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Title: Potential for biomagnification of contaminants within marine and freshwater food webs
Authors: Long-Term Effects of Dredging Operations Program (U.S.)
Kay, Stratford H.
Keywords: Aquatic ecology
Dredging spoil
Dredged material
Environmental aspects
Environmental effects
Heavy metals
Organic compounds
Water quality
Water pollution
Publisher: Environmental Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
Description: Technical Report
Executive Summary: The total yearly volume of materials dredged by combined Corps of Engineers and private operations averages about 290,000,000 m³. Pesticides and pesticide residues, nutrients, organic wastes, heavy metals, and other contaminants entering our waterways may associate strongly with particulate materials and eventually accumulate in the sediments. The presence of high levels of potentially toxic contaminants in some sediments has generated concern that dredging operations and the disposal of dredged material may cause the deterioration of the environment. Chemical residues which persist in the environment may be absorbed by plants and animals and accumulate within their tissues to levels that are greatly in excess of the ambient concentrations in their environment. Many of these substances have no known biological function and could accumulate to levels that are detrimental to the organism itself, or to its predators. Biomagnification may occur if the contaminant is persistent in biological systems and the food pathway is essentially linear, with the predominant energy flow from lower to higher trophic levels. (The meanings of biomagnification, bioaccumulation, and bioconcentration are defined as used in this text.) Although well known in terrestrial ecosystems, the occurrence of biomagnification in aquatic ecosystems is questionable and is the topic of considerable debate. The objectives of this report are multifold: review the literature on biomagnification of contaminants within aquatic ecosystems; determine the relative importance of food as a source of contaminants in aquatic food webs; pinpoint those contaminants which may significantly biomagnify within aquatic food webs; indicate the gaps in existing knowledge; and provide recommendations for future research on biomagnification of contaminants in aquatic systems. This report is part of a study to assess the potential impact of the open-water disposal of contaminated dredged material upon aquatic ecosystems and is limited in scope to water-breathing aquatic animals. The literature treating the bioconcentration of contaminants by and the toxicity of contaminants to marine and freshwater organisms is voluminous, in contrast to that regarding biomagnification. The available information suggests that mercury, particularly methylmercury, may be the only heavy metal that biomagnifies significantly within aquatic food webs. Food is also an important source of copper, zinc, and selenium, all of which are essential trace elements for animal metabolism, as well as arsenic, chromium, lead, and possibly cadmium, which are not known to have any biological functions. These metals do not biomagnify, however. Organic compounds which appear to have significant potential for biomagnification include polychlorinated biphenyls (PCBs), benzo[a]pyrene, the naphthalenes, and, possibly, a few organochlorine insecticides, such as dieldrin, endrin, kepone, and mirex. Relatively little food-chain information was available for other organic compounds, however. The data available indicate that biomagnification of contaminants in freshwater and marine food webs is not a dramatic phenomenon. As the biological availability of contaminants from sediments should be similar regardless of whether or not these sediments have been dredged and placed in an open-water disposal site, it appears unlikely that the open-water disposal of dredged material will have any substantial environmental impacts. Several important ideas regarding future research efforts have surfaced in this review and will now be summarized briefly. More emphasis needs to be placed upon using the proper experimental design to address the problem and upon using adequate numbers of experimental organisms to account for natural variation in the population. The concentration of a contaminant within living organisms should be expressed in as many ways as possible (fresh weight, dry weight, tissue, organ, lipid, etc.) to allow valid comparison with work done elsewhere. For the purpose of biomagnification studies specifically, the expression of contaminant concentrations on the basis of parts per million dry weight of the whole organism (with and without gut contents, where possible) is the most useful approach. From the perspective of field-oriented research, a number of recommendations have emerged. Trophic levels must be precisely determined using an accepted method, rather than by arbitrary assignment. When sampling in the field, all possible trophic levels should be collected at a given place and date, with a regular sampling schedule. Information on an organism's size, age, sex, and physiological state should be recorded, if possible. Gut contents should be analyzed chemically, and for species composition wherever possible. Data on physicochemical conditions should be taken at each place and on each date. Data from on-going field studies should be compared with those from any previous work at the same location. Laboratory studies need to concentrate upon those compounds which have very low water solubilities and high solubilities within specific tissue fractions, particularly fats or lipids. Chronic exposure to contaminants should be done at levels approximating those in nature and without the use of solvents, carriers, or chelators which may enhance water solubility and biological availability. Experimental food chains should include only species that actually are representative of those found in the natural ecosystem. Environmental conditions during exposure must also reflect as closely as possible those actually occurring in the natural ecosystem, or the data will have no valid application to a real system. Both the chemical species of a contaminant encountered in nature and its depuration following exposure must be considered. Background levels of the contaminant in the experimental organism and the organism's possible requirement for the contaminant (in the case of essential metals) must be evaluated in any bioaccumulation study. Finally, when using radioisotopes to follow the movement of a contaminant in a food chain, the data should be presented in terms of absolute concentration (parts per million, etc.), as well as in radiological terms such as disintegrations per minute.
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