An important criterion for the potential Persistent Organic Pollutants (POPs) requiring international management is the potential for the substance to biomagnify in food chains. Such biomagnification can result in increased and often unpredictable exposure to humans and higher predators in the environment. As discussed in Technical Background Sheet No. 3, biomagnification is of concern because the resulting increased concentrations could result in toxic effects.

In the initial screening of substances for nomination, the potential for biomagnification is addressed by evaluating a substance’s bioaccumulation potential, using measured or estimated bioconcentration and bioaccumulation factors, in aquatic organisms – especially fish. It may be asked whether this is a sufficiently protective evaluation or whether bioconcentration and/or bioaccumulation in terrestrial-based food chains, specifically in plants, should also be considered. Are additional criteria needed for the bioaccumulation in terrestrial plants?

To address this question, this note discusses several factors that influence the relative importance of bioconcentration and thus the potential for biomagnification of substances in terrestrial and aquatic ecosystems. How do these factors affect the relative ability of terrestrial plants and aquatic organisms to bioaccumulate POP-like substances and for these substances subsequently to biomagnify in humans and predators to levels that could cause adverse effects?

This discussion helps us decide whether it is sufficiently protective to consider only bioaccumulation and biomagnification in the aquatic environment, or whether separate consideration and thus separate criteria for bioaccumulation in vegetation and via terrestrial food chains is required.

The bioaccumulation of POP-like substances in terrestrial plants via root uptake from soil and soil water is much less than in aquatic organisms from water because the bioavailability of the substances is quite a bit lower in soil than water.

The lower bioavailability is mainly because these POP-like substances are highly sorbed to soils, and their solubility in soil water decreases with increasing organic carbon and clay content of the soil.

Furthermore, both the root uptake of substances from soil and also their subsequent translocation within the plant also depends on the water solubility of the substance. POP-like substances are extremely hydrophobic and have low water solubility. Therefore, these substances are only very slowly translocated within the plant from the roots to leaves and edible portions. For example, the maximum fraction transferred from soil to plants is only 1% for dioxins and furans, typical POP-like substances.

By contrast, in the aquatic environment there are fewer factors to decrease the solubility of these chemicals and their availability for uptake by aquatic organisms. The intimate contact of the aquatic organisms with water and suspended particles and the rapid flow of water across the gills of fish both contribute to efficient transfer of these substances from the water into the aquatic organisms. Only when the hydrophobicity is quite high (e.g. log Kow greater than about 7) and the water solubility extremely low are these substances to a large degree unavailable for uptake into aquatic organisms.

Studies in both temperate and Arctic ecosystems have shown that bioaccumulation is dependent on the length of the food chain to the organisms of interest.

Aquatic based food chains, whether freshwater or marine, tend to be quite long. A typical chain will consist of water - algae - invertebrate (1 or 2 steps) - fish - predators (1 or 2 steps). Moreover, critical pathways for the exposure of wildlife species via freshwater aquatic food chains often include benthic organisms. These organisms are important contributors of POP-like substances to food-chains. This is because sediments are often important sinks for hydrophobic POP-like substances and benthic organisms accumulate them directly from sediments and associated detrital food webs. By contrast, Arctic terrestrial food chains are usually very short, often consisting only of plants, a few herbivores and one or two main predators. The major food chain is air-plant- animal with perhaps one additional higher predator including man.

It is understandable, therefore, that field studies have confirmed that the organisms with the greatest exposure to POP-like substances, and consequently the greatest risk from biomagnification, include:

• the top carnivores of an aquatic food chain such as fish-eating raptors and fish eating colonial birds, (as well as birds such as peregrine falcons which feed on them).
• insectivores that feed on emerging aquatic insects; in fact, field studies have shown that birds that feed on terrestrial insect sources have lower body burdens than those feeding on insects emerging from aquatic environments.

Due to their hydrophobicity the dynamics of POP-like substances in aquatic and terrestrial food chains is closely related to the dynamics of lipids in the organisms. In fact, the concentrations of these substances in biota, especially in fish and predators, is determined by the concentration of these substances in the lipids of the foods that they consume and the efficiency of lipid absorption. Very little of these very hydrophobic substances is accumulated in organisms from gill respiration, for example, or inhalation of contaminated air.

In Arctic and marine food chains accumulation of POP-like substances is especially marked because high lipid levels are adaptations to the cold and cyclic annual productivity in these ecosystems. For example, a large portion of the body mass of marine mammals is fat or blubber which serves as both insulation and energy storage. The higher production, content and storage of lipids is thus particularly evident in arctic marine food webs. For example, the lipid content of primary producers, mainly diatoms, is very high and the organisms that feed on these primary producers have a strong propensity for storing these lipids during the growth season. Transfers to higher levels of the food chain occur over many years, as the relevant species, especially marine mammals, are very long-lived.

By contrast, within terrestrial ecosystems, POP-like substances tend to accumulate in and on the waxy cuticles of plants. This accumulation is a result of direct wet and dry deposition of particles containing these substances as well as exchange with fog and air. On the other hand, there is little transfer from soil to these waxy cuticles - due to sorption onto soil and the lack of translocation within the plant. Furthermore, in these ecosystems, POP-like substances are associated primarily with particles. Therefore much less is incorporated into the waxy cuticles of terrestrial plants than into the lipids of aquatic organisms.

For humans, as with predatory animals, exposure to POP-like substances is strongly related to the amount of lipid in the diet. Thus, it has been shown that for humans the major source of such substances is fish, especially fatty fish.

AMAP. 1998. AMAP Assessment Report: Arctic Pollution Issues: Arctic Monitoring and Assessment Programme. Oslo, Norway

Carey, J. P. Cook, J. Giesy, P. Hodson, D. Muir, J.W. Owens, and K. Solomon. 1998. Ecotoxicological Risk Assessment of the Chlorinated Organic Chemicals. Proceedings of Pellston Workshop. Society of Environmental Toxicology and Chemistry (SETAC). Pensacola, Florida.

Swedish Environmental Protection Agency. 1998. Persistent Organic Pollutants: A Swedish View of an International Problem. Stockholm, Sweden

Van Leeuwen, C.J. and J.L.M. Hermens. 1996. Risk Assessment of Chemicals: An Introduction.

Kluwer Academic Publishers.