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The source of all freshwater is rain or snow, and small upland streams therefore usually have very clean water. The water in your pond or stream, however, may have been held by natural processes in highland lakes or winter snow, passed through underground chambers, and filtered through soils. It may have picked up high levels of minerals (e.g. from mine workings), encountered abnormal organic enrichment (e.g. from livestock manure or untreated sewage), or been exposed to man-made chemical contamination (e.g. from fertilizer run-off or industrial waste).
Streams and ponds therefore sometimes become polluted. When this happens, some species cannot tolerate the contaminated water, whereas others can survive if the pollution is not too severe, and a few species actually thrive in polluted water. Therefore, the types of animals you find during your dipping can indicate whether your stream or pond is healthy or not. Here’s how you can use the results of your pond- or stream-dipping to become a ‘pollution detective’.
Make a note of which of the following animals you found when dipping in your pond or stream.
In a stream:
Group A: Mayfly nymphs or Stonefly nymphs
In a pond:
Group A: Dragonfly nymphs or Damselfly nymphs
In pond or stream:
Group B: Caddis fly larvae or Freshwater shrimps
Group C: Hoglice or Bloodworms
Group D: Worms (Tubifex) or Rat-tailed maggots
Group E: No animals found.
Then use the following chart to find out if your pond or stream is polluted.
You can download a PDF copy of this chart.
(Our thanks to Craig Macadam for letting us use his ‘Pollution Detective’ scheme.)
For a similar, but more advanced, method to calculate the ‘biotic index’ of a stream community as a measure of pollution based on the presence or absence of more than thirty types of animals, see ‘The Freshwater Name Trail’ published by the Field Studies Council.
