When people alter an environment, they shift the delicate balance that exists between the biotic and abiotic components. Most forms of development simplify an ecosystem, reducing the diversity of habitats, likewise reducing the abundance of plant and animal species. Food webs become unstrung and rewoven, with fewer energy-flow pathways. The work of decomposers is disturbed, interrupting vital nutrient cycles. In time, the environment may settle into a new equilibrium, but only if it does not incur constant cataclysmic changes, such as the drastic fluctuation of water levels above many dams. Plants and animals, which take long periods of time to adapt to environmental changes, cannot adjust instantly to a constantly changing freshwater environment. Consider the barren, rocky shores of many reservoirs where the water level vacillates several meters or more throughout the year.

Every action we take can affect the quality of our freshwater environments. Although we do not affect the quantity of water that circulates around our fragile planet, we clearly affect the purity of that life-giving element. And since 80 percent of our bodies are composed of fluid, water is truly, as Chief Seattle once forewarned us, “blood which unites one family.”

Ecological pollution consists of adding a substance into an ecosystem that is not naturally occurring, or increasing the amount or intensity of a naturally occurring substance in an ecosystem, or altering the level or concentration of a biological or physical component of an ecosystem.

When does a change or addition to a system become pollution? Ecological pollution is present when one of the parameters described above stresses the plants and animals in an ecosystem and requires responses that are beyond their normal range of resiliency. This causes sickness or death and results in a shift in the ecological fabric of that community.

Pollution is a dynamic intruder. The severity of its damage can be affected by environmental conditions such as temperature, water chemistry, season, and the surrounding topography and bedrock. For instance, during the growing season, wetland plants may intercept some pollutants, such as agricultural pesticides or fertilizer runoff, before they reach ponds and streams. When the ground is frozen, plants are dormant and soils impermeable. Pollution runs more freely into the open waterways. Naturally impermeable soils, such as those overlying bedrock, will intercept less pollution as it flows downhill. Hard water tends to he more reactive and can lessen the ecological damage done by pollution.

When several kinds of pollution are introduced into an aquatic ecosystem they can have synergistic effects, during which the combined effects can be greater than the sum of the effects of the individual pollutants. A fish that is under stress caused by low oxygen levels in the water is more susceptible to poisoning by insecticides or heavy metals. In some rivers, this combination of pollutants has created zones that are so deadly to fish that they act as walls blocking migration runs. Conversely, antagonism can also result when two compounds interact, decreasing the severity of a pollutant’s ill effects. Copper, for instance, has been shown to be less toxic in the presence of high levels of calcium such as would be found in hard waters over limestone.

Persistently high levels of pollution through time create communities that consist of many individuals of a few tolerant plants and animals. Some species of benthic (bottom-dwelling) invertebrates are often used as indicators of polluted waters. These are not “pollution species,” they are organisms that can tolerate conditions such as low levels of oxygen and high sedimentation; lacking competition for available resources, they multiply in great numbers.

There are many ways in which we alter the freshwater environment. A concentrated source of pollution, like the outflow of chemical and industrial wastes from a pipe, is called a point source, while general runoff, like that of excess fertilizer from an agricultural field, is called a nonpoint source. These are both direct sources of pollution that enter our waterways. Acid rain is an example of indirect pollution, where the activities of people can affect water quality several thousand miles away. Agriculture is responsible for 68 per cent of our water pollution, followed closely by human organic wastes and industry.

Among the most noxious kinds of pollution are those associated with the addition of inorganic nutrients and organic matter into aquatic ecosystems. Many people have experienced the murky water and offensive odors caused by heavy growth of algae in lake waters that are enriched by the leaching of septic systems from homes along the shore or fertilizer entering from bordering farmland. Phosphate and nitrate are common inorganic nutrients that are often introduced into fresh water via surface runoff that contains agricultural and domestic fertilizers. Organic pollution is caused by overburdening an aquatic ecosystem with excess organic matter, such as human sewage waste or manures from stockyards, and from the rich blooms of algae and other organisms that result when inorganic nutrients are introduced in great amounts. Organic matter refers to compounds that contain carbon, a basic building block of living things. Nutrients can enter in solution or in suspension, and organic matter can arrive in the form of solids as in leaves, sewage waste, or eroded soil sediments. The major sources of nutrient enrichment are:

• Agriculture, principally runoff and erosion

• Forestry practices: clear cutting and erosion on steep slopes where heavy machines are used, causing an input of mineral nutrients, organic matter, and sediments

• Sewage wastes: effluent from wastewater treatment plants, manure, leaching from septic fields

• Industry

• Pulp and paper mills

• Food-processing plants

• Fires: without protective plant cover, nutrients and soil are more easily washed into waterways

• Urban runoff: the impermeable surfaces of cement and asphalt result in rapid runoff, often introducing lawn and garden fertilizers, pesticides, petroleum products, and other toxics into nearby rivers and lakes

• Natural sources: organic matter entering lakes and ponds from swamps, bogs, and other environments

Healthy ecosystems experience a rough balance between the levels of activity of the oxygen producers and oxygen users. When high levels of organic matter are introduced, the population of aerobic decomposers increases and oxygen is used up faster than it is introduced into the system. Excess nutrients, such as phosphates, nitrates, silicon, and iron, stimulate algal blooms that also reduce DO levels via nighttime respiration and upon death and decomposition of the algae. Thus nutrient enrichment can result in an oxygen sag, where anaerobic conditions can occur, stressing fish and other aquatic life. In time, if the pollution does not persist, decomposers will consume the organic matter and the balance between oxygen production and consumption will be restored. Organic pollution is sometimes measured using biological oxygen demand (BOD). The BOD of water is an important indicator of how much oxygen-demanding decomposition and respiration will be required to fully consume the organic matter contained in that water.

In still waters where organic pollution is severe, the rates of decomposition arc high and DO levels are consistently low. These waters are especially vulnerable during the winter in areas where ice cover decreases atmospheric mixing and photosynthetic production of oxygen. This over fertilization of aquatic ecosystems is called eutrophication, and it can result in faster rates of aging in a pond or lake due to high levels of plant production and the buildup of organic remains.


Toxic elements damage the ability of plants and animals to carry out their life-sustaining functions. The disturbance of organisms short-circuits nutrient cycles and energy flow, sometimes causing disruptions in the workings of an ecosystem. The major sources of toxins arc agriculture, industry, paper mills, polluted precipitation, and urban runoff. Pesticides, herbicides, and industrial compounds are the chief contaminants in polluted waters.

Plants and animals have tolerance levels to toxic elements, which, if exceeded, will cause stress, lowered resistance to disease and other environmental hazards, and eventually death. The level of a substance that an organism can tolerate is affected by its initial health, its life history stage, and other environmental conditions such as the season and pollution that may already be present. A trout living in a stream at the upper end of its temperature tolerance range is more susceptible if pesticides enter via runoff from a cornfield upstream. The maximum acceptable tolerance concentration for an organism is determined by long-term studies of individual plants and animals under varying environmental conditions.

Biomagnification is a major ecological concern. This occurs when an element is introduced into an ecosystem and its concentration increases, moving up the fbod chain. Toxins having a long biological half-1ife, the time that a body takes to rid itself of one half of its load of a substance, are especially dangerous. Long-lived pesticides, such as chlorinated hydrocarbons, become increasingly concentrated in body tissues at higher trophic levels. Clear Lake in California was sprayed with DDD in the l940s and l95Os to control biting gnats. Plankton absorbed this chemical from the water as they grew and were in turn eaten by filter feeders, which were then consumed by frogs and sunfish. Fish can also absorb toxins through gill membranes. The pesticide became concentrated in sunfish at twelve thousand times the dosage sprayed in the surrounding water. The production of the lake’s population of grebes, who fed on these fish, dropped from one thousand actively-nesting grebes in the surrounding marshes in 1950, to no young being produced by l960. Another chlorinated hydrocarbon, dieldrin, decreases a sculpin’s ability to metabolize food.

Evidence shows that concentrations of radioactive substances can also be magnified in food chains. Strontium 90, a radioactive isotope, traveled through the atmosphere as a result of the above-ground testing of nuclear weapons in the 1950s and 1960s. The human body treats strontium 90 in a similar manner to calcium; it becomes concentrated in the bone tissues of those who eat contaminated food, especially milk. Strontium 90 has a biological half-life of one thousand days.

Acid precipitation is an airborne pollutant that has dramatic effects on aquatic life. Acid rain is a commonly used catchall term for acid precipitation, which includes rain and snow, sleet, hail, fog, and dry particles that fall from the sky. It is caused by the burning of fossil fuels, especially gasoline and high-sulfur coal and oil. The sulfur dioxide and nitrogen oxides that are produced react with water vapor to form sulfuric and nitric acids. Carbon dioxide, a naturally occurring gas, also reacts with water vapor, resulting in carbonic acid. Normal rainwater has a pH of 5.6; most fish die when lake water reaches a pH of 5 or lower. Since the pH scale is logarithmic, a decrease in one number means a tenfold increase in acidity, and a drop of two numbers indicates rain that is 100 times more acid than normal. Recent measurements in the northeastern United States indicate that rain water is averaging 40 times more acid than normal, often around a pH of 4.0, although readings do occasionally dip into the upper 3’s. Vinegar has a pH of 3.

Is acid rain a new problem? It has been recognized and documented in Europe since the mid-1950s. Air pollution from the heavily industrialized Ruhr Valley, including parts of the Federal Republic of Germany, Belgium, and the Netherlands, is carried on southerly winds into the Scandinavian countries to the north. This pollution forms a potent acid rain. In 1950 a study was begun of 266 Scandinavian lakes, at which time there were 48 that were devoid of fish life. This figure reached 75 by 1960; by 1975, fish were absent in 175 of these lakes. These are mostly soft-water lakes with little buffering capacity.

Acid precipitation turns rainwater into a powerful leaching agent. It can leach nutrients from leaves and can damage the waxy coating on ever-green needles, causing them to lose water and wither. Botanists at the University of Vermont in Burlington have concluded that acid rain is implicated in the death of 50 percent of the spruce trees on Camel’s Hump, a picturesque peak in the Green Mountains of Vermont, since 1965. The growth of living spruces has been stunted by one half.

Acid rain can leach copper, aluminum, and other heavy metals out of the soil and into runoff and drinking water. The toxicity of numerous heavy metals has been shown to increase in the presence of acid rain: lead, aluminum, mercury, zinc, cadmium, and copper. Copper concentrations of only 10—40 parts per billion (ppb) can kill fish in acid water. (One part per billion equals the volume of I liter placed in a lake that has the surface dimensions of a football field and a depth of 738 feet or 225 meters). Copper interferes with energy metabolism and enzyme function, causes a mucous covering on gills, and can damage kidneys, liver, and spleen. Aluminum is naturally present in the soil and lake bottoms. The increased acidity “mobilizes” this metal into solution, where it binds to Fish gills and causes suffocation. Heavy metals can also inhibit the hatching of fish eggs and cause deformed young fish and amphibians to be born, such as frogs and salamanders. If the waters become too acid, algae are killed, eliminating the most important plant food supplying aquatic life. Fish die along with the algae. Most fish do not eat algae directly, but they feed on animals and other small creatures that need algae to live.

Waters over hard rocks, such as granite, are most susceptible to acid rain because there is little buffering capacity. Limestone areas are more resilient. The congressional Office of Technology Assessment has determined that 3,000 lakes and 23,000 miles (37,015 kilometers) of streams and rivers are vulnerable to acid rain in the eastern United States alone.  Acid rain also dissolves the structures of buildings and cars, eroding away iron, steel, limestone, and more.

Ground-water pollution. There is another kind of pollution that, like acid rain, is often not apparent until its damage is done. Ground-water pollution really describes a place where all the previously described chemical and organic pollutants can go. But because our ground water is such a vital resource, it deserves special mention. About 50 percent of the people in the United States rely on ground-water supplies at home. And for the most part, when we turn on the tap, clear, clean, cool water comes gushing forth: 75 gallons (284 liters) per day for the average user.

Aquifers that are especially prone to contamination are those overlain by permeable soils, through which polluted water can percolate, and those in contact with a water table that is at or near the surface. In addition to the possibility of pollution from those sources already mentioned, ground water can be contaminated by the disposal of organic wastes, such as those from septic tanks and stockyard manure runoff; landfills (dumps) and toxic-waste storage sites from industrial waste; road salt use and storage piles; acid runoff from mines; leaks from underground storage tanks, such as those used for gasoline; and radioactive wastes. Landfills alone can be a source of leachate containing lead, copper, iron, steel, salt, and organic compounds. Ironically, sites that seem to be ideal for waste disposal, such as old gravel pits and natural depressions, are often places where the ground water is at or very close to the surface and is easily contaminated.


Each year more forested wetlands, marshes, and wet meadows are drained fir agricultural purposes or filled for construction. Rivers are dredged for navigation. Our rivers and lakes are tapped to provide drinking water and for irrigation, the latter being the largest single use of water in the United States. Dams are built to create reservoirs to supply this water and to generate hydroelectric power. Powerplants that run by heat-generated steam use millions of gallons of water as a coolant each year. Industries use water for production and for dissipating heat that is produced. For instance, one million gallons of water are needed during the production of one thousand barrels of jet fuel.

What are the effects of these environmental alterations? Besides the intended benefits that accrue to human society, the ecological changes wrought tend to cause the death of some ecosystems, a state of constant instability and flux in others, and can sometimes result in the creation of entirely new environments. Consider the common forest management prac tice that leaves clear-cuts reaching down to, and across, streams and rivers. Eroded sediments during the spring runoff can bury and suffocate the eggs of trout and other spawning fish, and silt can clog fish gills, causing death.   The increased turbidity of the water decreases sun-light penetration to algae and other green plants, reducing their productivity and resulting oxygen levels from photosynthesis. “What more?” you might ask. When the plants are removed from the banks, and shade is decreased, water temperatures often become too high for trout and many benthic invertebrates, such as stoneflies, to survive.

In places where the banks of streams are cleared and the streambed straightened, the natural values of associated wetlands are greatly reduced or eliminated.  The cleansing action of the floodplain marshes and swamps, flood-control capabilities, productivity of plants, and fish breeding grounds are all adversely affected. The uniform bottom of the channel itself offers a lower diversity of habitats for benthic invertebrates, which decreases their productivity and variety. Without the balanced cycle of plant production in riffles and decomposition in pools, productivity is reduced even more.  Channelization is an ecological disaster.

Water that is used as a coolant and then returned to the environment at a higher temperature creates thermal pollution. This is a serious problem in rivers downstream from the outflow and in ponds and small lakes. The composition of plants and animals in a river can take on the character of communities normally found in warmer water. In small bodies of water in temperate climates, the length of the period of ice cover each year can he shortened. Overall, the metabolic rate of plants and animals increases in warmer water, producing a greater demand for oxygen. Simultaneously, the efficiency of plants producing oxygen, and the amount of oxygen that the water can hold, are both reduced. Overall, there is less oxygen available and a higher biological demand for this limited oxygen.

The many benefits derived from dams—primarily recreation, electrical power generation, and water supply—also have their ecological costs. Many of these drawbacks are subtle. Early dams brought an abrupt halt to the spring runs of salmon, shad, smelts, and other fish that seek the cool, gravel-bottomed streams to spawn. In earlier times, little was understood about fish biology and, even if it had been, technology was not yet available to build large fish-transport mechanisms. In some large rivers, such as the Connecticut River, dams halted the salmon runs in the late 1700s. Salmon tried in vain to spawn for up to a decade after a sixteen-foot dam was built in 1798 at Miller’s River, 100 miles up from the ocean along the Connecticut River. Then the salmon disappeared. Prior to this time salmon were so common in some waterways that they were speared during spawning runs and used to fertilize agricultural lands along the banks. Along the Connecticut River today, and in other rivers, the construction of fish ladders and other means of clearing the dams, salmon breeding and reintroduction pro grams, and pollution controls are slowly bringing back the salmon.

The churning water below a dam is often supersaturated with oxygen. This can cause a gas bubble disease in migrating fish with effects similar to the condition known as the bends, which deep-sea divers experience when they surface too quickly: If water flows over the top of a dam, the temperatures downstream tend to be similar to those normally expected for the river below. Bottom-draining dams, however, especially in very deep lakes where the water comes from near the bottom, cause abnormally cold water downstream, creating localized cold-water communities. Fluctuating water levels, especially marked below flood-control dams, result in lower populations because of the extreme and rapid changes that occur—changes to which few plants or animals can adapt. Tall dams can prevent or inhibit the natural upstream migration of the adult stages of aquatic insects, possibly decreasing populations upstream.

Of course, human alteration of freshwater environments has not been all bad from an ecological perspective. Dams created new habitats for the plants and animals of lakes and associated wetlands. Over 2 million acres (809,389 hectares) of ponds have been created in the twenty-year period ending in mid-1970 alone. With our increasing knowledge of, and appreciation for, the values of freshwater resources, large natural areas have been set aside for their utility to people and wildlife.


Now, with a greater understanding of natural laws and the impact of human activities on freshwater environments, we will look at how plants and animals are adapted for survival in a world of water, and for life with one another. Discovering the infinitely diverse forms of aquatic life brings wonder and mystery, for each living thing is unique in its approach to freshwater life.

On any summer day you can gaze across a pond and see an environment alive with activity. A bass dimples the surface as it catches its meal, an unfortunate insect that has become trapped in the surface tension of the water after falling in from an overhanging branch. Water striders and whirligig beetles race and shoot over the surface, leaving small wakes as they hunt for a meal. The dreamy whir of dragonfly wings startles you as the dragonfly traces an aerial search pattern for food. Beneath the surface are hidden hundreds of plants and animals, giant and small, that form the threads of the pond food web.

In the United States alone over 1200 species of plants can be found that are associated with fresh water. These plants range from microscopic algae to towering cottonwoods.