16.3: Toxicity

Toxic substances are substances that have adverse effects on organisms, including humans. The term toxic chemical is now among the most emotion-evoking in our society. In fact, there is a widespread belief, fostered by the media and many environmental interest groups, that the release of any quantity of toxic chemicals to the environment, particularly the oceans, is harmful and must be stopped. This belief is unfounded (CC18). Many chemicals that are toxic at high concentrations are naturally occurring elements or compounds that have been present in the oceans throughout geological time. Some additional quantities, in many cases substantial quantities, of these substances and even synthetic chemicals that do not occur naturally can be safely accommodated by ocean ecosystems. However, if they are to cause no harm, wastes must be disposed of in ways that ensure that the ocean’s assimilative capacity is not exceeded either globally or locally. Marine pollution problems occur when chemical discharges exceed the assimilative capacity and cause adverse effects, including lethal or non-lethal toxicity to species within the ocean ecosystem. Therefore, determining this capacity for each local region into which toxic substances are discharged is an important scientific task.

A second widespread belief is that the ultimate solution to contamination by toxic chemicals is to recycle everything. Recycling is a highly desirable and rapidly growing practice that continues to reduce quantities of potentially toxic chemicals released to the environment. However, recycling can never be 100% effective. Chemicals that can be toxic at high concentrations are present in small quantities in almost every natural and synthetic product in our society, including human waste excretions. For many of society’s waste products, the energy costs and associated adverse environmental effects of recycling and removal of traces of potentially toxic chemicals may far outweigh the environmental costs of properly managed disposal of the waste. In addition, storing all toxic chemical–containing wastes in “secure” landfills is not feasible. In most instances, using valuable land to dispose of large-volume wastes that have low concentrations of toxic chemicals is inappropriate. Perhaps more importantly, no landfill can ever be secure on a geological timescale. Wastes containing high levels of long-lived potentially toxic substances that are buried in landfills will eventually release these substances to freshwater and groundwater and from there to the oceans. The Earth’s freshwater resources are severely limited, and any contamination of the freshwater poses a threat to human health and terrestrial ecosystems. Consequently, many environmental scientists believe that properly managed ocean disposal of wastes may be the environmentally preferable management option for wastes that cannot be efficiently recycled and that have low concentrations of potentially toxic chemicals.

The fate and effects of chemicals in the oceans are determined by how they are transported by currents, how they are removed and incorporated in sediments, and how they are taken up by the marine biota. Several chapters in this text describe the movements of ocean water and suspended sediment and the processes of marine sedimentation that determine the distribution and ultimate fate of dissolved and particulate constituents in the oceans.

Effects of Toxic Substances on Marine Organisms

CC18 describes the general principles that determine the toxicity of substances to living organisms. The essential point is that most substances are toxic to a specific organism only when their concentration in food or in solution in the surrounding water exceeds a certain level, above which, the concentration of the chemical within the organism is high enough to interfere with one or more biochemical processes critical to the organism’s life cycle. For all potentially toxic chemicals, there is a concentration below which the chemical has no adverse effects.

The only exception to this rule may be for compounds that are carcinogens (cancer-causing), mutagens (causing genetic changes in the offspring by altering the parents’ DNA), or teratogens (causing abnormal development of the embryo). There are conflicting views about whether a threshold concentration exists for such compounds. If there is no threshold concentration, any concentration of the compound will increase the incidence of disease. However, below a certain concentration, the number of individuals affected by a carcinogenic, mutagenic, or teratogenic chemical will be far smaller than the number similarly affected by naturally occurring compounds and natural radioactivity, which also have such effects. Hence, even for these compounds, there is a concentration below which they have no significant adverse effects.

Evaluating Toxicity 

Assessing the toxicity of substances to marine organisms is very difficult. Two general approaches are used. First, the distribution of species in an ocean region affected by inputs of that substance can be compared with the distribution in similar but unaffected regions or in the affected region before the anthropogenic inputs occurred. Second, bioassays can be performed in the laboratory to test the responses of organisms to various concentrations of the chemical (CC18). The laboratory results must be extrapolated to what might occur in the environment. Both approaches are difficult and prone to errors and uncertainties.

The field approach to toxicity evaluation requires a very detailed survey of the affected ecosystem and of control sites. Researchers must characterize the population levels and health of many species to be certain that the most susceptible species are included. Moreover, changes observed in the ecosystem may be caused by natural factors, such as climate variations. Hence, even if a significant difference in the ecosystem is observed between test and control sites, researchers usually cannot eliminate the possibility that it is natural and unrelated to contaminant toxicity. In addition, most contaminated locations receive inputs that contain a variety of toxic chemicals, all of which vary in concentration with time. Assessing which contaminant might be responsible for an observed change is difficult.

At present, we do not have a detailed understanding of the effects of toxic substances in the oceans, and we cannot predict with certainty the consequences of specific concentrations. Management of ocean uses that involve the release of toxic chemicals to the marine environment is therefore difficult and controversial. However, our understanding of the effects of toxic chemicals in terrestrial, freshwater, and groundwater ecosystems, and on human health, is not significantly better than our understanding of their effects in the marine environment. There is general agreement that we must exercise caution and, where practical, limit the release of potentially toxic chemicals to the environment. Nevertheless, wastes containing low concentrations of potentially toxic chemicals will always require disposal. If properly managed, ocean disposal of these wastes may be both safe and environmentally preferable to other disposal methods.

Bioaccumulation and Biomagnification

Two processes cause concentrations of substances that are potentially toxic to humans in marine organisms to become elevated in relation to concentrations in their food or environment (CC18): bioaccumulation and biomagnification.

Uptake and excretion of toxic substances by marine organisms are typically complex processes that involve transfers of the toxic substance among several different tissues of the organism, as well as to and from the organism’s food and the surrounding water. Hence, some toxic substances may be taken up quickly when the environmental concentration increases, but released much more slowly when the environmental concentration decreases. The complexity of these bioaccumulation equilibria generally increases as the organism becomes more complex.

Higher order animals tend to store toxic substances in tissues where they are least harmful. Toxic substances are excreted only very slowly from these tissues after the exposure is reduced. In some cases, the loss is so slow that the concentration of the toxic substance tends to increase progressively during the individual organism’s lifetime. The cumulative buildup of metals, such as lead, mercury, and arsenic, in human beings during their lifetime is an example. In such situations, short-term laboratory bioassay tests cannot accurately reflect the effects of long-term exposure to toxic substances or wastes. 

Biomagnification is a special situation in which substances that are taken up by an organism from food or water are stored in tissues and there is no route by which these stored substances can be released. Biomagnified substances not only accumulate throughout an organism’s lifetime but are transferred up the food chain so that the substance accumulates to higher concentrations with each step in the chain. 

Trace metals are not biomagnified in marine food webs, except in a few instances when the metal is organically combined, such as in the methylated form of mercury. Biomagnification in ocean food webs appears to be limited to compounds that are highly soluble in fatty tissues and have relatively low solubility in water. These mainly include synthetic organic compounds, of which DDT and PCBs, and perhaps a few synthetic or naturally produced metal-organic compounds, such as methylmercury and tributyltin, are the primary ones.

Biomagnification of DDT and its decomposition products was responsible for the decline of pelican, sea lion, and elephant seal populations off California and throughout the eastern Pacific Ocean during the 1950s and 1960s. Almost all uses of DDT were banned in the U.S. in 1971. Since then, each of the affected species has recovered steadily in the eastern North Pacific Ocean. However, DDT continues to be used elsewhere in the world, particularly in tropical regions. Both DDT and its toxic but longer-lasting decomposition products are still found at high concentrations in marine species at higher trophic levels in all parts of the oceans.

Synthetic and Naturally Occurring Toxins 

Two distinct classes of toxic substances are released to the marine environment by human activities: naturally occurring substances, including trace metals (e.g., copper, lead, cadmium) and petroleum hydrocarbons; and synthetic chemicals produced only by human industries (e.g., DDT and PCBs). These two classes should be viewed differently. 

Marine organisms are adapted to generally low but variable concentrations of natural toxic substances in their environment. For example, lead and other toxic metals are either safely stored within marine organisms’ tissues where they do not affect critical biochemical processes or safely excreted back to the environment. Petroleum hydrocarbons are metabolized to harmless substances by many fish species and used as food by many marine decomposers. Adverse effects occur only when these natural detoxification processes are overwhelmed by high concentrations of such compounds. This is fortunate, because total elimination of the releases of such compounds by all human activities is impossible.

Synthetic chemicals, such as DDT and PCBs, are not present naturally, and marine organisms may not have mechanisms to detoxify them. Consequently, these compounds have more unpredictable fates and effects in the marine environment. If they are persistent and not easily broken down by the chemical processes or metabolic processes of marine organisms, they tend to accumulate, persist, and cause adverse effects. Unlike contaminants that occur naturally, a synthetic chemical can be eliminated completely from our society and therefore from any further introduction to the environment. Alternate synthetic chemicals have now been designed and developed that adequately fulfill the purposes for which DDT and PCBs were intended but that, in contrast, are readily and rapidly decomposed in the environment. Unfortunately, developing, testing, and manufacturing new, effective, and rapidly biodegradable chemicals is very costly. The widespread adoption of such expensive replacement products is impeded primarily by economic factors, especially in poorer nations.