Many invertebrates live in freshwater, and those that live in streams can be used to indicate water quality.
Streams can become polluted by water entering from agricultural land or industrial sites, and the quality of the water will be reflected by the types of creatures that can survive. Some of the worms can manage in very heavily polluted water (Tubifex is a good example), while some of the insects require clean conditions.
Macroinvertebrates.
The larger freshwater invertebrates (macroinvertebrates) spend most of their time in the same part of the stream. They are numerous and easy to catch, and once caught and identified they can give clues to the pollution history of that particular part of the stream. By sampling different locations it is possible to locate the source of any polluting incident – upstream will not be damaged, but downstream will have been affected.
Biotic Index.
There is a general rule that better water has a larger number of different invertebrate species, and by counting the number of different types of creatures it is possible to get a rough idea of water quality. By going further and identifying each species found it is possible to quantify this information. It is possible to arrive at the Biotic Index. This is a figure which allows different stretches of water to be compared in an objective way.
Chemical Analysis.
The alternative way of studying water pollution is by direct chemical analysis, and while it will be specific about the nature of the pollutants it can only help when the stream actually contains them. Once the damage has swept downstream the water will be replaced, but the invertebrates living in the affected area will have been harmed, and they will take a long time to recover. This means that occasional sampling of stream macroinvertebrates can be used to discover pollution events that occurred in the past.
Indicator Species.
If you can recognise one or two of the ‘indicator species’ you will not have to go to the trouble of identifying all the other creatures. If Stonefly larvae are present then the water is good, if they are absent but there are still Mayfly larvae then the water is reasonable. If neither of these is present then there has probably been a problem and a more detailed study might be called for.
Collecting Macroinvertebrates.
You will need a net and a pair of waterproof boots. Most of the creatures you want live under stones so that kicking the stones around upstream of the net will dislodge them and allow the current to carry them downstream and into your net. (This is sensibly known as ‘Kick Sampling’!). Putting them into a white dish with a little water will rapidly show you what you have caught.
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