of mercury in our food supply.