16.2: Adverse Effects of Human Activities

Contamination and other human activities cause a variety of adverse effects, some of which are described here. In many instances, one activity can lead to more than one of the impacts discussed.

Interference with Photosynthesis and Primary Production

Many contaminant inputs to the oceans contain large quantities of suspended sediment and light-absorbing organic matter. If retained in the water column, high concentrations of these materials block the penetration of sunlight and reduce the amount of light available for photosynthesis (CC14).

Human-caused discharges of suspended particles to the oceans primarily affect coastal regions (Fig. 16-3), where much of the ocean’s primary production occurs. Benthic algae, both microalgae and macroalgae, are important primary producers in many coastal and estuarine areas. Excess suspended sediment reduces light penetration and, thus, primary production by benthic algae.

Figure 16-3. There are many sources of suspended sediment contaminant discharges to the oceans including some you might not expect. For example, sugar cane fields are often burned after harvest to clear the land for new planting. This practice increases erosion, and large amounts of suspended sediment are discharged in runoff. These fields are on the north shore of Hawaii. You can see the smoke from one of the fires and the plume of suspended sediment that spreads in the nearshore zone where there are reef-building coral formations that can be destroyed by excess sedimentation.

Adverse effects on primary production can be caused by discharges of large quantities of nutrients. Alterations in relative concentrations of nutrients can cause changes in phytoplankton species composition (Chap. 13). The result can have adverse effects on the food web if the newly advantaged phytoplankton species are unsuitable food for the zooplankton population.

Excessive nutrient discharges can increase primary productivity so dramatically that it exceeds the grazing capacity of consumers. In such cases, phytoplankton reproduce and grow rapidly in a bloom until the limiting nutrient is depleted. The bloom then collapses, and dead phytoplankton cells sink and decompose rapidly, removing some or all of the oxygen from the water. Reduction of oxygen concentration can adversely affect the respiration of fishes and invertebrates.

Oxygen depletion due to the presence of excessive nutrients or organic matter is called eutrophication and has long been a menace in lakes, rivers, and estuaries where nutrient inputs are large and residence times long. In many such areas, the nutrient and organic matter inputs are from sewage discharges. Sewage treatment has dramatically reduced eutrophication in U.S. lakes, rivers and estuaries since the early 1970s. However, nutrients remaining in the effluent after sewage treatment, when combined with other nutrient inputs, are now causing increased eutrophication problems (hypoxia and anoxia) in coastal oceans near estuaries. Hypoxia and anoxia that result from excessive nutrient inputs, and the increasing severity of these problems, were labeled in a 2000 report by the National Research Council of the National Academy of Sciences as the most pervasive and troubling pollution problem facing U.S. coastal waters.

Oxygen concentrations can also drop if photosynthesis is reduced. For example, increased concentrations of suspended sediment, copper, or other substances, such as herbicides, can inhibit photosynthesis and/or primary production without significantly affecting respiration in a marine ecosystem.

Human activities adversely affect oxygen concentrations in at least two special situations. First, when rivers are constrained within levees, the surface area is diminished but the river depth and currents increase. Often the photic-zone depth is naturally shallow and may be further reduced by higher turbidity caused by faster river currents that resuspend particulate matter. Because both the surface area and the depth of the photic zone are reduced, the total volume of the photic zone is smaller. Furthermore, the reduced surface area lowers the rate of oxygen resupply from the atmosphere.

In the second special case, oxygen concentrations are reduced because power plant, or other industrial, effluents have a higher-than-ambient temperature. Because oxygen has a lower saturation solubility at higher temperature, oxygen concentrations in the heated effluents are generally lower than ambient concentrations. This disparity is seldom an immediate problem, because the saturated oxygen concentration is sufficient to support respiration at all but the highest ocean water temperatures (above about 30°C). However, the low concentration of dissolved oxygen reduces the assimilative capacity for oxygen-demanding substances. Because tropical waters have high ambient temperatures and low ambient oxygen concentrations, oxygen depletion and sulfide production can be caused by smaller inputs of nutrients or organic matter in the tropics than in other latitudes. Hence, water bodies in tropical regions have a lower assimilative capacity for organic wastes than do water bodies with similar residence times at higher latitudes.

Habitat Alteration

Each species that lives in the oceans has its own requirements for the physical and chemical characteristics that constitute its habitat (Chaps. 14, 15). Benthic species are particularly dependent on the nature of sediments in or on which they live. Pelagic species require, or prefer, certain ranges of temperature, salinity, turbidity, and current or wave regime. Many pelagic species depend on the benthic environment for food or shelter during part of their life cycle, whereas many others depend on the shallow-water environments of mangrove swamps, coastal wetlands, rivers, and estuaries during their juvenile phases.

Human activities have caused the destruction of vast areas of coastal wetlands and have caused adverse habitat alteration in other areas. For example, rivers have been dammed, preventing anadromous fishes, such as salmon, and catadromous fishes, such as eels, from migrating to the upper reaches of the rivers on which they depend. Levees, marinas, and ports have substantially altered current patterns, and thus suspended sediment transport routes, in many estuaries. Vast quantities of freshwater have been removed from many rivers, thus changing the salinity and other chemical characteristics of estuaries. Coastal structures have interfered with sand transport along many coasts. Large volumes of wastes have been discharged directly to the oceans or through rivers, and many parts of the seafloor have been damaged by dredging and dynamite blasting, and by anchors and fishing gear. All of these activities cause changes in sedimentation rate, sediment grain size, and sediment chemistry on the seafloor of affected regions. Sandy seafloor can be turned into mud and vice versa, and stable rocky or reef bottoms can be covered with sediment or broken up and eroded away.

Whenever marine habitat is altered, the species composition changes. Some species are disadvantaged and others benefit. Most often, the alteration causes at least a temporary reduction in species diversity (CC17) and dominance by species that are less sensitive to changing habitat. Opportunistic species that are not normally a major part of the biota commonly move into and dominate an environment when it has been altered by human activity, especially if the disturbance is ongoing, such as waste disposal or dredging. Although in some instances opportunistic species can enhance the biomass of the natural food web, more often they are less desirable or worthless in the natural food web.

Community Structure Alteration

The many species that make up a marine community depend on each other for food and in many other ways (Chaps. 14, 15). The balance among species is determined by centuries or millennia of competitive and cooperative interaction that has enabled the community to reach a relatively stable state. If the balance is disturbed, the community structure can become unstable and change unpredictably.

The greatest direct human disturbances of community structure are caused by preferential exploitation of one or more species in a fishery or by the introduction, accidentally or otherwise, of nonindigenous species (discussed later in this chapter). Human activities may also introduce nonindigenous disease-causing organisms that affect some species important to the ecosystem. Other human influences, such as substrate alteration and introduction of potentially toxic substances, can also advantage or disadvantage certain species and cause community structure to be altered.

Marine communities are periodically subjected to habitat disturbances due to natural events, such as earthquakes and climate changes. Some additional human disturbance can be tolerated and accommodated by the ocean ecosystem. However, in some cases, anthropogenic disturbance may be more rapid or drastic than natural disturbances. Furthermore, human disturbances are often continuous or increase progressively, and their scale may be unprecedented in some coastal and estuarine areas.

Contamination of Food Resources 

Aquatic species can obtain elements and compounds from food and directly from solution. Therefore, substances that are toxic to humans introduced in dissolved form or associated with organic particles can be assimilated by most marine organisms and passed through the food web. Many marine species are tolerant of relatively high concentrations of potentially toxic elements or compounds in their environment or food, probably because their body surface and respiratory tissue are continuously exposed to seawater. Rather than building defenses against the absorption of potentially toxic substances from their environment, many marine species simply take up such compounds and store them in some organ where they cannot interfere with essential biochemical processes. This method of detoxification has limits, but it enables many marine species to tolerate high concentrations of some substances.

Fish and shellfish with high concentrations of stored potentially toxic substances may suffer no adverse effects but may still pose a significant risk to human health. Shellfish, such as oysters, are particularly adept at concentrating trace metals, and many synthetic organic compounds, such as DDT and PCBs which are both concentrated in fatty tissues of most marine animals.

High concentrations of metals and synthetic organic compounds have been found in the biota from many locations where human activities release such contaminants. In these locations, the fishery or shellfishery is closed, or people are advised to eat only limited amounts of seafood or a specific seafood species. Thus, the value of the fishery resource is diminished or lost. Fortunately, in only one recorded instance have people died from ingesting seafood contaminated with industrial toxic substances discharged into the oceans. That incident, in Minamata, Japan, is discussed later in this chapter.

Contamination of fish and especially shellfish with microorganisms that are human pathogens is a serious problem. Because some seafood is eaten raw or only lightly cooked, any microorganisms present will be passed on to the consumer. Seafood may be contaminated during handling and processing, although refrigeration and hygienic food-handling techniques have greatly reduced this problem in most developed countries. Most microbiological contamination of seafood now comes from the harvested waters. The problem areas are generally contaminated by discharges of raw or treated sewage or by animal feces carried in street and land runoff. The contamination is concentrated in estuaries and the coastal zone because human pathogens are progressively diluted and destroyed by marine microbes as they are transported to the open oceans. Unfortunately, shellfish beds are located mostly in coastal and estuarine zones.

Some pathogen-contaminated shellfish can be collected and cleansed by being kept in pathogen-free, constantly running and renewed seawater, but this procedure is expensive and requires a source of reliably pathogen-free seawater. Consequently, the only practicable way to prevent the spread of disease by contaminated shellfish is to prohibit harvesting in contaminated areas. At present, many potentially valuable shellfishing areas of the coastal oceans and estuaries in the contiguous U.S. are closed to shellfishing for at least part of the year. The total area closed is increasing, despite immense expenditures for sewage treatment and control of other contaminant sources since 1972, when the U.S. passed the Clean Water Act requiring secondary treatment of almost all sewage.

Toxin-producing phytoplankton blooms are also a growing contamination problem. These blooms, the toxins they produce, and their effect on marine ecosystems and seafood values are discussed in Chapter 13.

Beach Closures and Aesthetic Losses

Many people look to the coastal oceans as places of aesthetic beauty where they can renew their contact with the natural world through various recreational activities. As a result, one of the largest industries on the planet has developed around coastal recreation. However, the value of the coast for such activities is compromised and diminished in many locations by the presence of human structures that mar the natural beauty (Fig. 16-4) and interfere with natural processes.

Figure 16-4. Coastal industries tend to reduce the aesthetic value of the shoreline. (a) The Moss Landing Power Plant, a very prominent structure located on the coast in the middle of Monterey Bay, California. (b) An oil refinery at Anacortes, Washington. (c) A marina at Antioch, California, constructed on a former wetland in the San Francisco Bay Delta.

Many coasts that otherwise would be areas of high recreational value are sites of human industrial and residential developments that bar the public from reaching the shore. Such developments often cause major changes in the coastal form and function through direct modifications of the shoreline, such as bulkheads, piers, and groins, and through alteration of the coastal current, wave, and sediment transport patterns (Chapter 11). In addition, vast areas of biologically important coastal wetlands have been drained and filled to accommodate development (Fig. 16-2). Today there is a growing realization that many of these developments were ill-planned. Public opinion now favors protection of the coast from unreasonable degradation by development, although the legal system lags behind this imperative.

Outfalls discharge treated sewage, industrial waste, and storm-water runoff to rivers, estuaries, and oceans. Rivers also receive various materials that have been carelessly or deliberately dumped by humans. Vessels often discharge wastes directly into the ocean. Many beaches are periodically closed to swimming because the water is contaminated with pathogens from improperly treated sewage. In addition, floating debris mars beaches and, in extreme cases, leads to the temporary closure of beaches for recreational purposes. For example, medical wastes, including syringes, apparently dumped in nearby rivers or from vessels, have periodically washed up on New York and New Jersey beaches, prompting closure of the beaches for fear that the materials could carry pathogens, including the HIV virus.