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Life in Freshwater

Polluted Rivers

The main pollutants in rivers are

See also Pollution in Lakes, e.g. ochre pollution

Headwaters are normally pristine. It is the lower reaches of a river that we see the increase chance of pollutants entering the water from industries and urban areas.

Organic Matter & BOD

Organic matter, we have seen elsewhere, is an often essential component of river energy flow. The problem comes when this is excessive, sewage derived material. In lowland areas industrialisation has led to organic matter, normally from the population to be discharged into the river to be carried away. A hundred and fifty years ago the Thames was an open sewer supporting little animal life but with Sewage treatment methods salmon have started to return to the river and move past London in the last few decades. Treatment is just a way of concentrating the normal decomposition taking place in the river. Below is a now famous set of graphs that represent what happens along a river from the point of discharge. It has appeared in many exam papers too. This was from the work by Hynes who wrote an excellent book "The biology of polluted rivers".

Graph of the effect of organic matter discharge
A: this section shows that from the outfall there is a massive increase in suspended solids which decline down the river as they are decomposed but most importantly the oxygen level plummets before slowly recovering. This is the main problem with organic pollution - a loss in oxygen, that causes the death of most of the community especially fish and stoneflies.

B: this shows what the decay in the organic matter generates; initially a great deal of nitrogen in the form of ammonium ions and then phosphate and nitrate. This is the cause of eutrophication in lakes, i.e. a sudden bloom in algae

C: this eutrophication can be seen with the peak in algae soon after the nitrate peak. Most significantly see what has caused the oxygen depletion - a massive growth of the sewage fungus and bacteria.

D: this shows the communities present. The clean water species have died out within a short distance of the outfall. It was not immediate but when compared to graph A it correlates fairly exactly with the oxygen change. The dominant species replacing the clean water fauna is firstly Tubifex worms and then Chironomus midge larvae. Tubifex is able to survive almost nil oxygen conditions and thrives in organic matter. Chironomus larvae are the only insect type capable of producing haemoglobin. This blood pigment, in both groups, helps to store oxygen rather than transport it. When oxygen is available they will be able to absorb it and use it when the oxygen tension declines. Asellus (the Water Hoglouse) feeds and lives amongst organic matter but does not have the ability to survive the low oxygen levels. Hence it is found a bit further down river once the oxygen is recovering.

The effect seen in these graphs is applicable in most situations. The distance down stream that the effect will occur depends on both the size of the organic discharge and the size of the river

The treatment of sewage concentrates the decomposition process within sprinkler beds. The methods vary but most importantly it is aerated as much as possible to accelerate the decay. Once this has happened it will be discharged into the river. There will be less effect on the river water quality. Of course the river is important for kinds of reasons and one is to extract water for drinking. It is said that on a typical river in the UK the river water may be drunk four or five times before it arrives at the sea.

The most natural method of sewage treatment is to pass it into Phragmites reedbeds. A human community such as a village of 50-100 people could have the sewage naturally recycled by a reedbed with no affects at all. Unfortunately it is only viable with small numbers. Some of the field centres belonging to the Field Studies Council use this method.

Graph A above shows the change in BOD. This stands for Biological (also Biochemical) Oxygen Demand. Water can contain a variety of organic material included dissolved organic matter. The sudden removal of oxygen from the water spells disaster for the fauna living there but we saw above that removal does not happen immediately as the microbes have to multiply and start working. BOD is a scientific way of measuring the demand for oxygen by the microbes to decay the organic matter. Essentially, this method provides ideal conditions for microbe growth: a sample of water is taken from the river and the oxygen content is measured. The sample needs to be sealed so no oxygen can get in or out and then it is incubated at an ideal temperature for bacterial action, e.g. 30 degrees, for 48 hours. Then the oxygen content is measured again. If this has dropped significantly it suggests there is a problem developing. The Environment Agency might then have an early warning to start oxygenating the water to stop the fauna dying.

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Heavy Metals

This is a problem found in water bodies where heavy metals like lead, cadmium and copper enter rivers from slag heaps and other sources. This problem has been reduced in Europe in recent decades. However, a sudden discharge will cause problems especially for humans if it gets into the public water supply. Cadmium if consumed by humans will replace the calcium in bones and cause a severe brittle bone problem. Aluminium scares have occurred, the most recent being at Camelford in Cornwall in 1988. Aluminium is linked with premature aging although long term problems at Camelford are disputed.

Heavy metal pollution can be devastating to human populations but many invertebrate communities have adapted to survive in these waters. It varies with the pollutant and the length of time that it has percolated into the water system.

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Alien Species

It is accepted worldwide that one of the biggest threats to natural communities is without doubt the invasion of alien species from other countries. These lead to intense competition such that it could be the local, native species that becomes excluded. Various flatworm species from the Far East and New Zealand are getting well established in the UK and causing earthworms to decline as they eat them. On rivers there has been an incredible crash in water vole populations to the extent that they are endangered. This native species is disappearing due to the rise in wild mink. The coypu is another introduced mammal species now becoming commonplace.

Coypu
Coypu at the side of a river bank

Coypus are 40cms long and were introduced from South America for the fur trade.

There are significant numbers of plant species that have generated problems in freshwater bodies. Canadian pond weed is well established in the UK and can severely choke canals and slow moving water. The water fern is a similar problem on the surface of the water. Himalyan balsam and Japanese knotweed can be prolific on river banks. These all push out native species.

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