Biomagnification

All of the naturally occurring elements occur in the environment in at least trace concentrations. This ubiquitous contamination is always detectable, as long as the analytical chemistry methodology has detection limits that are small enough. About 25 of the elements are required by plants and/or animals, including the micronutrients copper, iron, molybdenum, zinc, and rarely, aluminum, nickel, and selenium. However, under certain ecological conditions these micronutrients can biomagnify to very large concentrations, and even cause toxicity to organisms.

One such case refers to serpentine soil and the vegetation that grows in it. Serpentine minerals contain relatively large concentrations of nickel, cobalt, chromium, and iron, and soils derived from this mineral can be toxic to plants. However, some plants grown on serpentine soils are physiologically tolerant of these metals, and can bioaccumulate them to very large concentrations. For example, the normal concentration of nickel in plants is about 1–5 ppm (parts per million, a concentration equivalent to mg/kg). However, on sites with serpentine soils much larger concentrations of nickel occur in plant foliage and other tissues. Nickel concentrations as large as 16% occur in tissues of a plant in the mustard family, Streptanthus polygaloides, in California, and 11-25% nickel occurs in the blue-colored latex of Sebertia acuminata on the island of New Caledonia in the Pacific Ocean. It is common for plants growing on serpentine soils to have nickel concentrations of thousands of parts per million, which is usually considerably larger than the concentration in soil.

Another case of biomagnification occurs on some sites in semiarid regions in which the soil is contaminated by selenium, which may then be hyperaccumulated (i.e., extremely accumulated) by specialized species of plants. These plants are poisonous to grazing livestock and other large animals, causing a toxic reaction called "blind staggers." The most important selenium-accumulating plants in North America are milk vetches in the genus Astragalus, in the legume family. There are 500 species of Astragalus in North America, of which 25 are accumulators of selenium. The foliage of these plants can contain thousands of ppm of selenium, to a maximum of about 15,000 ppm, much larger than the concentration in soil. Sometimes, accumulator and non-accumulator Astragalus species grow together, as in the case of a place in Nebraska with 5 ppm selenium in soil, and 5,560 ppm in Astragalus bisulcatus, but only 25 ppm in A. missouriensis.

Mercury can also be biomagnified from trace concentrations in the environment. In this case, trace concentrations of mercury in water can result in large contaminations of fish and other predators. For example, fish species known to bioaccumulate mercury in offshore waters of North America include Atlantic swordfish, Pacific blue marlin, tunas, and halibut, among others. These fish can accumulate mercury from trace concentrations in seawater (less than 0.1 ppm, equivalent to 0.0001 ppb or 0.1 mg/L) to concentrations in flesh that commonly exceed 0.5 ppm fresh weight (f.w.), the maximum acceptable concentration in fish for human consumption. The contamination of oceanic fish by mercury is probably natural, and is not only a modern phenomenon. Studies have found no difference in mercury contaminations of modern tuna and museum specimens collected before 1909, or concentrations in feathers of pre-1930 and post-1980 seabirds collected from islands in the northeast Atlantic Ocean. In this phenomenon of mercury biomagnification, there is a tendency for larger, older fish to have relatively large concentrations. In a study of Atlantic swordfish, for example, the average mercury concentration of animals smaller than 51 lb (23 kg) was 0.55 ppm, compared with 0.86 ppm for those 51–99 lb (23–45 kg) in weight, and 1.1 ppm for those heavier than 99 lb (45 kg). Large concentrations of mercury also occur in fish-eating marine mammals and birds that are predators at or near the top of the marine food web.

Mercury biomagnification has also been observed in many freshwater ecosystems, even in some relatively remote lakes where this may be a natural occurrence. Mercury concentrations exceeding 0.5 ppm are often measured in fish from lakes and rivers in many parts of the United States and Canada. For example, about three-quarters of 1,500 lakes monitored in Ontario had at least some fish that exceeded 0.5 ppm f.w. in flesh. In one remote lake in northern Manitoba, the mercury concentration averaged 2 ppm f.w. in northern pike, and one individual had 5 ppm. As in the marine case, fresh-water fish that are top predators have the largest concentrations of mercury, and older and larger individuals are the most contaminated. Because of the common occurrence of large mercury concentrations in fish in certain regions, some governments have developed fish-consumption advisories and/or restrictions. In Sweden about 250 lakes have been "blacklisted" in terms of fish consumption, and 9,400 other lakes are candidates for that status. In Ontario about 1,200 lakes have restrictions on fish consumption. Some fish-eating wildlife, such as loons and mink, may also be affected by mercury in their food.