Aquatic Oligochaeta of Southern England

Abstract

A brief account is given of the known distribution of aquatic oligochaetes in the waters of southern England. Four main water types are recognized on the basis of the geology, topography and usage of the areas concerned.

The small, acid streams of Devon, Cornwall and the New Forest are characteristically inhabited by Rhyacodrilus coccineus and Stylodrilus heringianus. In the highly calcareous streams of Wiltshire, Dorset and Hampshire a much more varied oligochaete fauna is present. Again R. coccineus and S. heringianus are important but Limnodrilus hoffmeisteri, Tubifex ignotus, Aulodrilus pluriseta, Psanunoryctides barbata and the enchytraeid Propappus volki are all abundant and show characteristic longitudinal zonations like those of many other organisms. In the large river Thames, Potamothrix moldaviensis is predominant with Psammoryetides barbata, Limnodrilus hoffmeisteri and the naid Stylaria lacustris, also abundant. The slow flowing Norfolk broads have a community characterized by Potamothrix hammoniensis. A note is given of the occurrence of a form of Limnodrilus hoffmeisteri.

For the purposes of this paper the area described as Southern England is approximately south of a line drawn from the Wash in the East to the Bristol Channel in the West. The geology of the area is diverse, but several natural sub-divisions of the watercourses can be made.

In the South-West (Devon and Cornwall) Palaezoic granites and sandstones predominate and the streams consequently have stony substrata with nutrient-poor, acidic, soft water. In the New Forest area of Hampshire there are streams of a similar nature, but in contrast, they receive drainage from Tertiary gravels and sands.

Stretching north-eastwards from Dorset is a wide band of soft Mezozoic strata, mainly chalk and limestones. These formations often contain large natural reservoirs or aquifers, which are the source of many of the streams. These streams are fast-flowing, with stony or gravel substrata. The gravel in some stretches can be entirely composed of flints. The water is rich in dissolved inorganic salts, particularly those of calcium. Diversity of stream-bed, and hence micro-habitat for invertebrates, comes from the annual cycle of algal and higher plant growth (mainly Ranunculus and Rorippa species) and associated deposits of silt and sand. A notable feature of this chalk stream system is the temporary streams, or winterbournes, which dry up or flow only intermittently and weakly in summer. This is a rigorous environment for aquatic oligochaetes and lumbricid species are encountered in the upper, drier reaches.

The River Thames is a sole large river. Much of its initial catchment is in the limestone region but the presence of extensive industrial and domestic effluents, the large volume of water, and the extensive tidal region make it notably different in character from most other rivers in the South.

Finally, north of the Thames, in the low-lying district of East Anglia are the Norfolk Broads. These are areas of shallow, slow-flowing, silter watercourses, often spreading into wide, shallow eutrophic lakes, with dense summer plant growth.