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Organic compounds in freshwater

Effects of organic pollution on freshwater ecosystems


Types of freshwater pollution

Toxicity response

Acids & alkalis









Organic pollution occurs when large quantities of organic compounds, which act as substrates for microorganisms, are released into watercources. During the decomposition process the dissolved oxygen in the receiving water may be used up at a greater rate than it can be replenished, causing oxygen depletion and having severe consequences for the stream biota. Organic effluents also frequently contain large quantities of suspendid solids which reduce the light available to photosynthetic organisms and, on settling out, alter the characteristics of the river bed, rendering it an unsuitable habitat for many invertebrates. Toxic ammonia is often present.
Organic pollutants consist of proteins, carbohydrates, fats and nucleic acids in a multiplicity of combinations. Raw sewage is 99,9 per cent water, and of the 0,1 per cent solids, 70 per cent is organic (65 per cent proteins, 25 per cent carbohydrates, 10 per cent fats). Organic wastes from people and their animals may also be rich in disease-causing (pathogenic) organisms.

What are the origins of organic pollutants?

Organic pollutants originate from domestic sewage (raw or treated), urban run-off, industrial (trade) effluents and farm wastes. Sewage effluents is the greatest source of organic materials discharged to freshwaters. In England and Wales there are almost 9000 discharges releasing treated sewage effluent to rivers and canals and several hundred more discharges of crude sewage, the great majority of them tot the lower, tidal reaches of rivers or, via long outfalls, to the open sea. It has been assumed, certainly incorrectly, that the sea has an almost unlimited capacity for purifying biodegradable matter.

The effects of organic effluents on receiving waters

When an organic polluting load id discharged into a river it is gradually eliminated by the activities of micro organisms in a way very similar to the processes in the sewage treatment works. This self-purification requires sufficient concentrations of oxygen, and involves the breakdown of complex organic molecules into simple in organic molecules. Dilution, sedimentation and sunlight also play a part in the process. attached micro organisms in streams play a greater role than suspended organisms in self-purification. Their importance increases as the quality of the effluent increases since attached microorganisms are already present in the stream, whereas suspended ones are mainly supplied with the discharge.

Effects on the biota

Organic pollution affects the organisms living in a stream by lowering the available oxygen in the water. This causes reduced fitness, or, when severe, asphyxiation. The increased turbidity of the water reduces the light available to photosynthetic organisms. Organic wastes also settle out on the bottom of the stream, altering the characteristics of the substratum. The general effects of fairly severe organic pollution are illustrated in

Suggested reading for the topic: Water pollution

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