16.1: Pollution versus Contamination

All too often, any human activity that releases wastes or introduces particulate or dissolved substances to the ocean, either by accident or incidental to other activities, is mistakenly reported as “pollution.” In many cases, however, such releases are benign or even beneficial to the ocean ecosystem and ocean resources. In such cases, the released material acts as a contaminant but no pollution has occurred. Only when the ocean ecosystem or ocean resources are damaged should the activity be called pollution. Hence, human activities may contaminate the oceans without polluting them.

One important note required here concerns the gross misuse of the term “pollutant.” This term is applied to many substances that can have, and may have had, adverse effects on some ecosystem in which the concentration of the substances has been elevated by human activity to a level that has caused adverse effects and, therefore, pollution. There has been a popular use of the term “pollutant” to describe such substances, and then a popular trend toward calling the specific substance (for example, arsenic or cyanide) a pollutant. This misuse of the term has led many to believe that these substances are toxic and unacceptable at any concentration. However, this is never true. There is no substance known to humans that is always toxic at any concentration in any ecosystem. For example, arsenic is a natural element that is present in humans and other living organisms almost always at concentrations that cause no adverse effect (arsenic is benign), was once used extensively for medicinal purposes, and is the source of energy (by oxidation of arsenate to arsenite) for some species of bacteria in certain ecosystems. There are many compounds called cyanides, but “cyanide” usually refers only to hydrogen cyanide, which is toxic to humans at low concentrations, but which is naturally occurring in many ecosystems, produced by certain types of bacteria, and can provide the only source of nitrogen for other types of bacteria in certain ecosystems. In summary, no chemical always causes adverse effects (pollution) regardless of concentration in all ecosystems so there is no substance that can, or should be, always called a pollutant.

For many years, the oceans were considered so vast that human populations could safely discharge all their wastes there and carelessly exploit ocean resources such as fisheries and mineral deposits without causing adverse effects. In recent decades, the recognition that the oceans can be harmed by human exploitation and waste disposal has led to another viewpoint: that ocean ecosystems are so fragile that they must be protected from any human influence that may change them, and that no inputs of any contaminants can be permitted.

Just as the historical view of the oceans as limitless was wrong, this new view, although idealistic, is also incorrect. In some cases, the new view may have led to political decisions that, although they may possibly have reduced ocean pollution, also increased human health risks and terrestrial pollution. The oceans are naturally changeable, and humans have caused many changes in the oceans, just as we have in the terrestrial environment. If human civilization is to continue, further changes in both the terrestrial and ocean ecosystems are inevitable and necessary. We must learn to view the planet as a whole and recognize that we must use its resources wisely. We must accept contamination where necessary or desirable, and avoid pollution wherever possible, not only in the oceans but also on land and in the atmosphere.

Assimilative Capacity

The oceans receive millions of tonnes per year of many dissolved elements and organic substances from rivers, dust, and rain (Chaps. 5, 6) and have done so since long before humans appeared on the Earth. These substances include many elements, such as copper, zinc, arsenic, and mercury, that are toxic to humans and other species if present at high enough concentration. The oceans also receive organic matter from soils, plants, and animal wastes, the composition of which is substantially the same as that of human fecal and urinary wastes.

The amounts of these substances introduced naturally must have varied substantially as plate tectonics modified and moved the continents and as the Earth’s climate changed. Hence, ocean ecosystems can and have changed to accommodate a range of input rates of these substances. The maximum rate at which the oceans can accommodate such inputs without adverse effects is called the assimilative capacity. Chapters 5 and 6 discussed some of the chemical and biological processes that remove substances from ocean waters to balance inputs and prevent concentrations from rising continuously. The assimilative capacity is exceeded if the input rate increases so rapidly or by so much that removal processes are overwhelmed, and the concentration rises to a level that is toxic or has other adverse effects on the ecosystem.

Besides naturally occurring compounds, the oceans have an assimilative capacity, albeit sometimes small, for synthetic organic chemicals produced by human civilization. Although these compounds are new to the oceans, all (including plastics) are broken down by decomposers and chemical reactions into other compounds and eventually inorganic compounds. Some are broken down quickly, whereas others, including DDT (dichloro-diphenyltrichloroethane), PCBs (polychlorinated biphenyls), and many plastics are broken down very slowly.

Although assimilative capacity is a useful concept, it is very difficult to apply. Each element or substance has its own unique residence time, natural concentration, concentration at which it becomes toxic, variable toxicity to different species, and chemical and biological decomposition and removal processes. Each of these factors must be understood before the ocean’s assimilative capacity for a single substance can be estimated. The oceans are not instantly and uniformly mixed. Consequently, the assimilative capacity can be exceeded for part of the oceans if inputs to a specific region exceed the rate at which they can be removed by chemical and biological processes and by mixing with the rest of the oceans. Hence, assimilative capacity and residence time are linked.

CC8 describes how residence times can be determined for individual substances. In a geographically distinct region, an increased input rate of a contaminating substance will cause the substance’s concentration in the water (and thus, generally, in the sediments and biota of the region) to increase. The increase in concentration is greater if the residence time is longer. Thus, for example, organic material in the large volumes of sewage of a major city can far exceed the assimilative capacity of a river or bay if the water body has a relatively long residence time. The discharged organic material can be decomposed and deplete dissolved oxygen faster than the oxygen can be replaced by mixing with oxygenated ocean or river water or resupplied from the atmosphere. As a result, the water becomes anoxic, and toxic hydrogen sulfide is generated by bacterial action (Chap. 12). Parts of San Francisco Bay (Fig. 16-2), the New York Harbor, and many other estuaries developed chronic sewage-induced anoxia in the 1950s and 1960s.

Figure 16-2. This map of San Francisco Bay and its delta shows the areas that were formerly wetland or part of the Bay but that have been filled or confined by levees in the past. The area of the Bay is now approximately one-half of what it was before 1850, and the area of remaining wetland is only a very small fraction of what it was then. Small areas that were previously filled have now been restored.

The anoxia that affected many estuaries in the U.S. and elsewhere only several decades ago has now been alleviated in most instances. Today, all sewage is treated to remove most organic matter before being discharged as effluent. In addition, some, but not all, treated sewage has been diverted to outfalls discharging directly to coastal oceans, where the residence time is much shorter and the assimilative capacity much greater. Unfortunately, even the reduced-contaminant inputs exceed the assimilative capacity in some estuaries. Much of the oxygen demanding organic matter is now removed from all sewage in most industrialized nations, and this has led to the revival of many estuaries that formerly were severely degraded. However, treated sewage along with agricultural runoff are still largely responsible for areas of anoxia or hypoxia, commonly called dead zones, in a number of estuaries such as Chesapeake Bay. In many locations globally, treatment of sewage effluents has revived an estuary where relatively short residence times and restricted light penetration prevent rapid algal bloom formation only to create expanding seasonal or permanent anoxia and dead zones in the adjacent coastal ocean areas where residence time is much longer than in the estuary and turbidity is lower so algae can bloom freely as long as the nutrients (principally nitrogen), most of which remains in the treated sewage effluent during treatment, are available to sustain the growth. Coastal zone anoxia was discussed in more detail in Chapter 13.

A critical lesson here is that a discharge (nutrients in treated sewage effluents) in one location (the estuary) can produce contamination but no pollution, whereas a discharge of identical volume and composition in a region with a longer residence time (the adjacent coastal ocean) can produce a serious pollution problem. Hence, residence time and assimilative capacity are important parameters that must be considered when evaluating any activity that releases or may release contaminants. Because these parameters are location-specific, a release of a given type or amount of contaminant that has caused pollution problems in one location may not cause the same problems at other locations. Also, a release of a given type or amount of contaminant that has not caused pollution problems at one location may cause problems at another location.