Water pollution

  1. Pollution is the contamination of the environment causing harm and damage to the ecosystem. It is usually the result of human activities.
  2. Water pollution occurs when pollutant’s are discharged directly into water without undergoing treatment. A common water pollutant is sewage.
  3. Sewage is waste matter from industries and homes. It consists mainly of organic wastes such as detergents, oils and fats, insecticides and herbicides, and debris.
  4. Inorganic substances from industrial waste include: leached nutrients and fertilisers (nitrates and phosphates) from farmland, ammonia, sulfur dioxide from power plants and heavy metals.
  5. Some of these pollutants can be directly toxic to the living organisms in the water, causing them to die. Others are carcinogenic and can harm humans who get in contact with the contaminated water.
  6. Contaminated water usually encourages growth of microorganisms such as bacteria, parasites (certain protozoa and worms) and viruses. These could lead to disease such as gastroenteritis, cholera, typhoid and parasitic infection.
  7. Other possible outcomes:
Water pollution and eutrophication

Sewage treatment

  1. Environmental biotechnology is when biotechnology is used to treat polluted environments or in environment-friendly processes such as green manufacturing technologies. Sewage treatment is an example of environmental biotechnology.
  2. In sewage treatment plants, sewage is drained into settling tanks and sedimentation tanks to allow some of the solid waste to settle and be removed.
  3. The sewage then enters the aeration tank, where pure oxygen is bubbled in and bacteria added. The bacteria oxidise carbon compounds to carbon dioxide, oxidise ammonium and nitrogen compounds to nitrates and eventually nitrogen gas and remove phosphates.
  4. The liquid from the aeration tank is then filtered and the solid contents are allowed to settle. Sewage water containing low levels of organic material and suspended matter remains. The sewage water is disinfected to reduce the number of microorganisms in the water before it is discharged back into the environment.
  5. The solid matter left behind from the sewage treatment process is known as sludge.
  6. Sludge undergoes a process of bacterial digestion to reduce the amount of organic matter and the number of disease-causing microorganisms present.
Typical large-scale sewage treatment plant


  1. Biological magnification or bioamplification is the increase in concentration of a substance up a food chain. Successive trophic levels contain high concentrations of the substance.
  2. Substances that tend to accumulate up a food chain share one or more of the following characteristics:
    • Non-biodegradable or slow biodegradation, so it persists in the environment and can be transported by water to other areas.
    • Cannot be broken down (detoxified) within organisms
    • Cannot be excreted by organisms (insolube in water)
  3. Examples of substances that biomagnify are mercury, arsenic and DDT (dichlorodiphenyltrichloroethane). DDT is a synthetic pesticide used to control mosquitoes. These chemicals are toxic, especially at high concentrations.
  4. Each trophic level has to consume a larger biomass than it possesses, from the previous trophic level due to energy loss at every level. Thus, although the toxin present in the lower trophic levels might be small, larger amounts of toxins will accumulate in the higher trophic levels since each top level consumer feeds on a large amount of organisms from the trophic level below it.
  5. DDT
    • DDT is non-biodegradable and is transported by water to far-reaching areas.
    • It is insoluble in water and cannot be excreted in urine which is water-based.
    • It is soluble in lipids and accumulates within the fatty tissues of animals. This process is called bioaccumulation, which is the increase in concentration of a substance due to absorption from food and the environment, in the tissues of organisms bodies.
    • The concentration of DDT increases at the higher trophic levels due to biomagnification.

Note: Biomagnification and bioaccumulation are words that are commonly used interchangeably. However they do not have the same meaning. Bioaccumulation occurs within an organism (within a trophic level) while biomagnification occurs in a food chain (across trophic levels).


Environmental impact of DDT

DDT is toxic to aquatic life and insects. It is less toxic to mammals but causes eggshell thinning in birds. DDT was used to control the spread of malaria by killing mosquitoes, which carry the protoctist parasites that cause the disease. Unfortunately, DDT remains in the environment after it has been sprayed and can be absorbed in sub-lethal doses by miscroscopic organisms. Hence, it can enter food chains and accumulate as it moves up them.

The concentration of insecticide often increases as it passes along a food chain. Clear Lake in California was sprayed with DDT to kill gnat larvae.

An example of how DDT concentrations increase up the tropic levels of an estuarine food chain

The insecticide made only a weak solution of 0.015 parts per million (ppm) in the lake water. The microscopic plants and animals that fed in the lake water built up concentration of about 5 ppm in their bodies. The small fish that fed on the microscopic animals had 10 ppm. The small fish were eaten by larger fish, which in turn were eaten by birds called grebes. The grebes were found to have 1600 ppm of DDT in their body fat and this high concentration killed large numbers of them.

Larger scale pollution of water by insecticides for instance by leakage from storage containers, may kill aquatic insects, destroying one or more levels in a food chain or food web, with serious consequences to the ecosystem.

A build-up of pesticides can also occur in food chains on land. In the 1950s in the USA, DDT was sprayed on to elm trees to try and control the beetle that spread Dutch elm disease. The fallen leaves contaminated with DDT, were eaten by earthworms. Because each worm ate many leaves, the DDT concentration in their bodies was increased ten times. When birds ate a large number of worms, the concentration of DDT in the birds bodies reached lethal proportions and there was a 30-90% mortality among robins and other song birds in the cities.

Even if DDT did not kill the birds, it caused them to lay eggs with thin shells. The eggs broke easily and fewer chicks were raised. In Britain, the numbers of peregrine falcons and sparrow hawks declined drastically between 1955 and 1965. These birds are at the top of a food web and so accumulate very high doses of the pesticides that are present in their prey, such as pigeons. After the use of DDT was restricted, the population of peregrines and sparrow hawks started to recover.

Pesticides may become more concentrated as they move along a food chain. The intensity of colour represents the concentration of DDT

These new insecticides had been thoroughly tested in the laboratory to show that they were harmless to human and other animals when used in low concentrations. It had not been foreseen that the insecticides would become more and more concentrated as they passed along the food chain.

Insecticides like this are called persistent because they last a long time without breaking down. This makes them good insecticides but they also persist for a long time in the soil, in rivers, lakes and the bodies of animals, including humans. This is a serious disadvantage.


Herbicides are used by farmers to control plants (usually referred to as weeds) that compete with crop plants for nutrients, water and light. If the weeds are not removed, crop productivity is reduced. However, if the herbicides do not break down straight away, they can leach from farmland into water systems such as rivers and lakes, where they can kill aquatic plants, removing the producers from food chains. Herbivores lose their food source and die or migrate. Carnivorous animals are then affected as well.

Leakage or dumping of persistent herbicides into the sea can have a similar effect on marine food chains. Herbicides tend to be non-specific: they kill any broadleaved plants they come into contact with or are absorbed by. If herbicides are sprayed indiscriminately, they may blow onto surrounding land and kill plants other than the weeds in the crop being treated. This can put rare species of wild flowers at risk.

If herbicides are sprayed indiscriminately, they may blow onto surrounding land and kill plants other than the weeds in the crop being treated. This can put rare species of wild flowers at risk.

The effects of ultraviolet radiation

Ultraviolet (UV) rays reaching the Earth can cause problems for a range of living organisms.

Fair-skinned people are warned of the effect of too much UV radiation on exposed skin and advised to protect themselves with sun-block creams to prevent sunburn. Overexposure can be extremely harmful to health because UV rays cause mutations in DNA, and are linked to various types of skin cancer such as malignant melanoma non-lethal carcinomas. Long-term exposure is also a risk factor for cataracts in the eyes.

Plants can also suffer from UV radiation. Not only is their DNA likely to be damaged, UV radiation also affects photosynthesis. The photosynthetic pathways are inhibited, resulting in lower productivity in exposed plants. DNA damage restricts plant growth so that both biomass and net productivity decrease. Many plants that grow at high altitude, such as in the Andes mountains, have evolved mechanisms to screen themselves from the intense radiation, but floating aquatic plants seem to be very susceptible to UV light. Since plants play such a fundamental role as producers in food chain, decreased photosynthesis has the potential to affect a whole ecosystem.

Nuclear fall-out

This can be the result of a leak from a nuclear power station, or from a nuclear explosion. Radioactive particles are carried by the wind or water and gradually settle in the environment. If the radiation has a long half-life, it remains in the environment and is absorbed by living organisms. The radioactive material bioaccumulates in food chains and can cause cancer in top carnivores.

Probably the worst nuclear accident in history happened at Chernobyl in Russia in April 1986. One of the reactor vessels exploded and the resulting fire produced a cloud of radioactive fallout, which was carried by prevailing winds over other parts of the Soviet Union and Europe. The predicted death toll, from direct exposure to the radiation and indirectly from the fallout is estimated to be at least 4000 people (and possibly much higher), with many others suffering from birth defects or cancers associated with exposure to radiation. The fall-out contaminated the soil it fell on and was absorbed by plants, which were grazed by animals. Farmers in the Lake District in England were still banned from selling sheep.

In 1971, 45 people in Minamata Bay in Japan died and 120 were seriously ill as a result of mercury poisoning. It was found that a factory had been discharging a compound of mercury into the bay as part of its waste. Although the mercury concentration in the sea was very low, its concentration was increased as it passed through the food chain. By the time it reached the people of Minamata Bay in the fish and other sea food that formed a large part of their diet, it was concentrated enough to cause brain damage, deformity and death.

High levels of mercury have also been detected in the Baltic Sea and in the Great Lakes of North America.

Another major nuclear disaster happened at the Fukushima nuclear power plant in Japan in March 2011. The plant was hit by a powerful tsunami, caused by an earthquake. A plume of radioactive material was carried from the site by the wind and came down onto the land, forming a scar like a teardrop over 30 kilometres wide. The sea around the power plant is heavily contaminated by radiation. This is absorbed into fish bones, making the animals unfit for consumption.

Fukushima nuclear power plant, destroyed by a powerful tsunami and fire

Chemical waste

Many industrial processes produce poisonous waste products. Electroplating, for example, produces waste containing copper and cyanide. If these chemicals are released into rivers they poison the animals and plants and could poison humans who drink the water. It is estimated that the River Trent receives 850 tonnes of zinc, 4000 tonnes of nickel and 300 tonnes of copper each year from industrial processes. Any factory getting rid of its effluent into water system risks damaging the environment.

River pollution. The river is badly polluted by the effluent from a paper mill

Oil pollution

Oil pollution of the sea has become a familiar event. In 1989, a tanker called the Exxon Valdez ran on to Bligh Reef in Prince William Sound, Alaska, and 11 million gallons of crude oil spilled into the sea. Around 400000 sea birds were killed by the oil and the population of killer whales, sea otters and harbour seals among others, were badly affected. The hot water high-pressure hosing techniques and chemicals used to clean up the shoreline killed many more birds and sea creatures living on the coast. Since 1989, there have continued to be major spillages of crude oil from tankers and off-shore oil wells.

Oil pollution. Oiled sea birds like this long-tailed duck cannot fly to reach their feeding grounds. They also poison themselves by trying to clean the oil from their feathers

Discarded rubbish

The development of towns and cities, and the crowding of growing populations into them, leads to problems of waste disposal. The domestic waste from a town of several thousand people can cause disease and pollution in the absence of effective means of disposal. Much ends up in landfill sites, taking up valuable space, polluting the ground and attracting vermin and insects, which can spread disease. Most consumable items come in packaging, which if not recycled, end up in landfill sites or is burned, causing air pollution. Discarded rubbish that ends up in the sea can cause severe problems for marine animals.


When nitrates and phosphates from farmland and sewage escape into water they cause excessive growth of microscopic green plants. This may result in a serious oxygen shortage in the water, resulting in the death of aquatic animals – a process called eutrophication.


Plants need a supply of nitrates for making their proteins. They also need a source of phosphates for many chemical reactions in their cells. The rate at which plants grow is often limited by how many nitrate and phosphate they can obtain. In recent years the amount of nitrate and phosphate in our rivers and lakes has been greatly increased. This leads to an accelerated process of eutrophication.

Eutrophication is the enrichment of natural waters with nutrients that allow the water to support an increasing amount of plant life. This process takes place naturally in many inland waters but usually very slowly. The excessive enrichment that results from human activities leads to an overgrowth of microscopic algae. These aquatic algae are at the bottom of the food chain. The extra nitrates and phosphates from the processes enable them to increase so rapidly that they cannot be kept in check by microscopic animals which normally eat them. So they die and fall to the bottom of the river or lake. As the plants die, some through lack of light because of overcrowding, aerobic bacteria decompose them and respire, taking oxygen out of the water. As oxygen levels drop, animals such as fish cannot breathe, fish and other organisms die from suffocation and the whole ecosystem is destroyed.

Processes leading to eutrophication

The main causes of eutrophication

Sewage (Discharge of treated sewage)

Diseases like typhoid and cholera are cause by certain bacteria when they get into the human intestine. The faeces passed by people suffering from these disease will contain the harmful bacteria. If the bacteria get into drinking water they may spread the disease to hundreds of other people. For this reason, among others, untreated sewage must not be emptied into rivers. It is treated at the sewage works so that all the solids are removed. The human waste is broken down by bacteria and made harmless (free from harmful bacteria and poisonous chemicals), but the breakdown products include phosphates and nitrates. When the water from the sewage treatment is discharged into rivers it contains large quantities of phosphate and nitrate, which allow the microscopic plant life to grow very rapidly. Figure below shows this sequence of events as a flow chart.

Growth of algae in a lake. Abundant nitrate and phosphate from treated sewage and from farmland make this growth possible

Use of detergents

Some detergents contain a lot of phosphate. This is not removed by sewage treatment and is discharged into rivers. The large amount of phosphates encourages growth of microscopic plants (algae).

Arable farming

Since the Second World War, more and more grasslands has been ploughed up in order to grow arable crops such as wheat and barley. When soil is exposed in this way, the bacteria, aided by the extra oxygen and water, produce soluble nitrates, which are washed into streams and rivers where they promote the growth of algae. If the nitrates reach underground water stores they may increase the nitrate in drinking water to levels considered ‘unsafe’ for babies.

Some people think that it is excessive use of artificial fertilisers that causes this pollution but there is not much evidence for this.

Factory farming

Chickens and calves are often reared in large sheds instead of in open fields. Their urine and faeces are washed out of the sheds with water forming ‘slurry’. If this slurry gets into the streams and rivers it supplies an excess of nitrates and phosphates for the microscopic algae.

The degree of pollution of river water is often measured by its biochemical oxygen demand (BOD). This is the amount of oxygen used by a sample of water in a fixed period of time. The higher the BOD, the more polluted the water is likely to be.

It is possible to reduce eutrophication by using:

  • detergents with less phosphates
  • agricultural fertilisers that do not dissolve so easily
  • animal wastes on the land instead of letting them reach rivers

Plastics and their environment

Plastics that are non-biodegradable are not broken down by decomposers when dumped in landfill sites or left as litter. This means that they remain in the environment, taking up valuable space or causing visual pollution. Discarded plastic bottles can trap small animals; nylon fishing lines and nets can trap birds and mammals such as seals and dolphins. As the plastics in water gradually deteriorate, they fragment into tiny pieces, which are eaten by fish and birds, making them ill. When plastic is burned, it can release toxic gases.

Plastic bags are a big problem, taking up a lot of space in landfill sites. In 2002, the Republic of Ireland introduced a plastic bag fee, called a PlasTax, to try to control the problem. It had a dramatic effect, cutting the use of single-use bags from 1.2 billion to 230 million a year and reducing the litter problem that plastic bags create. Revenue raised from the fee is used to support environmental projects.

Air pollution

Some factories and most motor vehicles release poisonous substances into the air. Factories produce smoke and sulfur dioxide; cars produce lead compounds, carbon monoxide and the oxides of nitrogen, which lead to smog and acid rain.

Air pollution by industry. Tall chimneys keep pollution away from the immediate surroundings but the atmosphere is still polluted.
DDT spraying
Photochemical ‘smog’ over a city

Sulfur dioxide and oxides of nitrogen

Coal and oil contain sulfur. When these fuels are burned, they release sulfur dioxide (SO2) into the air. Although the tall chimneys of factories send smoke and sulfur dioxide high into the air, the sulfur dioxide dissolves in rainwater and forms an acid. When this acid falls on buildings, it slowly dissolves the limestone and mortar. When it falls on plants, it reduces their growth and damages their leaves.

This form of pollution has been going on for many years and is getting worse. In North America, Scandinavia and Scotland, forests are being destroyed and fish are dying in lakes, at least partly as a result of acid rain.

Oxides of nitrogen from power stations and vehicles exhausts also contribute to atmospheric pollution and acid rain. The nitrogen oxides dissolve in rain drops and form nitric acid.

Effects of acid rain on conifers in the Black Forest, Germany
Plant leaves damaged by acid rain

Oxides of nitrogen also take part in reactions with other atmospheric pollutants and produce ozone. It may be the ozone and the nitrogen oxides that are largely responsible for the damage observed in forests.

One effect of acid rain is that it dissolves out the aluminium salts in the soil. These salts eventually reach toxic levels in streams and lakes.

There is still some argument about the source of the acid gases that produce acid rain. For example, a large proportion of the sulfur dioxide in the atmosphere comes from the natural activities of certain marine algae. These microscopic ‘plants’ produce the gas dimethylsulfide which is oxidised to sulfur dioxide in the air.

Nevertheless, there is considerable circumstantial evidence that industrial activities in Britain, America and Central and Eastern Europe add large amounts of extra sulfur dioxide and nitrogen oxides to the atmosphere.

Acid rain in Britain. The pollution comes from British factories, power station, homes and vehicles. Most emissions start as dry gases and are converted to dilute sulfuric and nitric acids

Control of air pollution

The Clean Air Acts of 1956 and 1968

These acts designated certain city areas as ‘smokeless zones’ in Britain. The use of coal for domestic heating was prohibited and factories were not allowed to emit black smoke. This was effective in abolishing dense fogs in cities but did not stop the discharge of sulphur dioxide and nitrogen oxides in the country as a whole.

Reduction of acid gases

The concern over the damaging effects of acid rain has led many countries to press for regulations to reduce emission of sulfur dioxide and nitrogen oxides.

Reduction of sulfur dioxide can be achieved either by fitting desulfurisation plants to power stations or by changing the fuel or the way it is burnt. In 1986, Britain decided to fit desulfurisation plants to three of its major power stations, but also agreed to a United Nations protocol to reduce sulfur dioxide emission to 50% of 1980 levels by the year 2000, and 20% by 2010. This was to be achieved largely by changing from coal-fired to gas-fired power stations.

Reduction of vehicle emissions

Oxides of nitrogen come, almost equally, from industry and from motor vehicles. Flue gases from industry can be treated to remove most of the nitrogen oxides. Vehicles can have catalytic converters fitted to their exhaust systems. These converters remove most of the nitrogen oxides, carbon monoxide and unburned hydrocarbons. They add £200-600 to the cost of a car and will work only if lead-free petrol is used, because lead blocks the action of the catalyst.

Another solution is to redesign car engines to burn petrol at lower temperatures (‘lean burn’ engines). These emit less nitrogen oxide but just as much carbon monoxide and hydrocarbons as normal engines.

In the long term, it may be possible to use fuels such as alcohol or hydrogen, which do not produce so many pollutants.

The European Union has set limits on exhaust emissions. From 1989, new cars over 2 litres had to have catalytic converters and from 1993 smaller cars had to fit them as well.

Regulations introduced in 1995 should cut emissions of particulates by 75% and nitrogen oxides by 50%. These reductions will have less effect if the volume of traffic continues to increase. Significant reduction of pollutants is more likely if the number of vehicles is stabilised and road freight is reduced.

Protecting the ozone layer

The appearance of ‘ozone holes’ in the Antarctic and Arctic, and the thinning of ozone layer elsewhere, spurred countries to get together and agree to reduce the production and use of CFCs (chlorofluorocarbons) and other ozone-damaging chemicals.

1987 saw the first Montreal protocol, which set targets for the reduction and phasing out of these chemicals. In 1990, nearly 100 countries, including Britain, agreed to the next stage of the Montreal protocol, which committed them to reduce production of CFCs by 85% in 1994 and phase them out completely by 2000. Overall, the Montreal protocol has proved to be very successful: by 2012, the world had phased-out 98% of the ozone-depleting substances such as CFCs. However, the chemicals that were used to replace CFCs (HCFCs) are not as harmless as they were first thought to be, as they contribute to global warming.

The ‘greenhouse effect’ and global warming

The Earth’s surface receives and absorbs radiant heat from the Sun. It re-radiates some of this heat back into space. The Sun’s radiation is mainly in the form of short-wavelength energy and penetrates our atmosphere easily. The energy radiated back from the Earth is in the form of long wavelengths (infrared or IR), much of which is absorbed by the atmosphere. The atmosphere acts like the glass in a greenhouse. It lets in light and heat from the Sun but reduces the amount of heat that escapes.

If it were not for this ‘greenhouse effect’ of the atmosphere, the Earth’s surface would probably be at -18C. The ‘greenhouse effect’, therefore is entirely natural and desirable.

Not all the atmospheric gases are equally effective at absorbing IR radiation. Oxygen and nitrogen, for example, absorb little or none. The gases that absorb most IR radiation, in order of maximum absorption, are water vapour, carbon dioxide (CO2), methane and atmospheric pollutants such as oxides of nitrogen and CFCs. Apart from water vapour, these gases are in very low concentrations in the atmosphere, but some of them are strong absorbers of IR radiation. It is assumed that if the concentration of any of these gases were to increase, the greenhouse effect would be enhanced and the Earth would get warmer.

In recent years, attention has focused principally on CO2 . If you look at the carbon cycle, you will see that the natural processes of photosynthesis, respiration and decay would be expected to keep the CO2 concentration at a steady level. However, since the Industrial Revolution, we have been burning ‘fossil fuels’ derived from coal and petroleum and releasing extra CO2 into the atmosphere. As a result the concentration of CO2 has increased from 0.029 to 0.039% since 1860. It is likely to go on increasing as we burn more and more fossil fuel. According to NOAA data, CO2 levels rose 2.67 parts per million in 2012, to 395 ppm. This was the second largest increase since 1959, when scientists first began measuring atmospheric CO2 levels.

Although it is not possible to prove beyond all reasonable doubt that production of CO2 and other ‘greenhouse gases’ is causing a rise in the Earth’s temperature, i.e. global warming, the majority of scientists and climatologists agree that it is happening now and will get worse unless we take drastic action to reduce the output of these gases.

Predictions of the effects of global warming depend on the computer models. But these depend on very complex and uncertain interactions of variables.

Changes in climate might increase cloud cover and this might reduce the heat reaching the Earth from the Sun. Oceanic plankton absorb a great deal of CO2. Will the rate of absorption increase or will a warmer ocean absorb less of the gas? An increase in CO2 should, theoretically, result in increased rates of photosynthesis, bringing the system back into balance.

None of these possibilities is known for certain. The worst scenario is that the climate and rainfall distribution will change, and disrupt the present pattern of world agriculture; the oceans will expand and the polar icecaps will melt, causing a rise in sea level; extremes of weather may produce droughts and food shortages.

An average of temperature records from around the world suggests that, since 1880, there has been a rise of 0.7-0.9C, most of it very recently see Figure, but this is too short a period from which to draw firm conclusions about long-term trends. If the warming trend continues, however, it could produce a rise in sea level of between 0.2 and 1.5 metres in the next 50-100 years.

Annual average global temperatures and carbon dioxide levels since 1880

The first Kyoto Conference (Japan) in 1997 set targets for the industrialised countries to reduce CO2 emissions by an average of 5.2% by 2010. Europe, as a whole agreed to cuts of 8%, though this average allowed some countries to increase their emissions. The countries committed to the Kyoto convention, excluding the USA, eventually modified the targets, but agreed to make cuts of 4.2% on average for the period 2008-2012.

Britain planned to reduce emissions by 20% of 1990 levels by 2010 but really needed an overall cut of 60% to halt the progress of global warming. The big industrialised countries who contribute 80% of the greenhouse gases, particularly the USA, are opposed to measures that might interfere with their industries, claiming that global warming is not a proven fact.

The precautionary principle suggests that, even if global warming is not taking place, our supplies of fossil fuel will eventually run out and we need to develop alternative sources of energy now.

The generation of energy using fossil fuels is the biggest source of CO2 released by humans into the atmosphere. The alternatives are nuclear power or methods such as wind farms and solar energy. The experiences of Chernobyl and Fukushima have made people around the world very vary of the nuclear option. Not all countries have climates and weather suited to alternative energy and their environmental impact (visual and sometimes through the noise they can create) creates opponents to these methods.

The consequences of global warming could be pretty serious

There are several reasons to be worried about global warming. Here are a few:

  1. Higher temperatures cause seawater to expand and ice to melt, causing the sea level to rise. It has risen a little bit over the last 100 years. If it keeps rising it’ll be bad news for people and animals living in low-lying places. It will lead to flooding, resulting in the loss of habitats (where organisms live).
  2. The distribution of many wild animal and plant species may change as temperature increases and the amount of rainfall changes in different areas. Some species may become more widely distributed, e.g. species that need warmer temperatures may spread further as the conditions they thrive in exist over a wider area. Other species may become less widely distributed e.g. species that need cooler temperatures may have smaller ranges as the conditions they thrive in exist over a smaller area.
  3. There could be changes in migration patterns e.g. some birds may migrate further northern areas are getting warmer.
  4. Biodiversity could be reduced if some species are unable to survive a change in the climate, so become extinct.

Pollution by contraceptive hormones

When women use the contraceptive pill, the hormones in it (oestrogen or progestrone) are excreted in urine and become present in sewage. The process of sewage treatment does not extract the hormones, so they end up in water systems such as rivers, lakes and the sea. Their presence in this water affects aquatic organisms as they enter food chains. For example, male frogs and fish can become ‘feminised’ (they can start producing eggs in their testes instead of sperm). This causes an imbalance between numbers of male and female animals (more females than males).

Drinking water, extracted from rivers where water from treated sewage has been recycled, can also contain the hormones. This has been shown to reduce the sperm count in men, causing a reduction in fertility.

It should be noted that the contraceptive pill is not the only source of female hormones in water systems: natural hormones are also present in urine from cattle, for example, and these can enter the water with run-off from farms.