A build up of large quantities of these nutrients in the rivers and lakes causes an accumulation of nutrients at a rate greater than they can be used through the natural processes of decomposition and photosynthesis from the living organisms in the water.
Mainly in still, stagnant water the build up of nutrients encourages the growth of algae on the surface and this can cause blue-green algae to grow which can be hazardous to animals and humans in that it produces toxins. High levels of phosphorus can cause these algae to grow. (1)
Because light oxygen and heat are unable to penetrate through the surface, the algal cells die off from the surface, and bacteria decompose the cells. The bacteria increase in number to cope with the many algal cells and as they respire a lot of oxygen is used up, and other organisms requiring this oxygen subsequently die off as there is not enough.
In some areas of Britain research has shown that rivers contain in excess of the recommended 11.3mg 1‾ ¹ of nitrates, and this figure was set according to health risks from nitrate pollution. (2) This shows that nitrate pollution is a serious issue.
From a source by Dix in 1981 it was shown that water draining from cropland can contain 10-15mg 1‾ ¹ of nitrate, on average from a year’s discharge being studied. (2)
Although it is largely from the result of using fertilisers, it is also from point sources of human sewage and urban industrial waste. In Lake Wisconsin studied by Hasler in 1974, it was shown that 24.5% of the nitrogen came from treatment factories alone, compared to 53.8% from rural sources, which includes manure use, cropland, pasture, forest land and groundwater. (3) This shows that the agricultural chemicals are clearly affecting the eutrophication, but that there are many other sources that pose a problem as well.
Organic waste also contain chemicals, nutrients and pesticides in varying amounts from farmyard manure, fertilizer, the excreta from the livestock animals and soil nitrogen and can cause environmental problems when there is too much and it pollutes rivers from surface run-off. The total amount of nitrogen from these sources is 3000 tonnes × 10‾ ¹, from approximate estimates based on annual UK farming. (2) This can be prevented from waste being disposed of properly.
The use of pesticides to control the level of pests by using chemicals can also cause environmental problems. The problems from pesticides include chemicals which are spread in sprays which can become airborne and spread to areas not intended for contact with the pesticide. Also the pesticides can seep through the soil from the crop they were applied on and get into the groundwater. Today, many chemicals, including some pesticides, have been detected in groundwater. (6)
In the past natural organic pesticides were used much more such as nicotine and they were used in smaller amounts and irregularly applied. Nowadays however, more complex synthetic chemicals are used which control the level of pests more effectively but also can affect plants and animals that they were not intended for.
It has been shown the actual efficiency of pesticides can be very low, only 20% of the pesticide material reaching its target pest. (2)
The persistent types of pesticides are broken down slowly and remain in the insects body for longer. When larger animals such as birds eat the insect they have the chemical inside them and this can build up into concentrations capable of affecting the animals that they weren’t intended for. In a study made by Rachel Carson in 1963, the threat of these chemicals is outlined. Wildlife, fish and domestic animals including man were shown to contain various amounts of DDT which was a commonly used insecticide, and the source was found to be through plant food and in the case of fish, absorbed through the water. High and toxic levels of this chemical were found in fish and is an example of biological concentration. (2)
The problem of persistent chemicals has been addressed by most of such chemicals being taken out from the market and pesticides that break down quickly into harmless substances are encouraged for use. (6)
A solution used in agricultural that deals with the many problems like fertiliser use and pesticides is a system called integrated crop management. The overall benefits through strategies of crop rotations, organic manures, soil sampling, and disease assessment of crops include a reduce in nitrate pollution and less chemicals being used. (4) This system is very beneficial to the environment.
Bibliography
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2. Agricultural Ecology by Joy Tivy, Published 1990 by Longman Group
3. Impact on the Ecosystem by Greg O’Hare and Joy Tivy. Editor: W.E Marsden Published 1981.
4. Mr Roger Couzens, Speaker about Integrated Crop Management.
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