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Invertebrates and The Pollution of Rivers
The sewage discharged from towns, villages and small groups of houses constitutes one of the main sources of pollution of our rivers and burns. Inadequate treatment such as the discharge of septic tank effluents to small burns causes considerable local pollution and the replacement of a healthy community of organisms with one of only a simple species such as certain Chironomidae (midge larvae) and Tubifex (a small worm) or Assellus (the water hog louse).
Plecoptera (stoneflies) are the most intolerant of pollution from organic sources and are common in most Caithness rivers and the cleaner burns. Fourteen different species are known to be present in Caithness rivers, but it is likely that, with further study, many more will be found. The cast-off skins of the larger species such as Perla bipunctata and Dinocras ce?alotes are very notice- able on exposed stones in the upper reaches of the Thurso and Forss Rivers in the early summer. The nymphs can be found under stones in most Caithness rivers from late summer onwards, but are less frequent in the Wick River and completely absent from many small burns which are polluted by septic tank and farm effluents.
Ephemeroptera (mayflies) are another group intolerant to organic pollution. Of the large number of species present in the British Isles, the Freshwater Biological Association Key to Ephemeroptera lists only two species, Baetis rhodani and Centrophilum luteolum, as being found in Caithness. This is probably due to lack of published information rather than pollution, as in the last four years the Highland River Purification Board has recorded thirteen species from Caithness rivers. Baetis rhodani appears to be by far the most common species, but it prefers to keep out of the main current and is usually found close to the edge of the river. Ecdyonuridae, a family of mayfly nymphs which are considerably flattened in shape, and are able to live in fast-flowing stretches of river, are well represented in Caithness. Of the species of Ecdyonuridae recorded in the British Isles, five of these are present in Caithness.
Tricoptera (caddis flies) are another group generally intolerant of organic pollution. They are usually divided into two groups, the caseless net spinners such as Hydropsyche, and Pyacophila, and those that live in cases. Pyacophila is a bright green caddis larva and, along with Hydropsyche, is very common in Caithness rivers. Cased caddis construct a tube of sand grains, small stones or vegetable matter around themselves and carry their home with them wherever they go. Although present in most rivers, they are particularly abundant downstream of Loch Watten where a number of different species of the family Limnephilidae are found.
Crustacea which are represented in Caithness by Gammarus pulex (the freshwater shrimp) and Assellus aquaticus (the water hog louse) react quite differently to organic pollution. Most literature indicates that Gammarusis intolerant of pollution and low oxygen conditions, whereas Assellus prefers the organic enrichment of pollution. In Caithness, Gammarus is not common in the Thurso, Forss and Berriedale Rivers. Downstream of the cemetery footbridge in Thurso, where the river is affected by the tide there is a rapid increase in the number of Gammarus. This may indicate that Caithness rivers, with their very low mineral content, are unsuitable for crustaea. In the Wick River, where the mineral content of the water is greater, Gammarus is more common. Assellus is not usually present in the Thurso, Forss and Berriedale Rivers, but in the lower reaches of the Wick River is present in small numbers. In many of the smaller burns, however, into which septic tank and farm effluents are discharged, large numbers of Assellus are present.