Feeding relationships are more complicated than the simple definition 'food chain' suggests. Some species feed on organisms in two or more trophic levels and change their food sources during their life history. Studies in this area, that include both an assessment of the trophic position and the body burden of the contaminants of interest, are scarce. An extensive work has been done in recent years in the Baltic Sea. Pollutant concentrations in aquatic animals are determined by the levels both in the surrounding water and in their food. Most persistent organic pollutants are found in greater abundance in aquatic animals than in the planktonic algae, but their concentration levels are strongly influenced by their chemical-physical characteristics. According to the study carried out in the Baltic Sea, two of the more toxic PCB congeners occur in higher concentrations in consumer organisms such as herring and cod than in phytoplankton. The same tendency has been demonstrated for the three most toxic PCDDs, including TCDD, but other PCDD congeners exhibit a different behavior. Zooplankton and fish, for instance, show significantly lower levels of the fully chlorinated octachlorodibenzo-p-dioxin (OCDD) than phytoplankton. In general, the bioavailability of such a molecule appears to be limited by its comparatively large size and its extremely low water solubility, which can limit its passage from one link to another in the food chain; biological barriers are more difficult to cross for molecules with such properties. As a measure of positions in these food chains, concentrations of the nitrogen isotope 15N, which becomes enriched compared with i4N in conjunction with metabolism, have been determined. Fish are the main constituents of the diet of some mammals such as seals, otter (Lutra lutra), mink (M. viso) and in bird species, particularly birds of prey and water birds. It is among these animals at the top of aquatic food chains that the most serious toxic pollutant problems have been found. A difference in biomagnification potentiality exists between aquatic and terrestrial food chains. In aquatic gill-breathing organisms, POPs do not undergo a marked biomagnification through the food chains because the species live in such close contact with the aquatic environment that the tendency for concentrations of pollutants to increase is counteracted by a constant tendency to achieve equilibrium with pollutant levels in the surrounding water (Figure 1). On the contrary, food is the principal source of persistent organochlorine insecticides for most trophic levels in terrestrial systems. Unlike gill-breathing animals, aquatic mammals do not take up persistent pollutants from the water, and they are equally unable to rely on the reverse process of their direct outward release from tissues into the water. In mammals and birds, concentrations of toxic substances are instead determined by the balance between their intake in food and detoxification mechanisms, which for persistent and bioaccumulating substances are slow. A further risk for nonaquatic animals is that many detoxification mechanisms include the formation of more polar metabolites to be excreted by water-based fluids, but the need for them to conserve water reduces this possibility. Among them, herbivores have more efficient systems for metabolizing foreign compounds than carnivores because of the nature of their diet. A critical window in the lifetime of these organisms is when they suffer starvation or a serious disease resulting in the use of their fat reserves, the tissue which have accumulated the highest levels of the contaminants, releasing a significant dose to reach the vital organs.
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