What is pollution? Where does it come from? How does it impact our watershed? What impact does human acitivity have on specific organisms within a watershed? How do these impacts threaten and/or endanger the survival of entire species? These are some of the topics we shall discuss in this webpage.
Humans have been using and sometimes misusing rivers and other water sources for many years. One of the ways we misuse water in our watersheds is to pollute it. Pollution is when either a "foreign substance" enters a system (water, air, soil or organism) or when a naturally occurring substance enters a system in an excessive quantity.
To clarify, pollution is when things like trash, paint, detergents and sewage ("foreign substances") get dumped into a water body or onto the ground. Chemicals emitted from smokestacks and the exhaust from cars are forms of air pollution. The second half of the definition refers to pollution as being excessive amounts of naturally occurring substances. An example of this type of pollution is when the top soil from a newly plowed field washes into a river during a heavy rain storm.
Pollution is a complicated topic, and it doesn't get any easier when we try to understand the possible sources of pollution. Pollutants can enter a system either directly or indirectly; and from either specific, identifiable soures (e.g. pipes) or from diffuse, unidentifiable sources (e.g. storm water runoff). For example, if a person walks down to a river and dumps a gallon of used oil into the river, they have directly polluted the river. If that same person dumps the oil in their yard and it runs into the river, then they have indirectly polluted the river, but directly polluted the ground surface.
One of the biggest concepts in "pollution lingo" is the difference between point source pollution and nonpoint source pollution. Getting a grasp on the concept of point source versus nonpoint source pollution will help in understanding where the polllution that impacts our watersheds comes from. Point source pollution and nonpoint source pollution refer to where pollution comes from and how it enters the air, water or soil.
The terms point source pollution and nonpoint source pollution are most often used in referring to water pollution. Point source pollution refers to pollution sources that you can actually point to such as pipes carrying waste streams from industrial plants, chemical plants, and waste water (sewage) treatment plants to rivers. A hole in the hull of an oil tanker that spills oil into the ocean could also be considered a point source, although the term is more often used in referring to stationary sources. Concentrated sources of pollution are point sources of pollution.
Nonpoint source pollution is the opposite of point source pollution and can be defined as contamination from sources that are difficult to identify and locate. Nonpoint sources of pollution are more diffuse. For example, pesticides found in a river probably entered the river in storm water or irrigation runoff from a farm or lawn. The exact spot where the pesticides entered the river cannot be identified. Likewise, acid rain and airborne contaminants that fall to the ground in New England are believed to be the result of factories further west, although a specific plant cannot be held solely responsible. These are big examples, but many of the things we do every day, like fertilizing lawns, walking pets, changing motor oil and driving, also contribute to nonpoint source pollution. These activities deposit pollutants (such as soot, dust, oil, animal waste, litter, sand, salt and chemicals) on lawns, streets, parking lots and into the air. The rain then washes the pollutants into streams or into storm water systems that evenually flow into larger water bodies. Some of the pollutants will also seep into the ground with the rain water and end up in the ground water, which eventually dischares to a surface water body.
Chemical Pollutants: Pollution may disrupt the delicate balance within an ecosystem, killing off some species and prompting others to grow out of control. For example, chlorine (a biocide) is believed to have changed the chemical balance in many of Maine's rivers so much that entire populations of several abundant fish species such as alewife, salmon and smelt have (1) reproduced in smaller numbers, and/or (2) died off in large nulmbers. Chlorine is used as a biocide in many sewage treatment plants, swimming pools, and is found in many household cleaners. It is also used as a bleaching agent in paper mills.
Pollution also threatens the health of humans and other organisms. It can cause economic, aesthetic and recreational damage to the surface water bodies within our watersheds. Some of the most well known examples of how pollution impacts organisms relate to the impact of pesticides on bird populations. Some pesticides, such as the chlorinated hydrocarbon DDT, cause birds to lay eggs with very thin shells, reducing the chance of successful reproduction. What many people do not understand is how the pesticides get into the birds. Pesticides are applied to crops (usually sprayed on); when it rains some of the pesticides are washed off the crops and/or soil and then run into streams and rivers. Once in the stream, the pesticides may settle on plants or other substrates, or may remain in the water column. These pesticides are eventually ingested by fish and other organisms (either directly from the water or in their food). Some of the fish are then eaten by birds or other fish, passing the contaminant up the food chain. Humans may also eat the fish or the birds and are therefore also impacted by things like pesticides that are passed up the food chain. In fact, many pesticides are found in higher concentrations in organisms higher on the food chain (bioaccumulation). The effects of these chemicals are not always immediate, many of them can be stored in our bodies for years before the effects become evident. Once the dangers of DDT were recognized, its use was banned in the United States (but we still manufacture it and ship it to other countries).
Human and Animal Waste: These wastes contain lots of nutrients (nitrates, phosphorous, etc.) that plants need in order to grow. At first that sounds good, but when a lot of these nutrients enter a water body it can cause an abundance of phytoplankton or algae to grow. After a time these organism will die off in large numbers and begin to decompose. The decomposers (aerobic bacteria and fungi) may deplete the dissolved oxygen in the water. If the dissolved oxygen level gets too low other species will not be able to "breathe."
In addition to containing a lot of nutrients, human and animal wastes also contain many different kinds of bacteria. Some bacteria are good and help break down the wastes, but too many bacteria, or the wrong kinds of bacteria, can spread disease to animals and people. Disease producing organisms are called pathogens. As with the pesticides, organisms in the water will ingest the bacteria with their water or food. The bacteria might kill the organism, or it may be passed along to whatever eats it.
Thermal Pollution: This occurs when the temperature of a water body is raised or lowered beyond its normal range of temperature. A drastic change in the water temperature can be due to either the addition of heated or cooled water to the water body, or to a change in the amount of solar radiation reaching the water surface.
We all use electricity in our homes and schools, and in most cases that electricity comes from a power plant. Power plants, and many other industries, use water to cool the machinery in their plants. This water is taken from a river, stream or lake, and is usually returned to the water body at a higher temperature than when it was withdrawn. In fact, unless the water is effectively cooled before being returned to the water body, it will be an average of 10 degrees Celsius (C) warmer than when it was withdrawn. This may not seem like a lot to us, but it can cause thermal shock in many organisms, resulting in temporary suspension of life processes (e.g. breathing, feeding) or even in death. This drastic temperature change can also lead to changes in the metabolic and reproductive rates of plants and animals.
Another effect of thermal pollution is that heated water holds less oxygen and also leads, indirectly, to an increased oxygen demand in the organisms living in the area of the outfall (where the water is returned to the stream). In other words, there is less oxygen in the water at the same time the organisms need to use more. This can leave them less energy for things like reproduction and avoiding predation.
Another way humans contribute to thermal pollution is by changing the amount of solar radiation reaching the surface of water bodies. The relationship between water temperature and solar radiation is fairly simple: surface water absorbs heat from the sun rays. The greater the amount of water exposed to direct sunlight, the more heat the water will absorb and the higher the water temperature will be. One way people change the amount of solar radiation reaching water surfaces is by cutting down trees along river banks. Normally, in small to moderate size streams and rivers, a fair percentage of the stream is shaded by tree branches that hang out over the water. These branches provide shade and reduce the amount of solar radiation reaching the water surface. When these overstory trees along a river bank are removed for such things as lumber or clearing for construction, the amount of shading is greatly reduced. When this happens the amount of solar radiation reaching the surface of the water is increased and the water heats up. This increase in water temperature has the same impacts on fish as thermal pollution from other sources.
Construction Sites and Dams: People have undertaken some very big projects that directly affect rivers and streams within our watersheds. These big projects include things like building dams or altering stream channels. A dam is a structure that blocks a river channel and restricts the amount of water flowing downstream. Dam construction projects are undertaken for a number of reasons, including flood control, generation of hydroelectric power and the creation of lakes or reservoirs. Since a dam blocks a stream or river and only lets some of the water flow downstream, there is a backup of water behind a dam. This backed up water floods a lot of land behind the dam and creates a lake or reservoir. Over the years we have learned that not all dams work well at controlling flooding. When there is a flood larger than the design flood for the dam, the dam cannot control all of the flood waters. Dams will generally work in controlling the normal high water flow from rain storms and snow melt. However, reservoirs have limited storage capacity. When there is an unusually big storm, or all the snow on the surrounding hills melts at once, the water storage capacity could be exceeded and the water may overflow (breach) the dam. When a dam is breached, it causes worse flooding than if the dam had not been there because it has so much water stored up behind it. Not only that, but when there is a dam upstream, people will start to build houses and stores on the flood plain (the flat area on either side of the river channel that acts as the stream bed at times of high water).
The construction of dams impacts aquatic organisms and changes the nature of the stream itself. As water slows down and backs up behind a dam, it deposits the sediments it had been carrying. This causes a buildup of sediment behind the dam, often changing the composition of the steam bottom (i.e. a stream that may have been rocky will become sandy as the excess sand is deposited by the blocked stream waters).
There are other human activities that alter the flow of streams and the composition of the stream bottom. For example, channelizing a stream (straightening a section of a steam and even making the walls and bottom of the stream concrete, building retention walls along the banks of streams, and even entirely rerouting streams. Channelizing streams can cause problems because it takes less energy to flow through a straight concrete "river bed" than through a natural, curved, irregular river bed. More energy is retained by the stream and then expended on the banks of the natural river downstream. Retention walls do much the same thing, although if they are only on one side, the river will cut away at the opposite bank, gradually changing the course of the river.
Soil Erosion and Deposition: Most people do not think of soil as a pollutant. However, soil washing into streams, from things like construction sites and newly plowed fields, impacts organisms that use the stream in much the same way excess nutrients from human and animal wastes do. Excess soil (sediment) will increase the turbidity of a stream, making it difficult for fish to see their prey and reducing the amount of sunlight that reaches the plants growing in the water. If the water gets too cloudy plants won't have sunlight for photosyntesis. If the plants die then the insects and small fish that eat those plants will die too, and animals may also die because they cannot find food.
Aesthetic Impacts: We mentioned earlier that pollution can cause economic, aesthetic and recreational damage to the water bodies within our watershed. What does aesthetic mean? Well, it's easier to give examples. Who wants to swim in a murky pond or stream? Who wants to drink brown water? Who wants to buy a home next to a pond that is covered with algae all summer? The list of such questions is endless. These are questions about aesthetics.
Habitat Alteration: There are many other human activities that impact organisms within our watersheds. For example, replacing open fields or forests with houses or shopping malls reduces the amount of land available for wildlife habitat. It also increases the amount of storm water runoff entering surface water bodies. Animal habitats are often divided during the construction of roads through woodlands or fields. Many animals are killed on these roads as they move from one area of their habitat to another.
As we have discussed in other webpages, fish such as salmon and trout need to get back to the habitats where they were born in order to spawn. The fish must overcome fish hooks, nets, dams and various kinds of pollution as well as the natural hazards of the journey. The problem is particularly severe for Atlantic Salmon which must return from the ocean to the headwater streams where they were born. In fact, the human-created obstacles were the primary cause of the decline and extirpation of Atlantic Salmon from many of the rivers it once inhabited.
Another cause of the decline of salmonid fishes is fishing. Over fishing is one of the top causes for decline in many of our salmonid species. Overfishing is when so many fish are caught each year that there are not enough fish left to reproduce at a rate that will sustain the population. Over a period of years the fish population declines until there aren't any fish left. This is a complex problem, especially in species like the Atlantic Salmon where fishing occurs both in the sea and in the rivers as it journeys upstream to spawn. Many states have passed laws and regulations designed to protect salmon and trout in the rivers and ponds. In the ocean Atlantic Salmon are being protected by government programs which pay fishermen not to fish. You may have bought salmon or trout from a grocery or in a resturant, but today most of those fish come from hatcheries/fish farms.
Dams have also been a major contributor to the decline of salmon and trout. They pose very obvious physical barriers. Although fish ladders, fish lifts or downstream bypasses have now been built at many dams, this was not always the case. When swimming either upstream or downstream salmon are attracted to the strongest water current. This has posed a particular problem in the downstream journey where there are dams that generate hydroelectricity. At these dams the strongest current is usually at the turbine intake. When this is the case the salmon follow the water current into the turbine and a large percentage of them are killed. This problem has been lessened by putting in downstream bypasses. These structures basically consist of a tunnel from one side of the dam to the other, through which the fish can pass unharmed. These structures are only effective if the current in the bypass is strong enough to attract the fish.
Alteration of the stream bed composition has also resulted from dam construction, as well as building construction and agriculture. Salmon and trout are impacted by this change. In October or November the female salmon and trout prepare a redd in the gravel bottom of a cold water stream. The redd consists of a depression in the gravel with a gravel cover over the eggs once they are laid and fertilized. However, if the stream bed has been eroded there will be no gravel left in which to prepare the redd. Alternately, if a lot of sediment has been deposited, the salmon won't even attempt to spawn.
One additional way in which the construction of dams impacts salmonid populations has to do with the level of the river flow. Dam operators periodically vary the amount of water flowing through a dam and therefore, through a river channel. A salmon or trout may build a redd when the flows are high and there is a lot of water. However, if a dam operator decides to decrease the amount of water flowing through the dam, what was once a perfectly healthy redd of eggs may end up above the water line during a period of low flow and all of the eggs may die.
Salmonids, like all wildlife, are sensitive to chemical contaminants in the environment. Various types of water pollution (physical, chemical, thermal) impact the fish in different ways.
Just as pollution, over fishing and habitat destruction have impacted salmon and trout, the same types of human activities have impacted many other species of plant and animals. In fact, thousands of plant and animal species have become extinct (disappeared forever) over the last few hundred years and the majority of these disappearances can be linked to human activity. Why does this happen? Why can't these species survive? Those are questions without a single answer. In many cases human activities have destroyed the natural habitat of the organisms and left them with no place to live. In other cases human activity has eliminated or contaminated (e.g. through use of pesticides) the food supply of the organisms.
Nothing can be done to bring back those species that are already extinct, but steps can be taken to keep more species from becoming extinct. Species that haven't yet disappeared, but face great peril, are called either threatened or endangered. These terms are used and defined in the Endangered Species Act of 1973, which among other things, gives the U. S. Fish and Wildlife Service the responsibility for protecting and managing threatened and endangered species. An endangered species is one that is at risk of becoming extinct in the very near future. A threatened species is one with a lesser risk of becoming extinct in the short term, but one which is likely to become endangered if nothing is done to deal with the problems threatening it in the first place. In either case, action is needed to prevent permanent loss of these plants and animals.