Understanding water quality
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# 2.2 Living organisms

Besides the living organisms that form part of the natural cycle in rivers, there are other organisms that are less desirable. Their presence is generally due to human activities, and they are a form of pollution. Many of these organisms are pathogenic bacteria, which can cause disease. The most common source of pathogenic bacteria is sewage, and the purpose of the 19th Century legislation in England and Wales prohibiting the discharge of raw sewage into rivers was to prevent the spread of disease. Pathogenic bacteria are adapted to body temperatures so they die off relatively quickly in cold river waters. For example, typhoid bacteria die within seven days in river water at the temperatures found in Britain—but a week is long enough to spread infection.

As well as bacteria, there are other aquatic organisms that may be harmful. Diseases may also be transmitted by protozoa, worms, snails and insect larvae (Figure 7).

Figure 7 Mosquito larvae hanging under a water surface. The adult mosquito hatches after a period of pupation and can transmit diseases such as malaria.

Non-pathogenic bacteria are also part of the aquatic system. One such bacterium, Escherichia coli (E. coli), has various strains, most of which are non-pathogenic and exist in human and animal intestines in a symbiotic relationship, making substances needed by humans and animals. The presence of E. coli in river water is therefore a useful indication of faecal pollution. The coliform count (the number of coliform bacteria, of which E. coli is one, present in a fixed volume of water) is used as a measure of the extent of such pollution.

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