In order to understand how contaminants travel through the various trophic levels of the ecosystem, the risk assessment team must be knowledgeable of the potential for the chemicals of concern to bioaccumulate. Bioaccumulation refers to the degree to which an organism takes up and retains a contaminant from all applicable exposure routes. Bioaccumulation takes into account that organisms may accumulate contaminants through multiple exposure routes and that the total accumulation will depend upon the rate of intake versus the rate at which the organism is capable of eliminating (through urine or feces) or breaking down the chemical through metabolic processes.
Bioconcentration refers to the absorption or uptake of a chemical from the media to concentrations in the organism's tissues that are greater than in surrounding environment. The degree to which a contaminant will concentrate in an organisms is expressed as a bioconcentration factor (BCF), which is defined as the concentration of a chemical in an organism's tissues divided by the exposure concentration. Thus, a BCF of 100 means that the organism concentrates that chemical to a concentration 100 times greater than in the surrounding media. Bioconcentration factors are most commonly applied to aquatic organisms such as fish or aquatic invertebrates. Within a species, bioconcentration factors differ for different chemical compounds. For example, the BCF for the water flea (Daphnia magna) for three chemicals is shown in Table 2.1. BCFs also vary among species for the same contaminant and site-specific environmental conditions can affect BCFs. Consequently, the data needed to calculate site-specific BCFs for particular species is collected for some ecological risk assessments. This usually involves analyzing the concentrations of contaminants in an organism's tissues and comparing this to the concentrations of those contaminants in the surrounding media.
Biomagnification refers to the tendency of some chemicals to become increasingly concentrated at successively higher trophic levels of a food chain or food web. As discussed in Section 1.3, producers take up nutrients from the surrounding environment in order to synthesize the complex molecules required for various biological processes. Because the available supply of many nutrients tend to be limited in the environment, plants often utilize considerable energy to actively pump these nutrients into their cells. They may even take up more than immediately needed and store them for future use. Thus, plants often have tissue concentrations of important nutrients that are higher than concentrations in the surrounding media. In some cases, pollutants that are chemically similar to some of these inorganic nutrients are present in the surrounding environment and are taken up and stored in plant tissues as well.
The first step in biomagnification occurs when contaminants are stored in producer tissues at concentration higher than in the surrounding environment. The second stage of biomagnification occurs when the producer is eaten by a consumer. Recall from Section 1.4, that relatively little energy is available from one trophic level to the next. This means that a consumer (of any level) has to consume a lot of biomass from lower trophic levels. If that biomass contains a contaminant, the contaminant will be taken up in large quantities by the consumer. Contaminants that biomagnify have another characteristic. Not only are they taken up by the producers, but they are absorbed and stored in the bodies of the consumers. This often occurs with contaminants that are soluble in fat, such as DDT or PCB's. These materials are digested from the producer and move into the fat of the consumer. If the consumer is eaten by another consumer, its fat is digested and the contaminant is then stored in the tissues of the new consumer. In this way, the contaminant builds up in the fatty tissues of the consumers and the concentration of the contaminant in the tissues of consumers becomes higher with each trophic level. Water-soluble pollutants usually cannot biomagnify in this way because they would dissolve in the bodily fluids of the consumer. Since every organism loses water to the environment, as the water is lost the pollutant would leave as well.
A classic case of biomagnification is that of DDT. During the years that it was used in the U.S., the pesticide DDT caused thinning of eggshells in many birds-of-prey, including hawks and eagles. DDT was present at low concentrations in water due to runoff from agricultural fields. However, DDT is very persistent (long-lived) and accumulates in fats. This resulted in biomagnification in the food chain, beginning with aquatic plants and invertebrates, then moving through fish, and finally to fish-eating birds. The lower doses in lower trophic levels resulted in no observable adverse effects. However, the high doses accumulated by fish-eating birds caused thinning of eggshells and reduced reproductive success, resulting in drastic declines in populations of these species nationwide.