|MadSci Network: Environment|
You are asking us to write a book for you and we cannot do that. However, I will give you some pointers:
When a stream becomes polluted with organic material, the bacteria multiply because they have so much food available. The bacteria use up all the available oxygen depriving invertebrates in the process. Some invertebrates, such as tubifex worms, can tollerate low oxygen levels. Many others require high levels of O2 and can only live in water that has a low bacterial population. As the polluted water moves downstream, the food supply for the bacteria gets used up and the oxygen consumption decreases. This results in the appearance of invertebrates such as beetle larvae, amphipods and snails. Eventually all the organic material is consumed by bacterial population, which then dwindles due to lack of a food supply, and the normal biotic community appears. These effects can be seen for many months after a pollution event has occured and provides a quick system for field monitoring of water quality.
A widely recognised system in the UK for monitoring stream quality is the BMWP score. For more information on this see my previous response: Re: what is a biomarker? Insects as drosophila can be used as biomarkers?
Epping forest is a lowland area and you would not expect a particulary high score. Clean water in lowland streams naturally contains more organic material and has lower oxgyen levels.
I suggest you check out the following references:
Allan, J.D. 1994. Stream Ecology. Chapman & Hall. (I)
Horne, A.J. & Goldman, C.R. 1993. Limnology. McGraw Hill. (E)
Mos, B. 1988. Ecology of freshwaters. Blackwells. (E)
Lalli, C.M. & Parsons, T.R. Biological Oceanography - an introduction (second edition). Butterworths/Heinemann. (E)
And here are some websites:
water quality and water pollution assessment
Try the links in the MadSci Library for more information on Environment .