Water Pollution

Activities carried out by the human population to supply food, power, and industrial needs have a considerable effect on the environment. These effects include atmospheric and water pollution, and destroying habitats.

Water pollution is mainly caused by “run off”, containing fertilisers, enters rivers from fields. It is known as leaching when the fertilisers get washed into the river or lake. Similarly, sewage also pollutes rivers. The polluting effects of fertilisers and sewage are caused by the constituent ions such as nitrates and phosphates. They result in eutrophication.

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Nitrates and phosphates enter the river and are absorbed by algae and other autotrophies. This causes them to grow rapidly at a much faster rate than usual. An algae bloom will then occur creating a blanket on the top of the water, this will block out the light and other plants on the bottom of the lake will die because they can’t carry out photosynthesis. Bacteria and other decomposers will then begin to breakdown the dead plants and any of the algae that have died. The bacteria population will now increase rapidly as the amount of organic matter increases. As the bacteria grows and performs respiration they use up oxygen in the water. The lack of oxygen will then cause any fish that are very oxygen sensitive to begin to die off. The rotting dead fish will also contribute to even lower oxygen levels, again aerobic bacteria are responsible.

This process causes there to be a reduction in biodiversity as fish and other animals that are oxygen sensitive will die off. Although, organisms that are not so sensitive will increase due to a lack of competition and the lack of predators. An example of this is bloodworms. They process proteins similar to hemoglobin which helps them to take in enough oxygen for survival, even at low concentration. Without fish to eat them the numbers of bloodworm grow even more.

Eutrophication won’t just affect animals in the river and ponds. Primary consumers could be affected by the decrease in primary producers and could cause numbers in that food chain to decrease due to a lack of energy being passed on to the next tropic level. Some algal blooms are toxic to plants and animals. Toxic compounds they produce can make their way up the food chain, resulting in animal mortality. Freshwater algal blooms can pose a threat to livestock. When the algae die or are eaten they can pass up the food chian and may pose a threat to humans.

There are many ways to measure both pollutants and their effects. Populations of algae, bloodworms or fish can be estermated. A key measurment is biological oxygen demand (BOD). Clean water takes up much less oxygen than that polluted with organic material. Aerobic bacteria take up a large proportion of this oxygen. A lake, heavily heavily poluted with organic matter, has a very high BOD.

Another type of water polution is acifd rain. This is caused by the combustion of fossil fuels. This releases a number of acidic gases which dissolve in rain water. The two most segnificant is nitrogen oxideS and sulphur dioxide.

SO2 + H2O = Sulphuric Acid

NO� + H2O = Nitic Acid

Rain of a low pH can have a devastating effect on the organisms of an ecosystem. The low pH results in many mineral ions being less soluble and consequently less avalable to plants. Phophates ions become bound to clay particles and are unavailable to plants while other possitivlty charged ions such as calcium are more easily leached. Aluminium ions are an exception and may accumulate to a toxic level. This can cause plants to be defoliated and die. This has a sequential effect on all consumers which rely directly or indirectly on the plants in the food web. In lakes, the low pH destroys because the pH cause the fish to create too much mucas over their gills and this prevents gaseos exchage of oxygen. Levels of other minerals are normaly low and cannot sustain much plant growth. A lake is said to be oligotrophic in this condition.

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