Biomagnification

Biomagnification, also known as bioamplification, or biological magnification is the increase in concentration of a substance, such as the pesticide DDT, that occurs in a food chain as a consequence of:

* Food chain energetics
* Low (or nonexistent) rate of excretion/degradation of the substance.

Although sometimes used interchangeably with 'bioaccumulation,' an important distinction is drawn between the two, and with bioconcentration.

* Bioaccumulation occurs within a trophic level, and is the increase in concentration of a substance in an individuals' tissues due to uptake from food and sediments in an aquatic milieu.
* Bioconcentration is defined as occurring when uptake from the water is greater than excretion (Landrum and Fisher, 1999)

Thus bioconcentration and bioaccumulation occur within an organism, and biomagnification occurs across trophic (food chain) levels.

Lipid soluble (lipophilic) substances cannot be excreted in urine, a water-based medium, and so accumulate in fatty tissues of an organism if the organism lacks enzymes to degrade them. When eaten by another organism, fats are absorbed in the gut, carrying the substance, which then accumulates in the fats of the predator. Since at each level of the food chain there is a lot of energy loss, a predator must consume many prey, including all of their lipophilic substances.

For example, though mercury is only present in small amounts in seawater, it is absorbed by algae (generally as methylmercury). It is efficiently absorbed, but only very slowly excreted by organisms (Croteau et al, 2005). Bioaccumulation and biomagnification result in buildup in the adipose tissue of successive trophic levels: zooplankton, small nekton, larger fish etc. Anything which eats these fish also consumes the higher level of mercury the fish have accumulated. This process explains why predatory fish such as swordfish and sharks or birds like osprey and eagles have higher concentrations of mercury in their tissue than could be accounted for by direct exposure alone. For example, herring contains mercury at approximately 0.01 ppm and shark contains mercury at greater than 1 ppm (EPA 1997).

Current status

In a review of a large number of studies, Suedel et al (1994) concluded that although biomagnification is probably more limited in occurrence than previously thought, there is good evidence that DDT, DDE, PCBs, toxaphene, and the organic forms of mercury and arsenic do biomagnify in nature. For other contaminants, bioconcentration and bioaccumulation account for their high concentrations in organism tissues. More recently, Gray (2002) reached a similar conclusion. However, even this study was criticized by Fisk et al., (2003) for ignoring many relevant studies. Such criticisms are spurring researchers to study carefully all pathways, and Croteau et al. (2005) recently added Cadmium to the list of biomagnifying metals.

The above studies refer to aquatic systems. In terrestrial systems, direct uptake by higher trophic levels must be much less, occurring via the lungs.

This critique of the biomagnification concept does not mean that we need not be concerned about synthetic organic contaminants and metal elements because they will become diluted. Bioaccumulation and bioconcentration result in these substances remaining in the organisms and not being diluted to non-threatening concentrations. The success of top predatory-bird recovery (bald eagles, peregrine falcons) in North America following the ban on DDT use in agriculture is testament to the importance of biomagnification.

Substances that biomagnify

There are two main groups of substances that biomagnify. Both are lipophilic and not easily degraded. Novel organic substances are not easily degraded because organisms lack previous exposure and have thus not evolved specific detoxification and excretion mechanisms, as there has been no selection pressure from them. These substances are consequently known as 'persistent organic pollutants' or POPs.

Metals are not degradable because they are elements. Organisms, particularly those subject to naturally high levels of exposure to metals, have mechanisms to sequester and excrete metals. Problems arise when organisms are exposed to higher concentrations than usual, which they cannot excrete rapidly enough to prevent damage. These metals are transferred in an organic form.

Novel organic substances

Further information: Persistent Organic Pollutant

* DDT
* PCBs
* Toxaphene
* Monomethylmercurysextonnotxes

Inorganic substances

* Arsenic
* Cadmium
* Mercury