Contaminant Sources And Effects

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The purpose of this section is to identify and describe the effects of significant environmental contaminants and other anthropogenic processes capable of disrupting ecosystems. We have focused on major pesticides (including organophosphorus and carbamate anticholinesterases and persistent organochlorines), petroleum and PAHs, heavy metals (lead and mercury), selenium, polyhaloge-nated aromatic hydrocarbons, and urban runoff. Toxicity of other metals and trace elements is included in Chapter 40 on amphibian declines, Chapter 44 on trace element interactions, and in three of the case history chapters. Chapters in this section on other important anthropogenic processes include nuclear and thermal contamination, global effects of deforestation, pathogens and disease, and abiotic factors that interact with contaminants.

About 200 organophosphorus (OP) and 50 carbamate (CB) pesticides have been formulated into thousands of products that are available in the world's marketplace for control of fungi, insects, herbaceous plants, and terrestrial vertebrates following application to forests, rangelands, wetlands, cultivated crops, cities, and towns.15,16 Though most applications are on field crops and other terrestrial habitats, the chemicals often drift or otherwise translocate into nontarget aquatic systems and affect a much larger number of species than originally intended. Hill (Chapter 12) provides an overview of the fate and toxicology of organophosphorus and carbamate pesticides. More attention is given to practical environmental considerations than interpretation of laboratory studies, which were detailed in the first edition of this book. Invertebrates, fish, amphibians, and reptiles are exemplified as ecosystem components and for comparison with birds and mammals. The focus is on concepts of ecological toxicology of birds and mammals related to natural systems as affected by pesticidal application in agriculture and public health. The environmental fate of representative OP and CB pesticides, their availability to wildlife, and toxicology as related to ambient factors, physiological cycles and status, product formulations and sources of exposure are discussed.

It is unlikely that any other group of contaminants has exerted such a heavy toll on the environment as have the organochlorine (OC) pesticides. Blus (Chapter 13) discusses the nature and extent of ecotoxicological problems resulting from the use of organochlorine pesticides for over a half century as well as the future relevance of these problems. Toxicity of OCs is described as influenced by species, sex, age, stress of various kinds, formulations used, and numerous other factors. The eggshell-thinning phenomenon, depressed productivity, and mortality of birds in the field led to experimental studies with OCs, clearly demonstrating their role in environmental problems. An assessment of the environmental impact of OCs leads to the conclusion that the ecotoxicologist must integrate data obtained from controlled experiments with those obtained from the field. In this manner through the use of the "sample egg technique" and other such innovative procedures, controversies over whether DDE or dieldrin were more important in causing a decline of peregrine falcons and other raptors in Great Britain could have been resolved. Although most of the problem OCs have been banned in a number of countries, exposure, bioaccumulation, and ecotoxicological effects will linger far into the future because of the environmental persistence of many compounds and their continued use in a fairly large area.

Petroleum and individual PAHs from anthropogenic sources are found throughout the world in all components of ecosystems. Chapter 14 (Albers) discusses sources and effects of petroleum in the environment. Less than half of the petroleum in the environment originates from spills and discharges associated with petroleum transportation; most comes from industrial, municipal, and household discharges, motorized vehicles, and natural oil seeps. Recovery from the effects of oil spills requires up to 5 years for many wetland plants. Sublethal effects of oil and PAHs on sensitive larval and early juvenile stages of fish, embryotoxic effects through direct exposure of bird eggs, and acute effects in vertebrates are discussed. Evidence linking environmental concentrations of PAHs to induction of cancer in wild animals is strongest for fish. Although concentrations of individual PAHs in aquatic environments are usually much lower than concentrations that are acutely toxic to aquatic organisms, sublethal effects can be produced. Effects of spills on populations of mobile species have been difficult to determine beyond an accounting of immediate losses and, sometimes, short-term changes in local populations.

Lead (Pb) is a nonessential, highly toxic heavy metal, and all known effects of lead on biological systems are deleterious. According to Pattee and Pain (Chapter 15), present anthropogenic lead emissions have resulted in soil and water lead concentrations of up to several orders of magnitude higher than estimated natural concentrations. Consequently, lead concentrations in many living organisms, including vertebrates, may be approaching adverse-effect thresholds. The influence of the chemical and physical form of lead on its distribution within the environment and recent technology to accurately quantify low lead concentrations are described. The chapter also discusses the most significant sources of lead related to direct wildlife mortality and physiological and behavioral effects detected at tissue lead concentrations below those previously considered safe for humans.

The widespread geographic extent and adverse consequences of mercury pollution continue to prompt considerable scientific investigation. Globally increasing concentrations of methylmercury are found in aquatic biota, even at remote sites, as a consequence of multiple anthropogenic sources and their releases of mercury into the environment. For example, in the marine food web of the North Atlantic Ocean, analysis of feathers of fish-eating seabirds sampled from 1885 through 1994 have shown a steady long-term increase in concentration of methylmercury.17,18 Wiener et al. (Chapter 16) characterize the environmental mercury problem, critically review the ecotoxicology of mercury, and describe the consequences of methylmercury contamination of food webs. Topics include processes and factors that influence exposure to methylmercury, the highly neurotoxic form.

This form readily accumulates in exposed organisms and can biomagnify in food webs to concentrations that can adversely affect organisms in upper trophic levels. Emphasis is given to aquatic food webs, where methylmercury contamination is greatest.

Reproductive impairment due to bioaccumulation of selenium in fish and aquatic birds has been an ongoing focus of fish and wildlife research, not only in the western United States but also in other parts of the world. Selenium is a naturally occurring semimetallic trace element that is essential for animal nutrition in small quantities, but becomes toxic at dietary concentrations that are not much higher than those required for good health. Thus, dietary selenium concentrations that are either below or above the optimal range are of concern. Chapter 17, by Ohlendorf, summarizes the ecotoxicology of excessive selenium exposure for animals, especially as reported during the last 15 years. Focus is primarily on freshwater fish and aquatic birds, because fish and birds are the groups of animals for which most toxic effects have been reported in the wild. However, information related to bioaccumulation by plants and animals as well as to effects in invertebrates, amphibians, reptiles, and mammals is also presented.

PCBs, dioxins (PCDDs), and dibenzofurans (PCDFs) are all similar in their chemistry and manifestation of toxicity, including a high capacity for biomagnification within ecosystems. Mammals, birds, and fish all have representative species that are highly sensitive, as well as highly resistant, to dioxin-like adverse effects, especially chronic reproductive and developmental/endocrine effects. Aquatic food chain species (seals, dolphins, polar bears, fish-eating birds, and cold-water fish species) with high exposure potential through biomagnification are particularly vulnerable. Rice, O'Keefe, and Kubiak (Chapter 18) review the fate of these environmentally persistent compounds and their toxicity, which is complex and often chronic rather than acute. As for PCBs the complexity begins with the large number of compounds, with varying toxicities, that are regularly detected in the environment (100 to 150). With dioxin- and dibenzofuran-related compounds there are fewer commonly measured residues (< 20). However, environmental problems are confounded since they are not directly manufactured but occur as unwanted impurities in manufacturing and incineration.

Urban runoff investigations, which have examined mass balances of pollutants, have concluded that this process is a significant pollutant source. Some studies have even shown important aquatic life impacts for streams in watersheds that are less than 10% urbanized. In general, monitoring of urban stormwater runoff has indicated that the biological beneficial uses of urban receiving waters are most likely affected by habitat destruction and long-term exposures to contaminants (especially to macroinvertebrates via contaminated sediment), while documented effects associated with acute exposures of toxicants in the water column are less likely. Pitt (Chapter 19) recommends longer-term biological monitoring on a site-specific basis, using a variety of techniques, and sediment-quality analyses to best identify and understand these impacts, since water column testing alone has been shown to be very misleading. Most aquatic life impacts associated with urbanization are probably related to long-term problems caused by polluted sediments and food web disruption.

In addition to natural background radiation, irradiation occurs from the normal operation of nuclear power plants and plutonium production reactors, nuclear plant accidents, nuclear weapons testing, and contact with or leakage from radioactive waste storage sites. Assessing the impacts of nuclear power facilities on the environment from routine and accidental releases of radionuclides to aquatic and terrestrial ecosystems is important for the protection of these ecosystems and their species component. The impacts of power-plant cooling systems — impingement, entrainment, elevated water temperatures, heat shock, and cold shock — on aquatic populations and communities have been intensively studied as well. Discussion in Chapter 20 (Meyers-Schone and Talmage) focuses on basic radiological concepts and sources as well as the effects of radiation on terrestrial and aquatic populations and communities of plants and animals. Radiation effects in this chapter focus on field studies, with supporting information from relevant laboratory investigations. Selected examples attempt to relate estimated doses or tissue levels to potential effects; however, dose estimates in the field are often imprecise, and observations are further confounded by the presence of other contaminants or stressors. Thermal toxicity is related to power-plant cooling systems.

Nearly 17 million ha of tropical forests are being cleared each year for new agricultural lands, equivalent to clearing an area the size of the state of Georgia or Wisconsin annually.19 Global effects of deforestation include irreplaceable loss of species, emissions into the atmosphere of chemically active and heat-trapping trace gases (carbon dioxide, methane, nitrous oxide, and carbon monoxide), and consequent global warming. Current emissions of greenhouse gases from deforestation account for about 25% of the global warming calculated to result from all anthropogenic emissions of greenhouse gases. Continued emissions of greenhouse gases from both deforestation and industrial sources will raise global mean temperature by an estimated 1 to 3.5°C by the end of this century. Houghton (Chapter 21) reviews the contribution of deforestation and subsequent land use with respect to the increasing concentrations of greenhouse gases in the atmosphere and projected global warming. Suggested remedial and preventative actions include (1) a large (>60%) reduction in the use of fossil fuels through increased efficiency of energy use and a much expanded use of renewable energy sources, (2) the elimination of deforestation, and (3) reforestation of large areas of land, either to store carbon or to provide renewable fuels to replace fossil fuels.

Pathogenic organisms are life forms that cause disease in other life forms; they are components of all ecosystems. Although ecotoxicology is often considered to be the study of chemical pollutants in ecosystems, pathogenic organisms and their diseases are relevant in this context in at least several different ways, as described by Leighton (Chapter 22). Pathogens can be regarded as pollutants when they are released by humans into ecosystems for the first time or when they are concentrated in certain areas by human activity. Four situations in which human activities can alter the occurrence of diseases in the environment include: (1) translocation of pathogens, including manmade ones, host species, and vectors, to new environments; (2) concentration of pathogens or host species in particular areas; (3) changes in the environment that can alter host-pathogen relationships; and (4) creation of new pathogens by intentional genetic modification of organisms.

Environmental factors have long been shown to influence the toxicity of pollutants in living organisms. Drawing upon controlled experiments and field observations, Rattner and Heath (Chapter 23) provide an overview of abiotic environmental factors and perspective on their ecotoxicological significance. Factors discussed include temperature, salinity, water hardness, pH, oxygen tension, nonionizing radiation, photoperiod, and season. Free-ranging animals simultaneously encounter a combination of environmental variables that may influence, and even act synergistically, to alter contaminant toxicity. It is not possible to rank these factors, particularly since they are oftentimes interrelated (e.g., temperature and seasonal rhythms). However, it is clear that environmental factors (particularly temperature) may alter contaminant exposure and toxicity (accumulation, sublethal effects, and lethality) by more than an order of magnitude in some species. Accordingly, it is concluded that effects of abiotic environmental variables should be considered and factored into risk assessments of anthropogenic pollutants.

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