A quantitative study of the invertebrate fauna of the River Tees below Cow Green Reservoir

Summary Monthly samples were taken in the River Tees below Cow Green dam from four sites differing in flow conditions, by means of trays containing stones from the river, which were immersed for a period of 1 month. Information on the distribution, abundance and biomass of the benthos of the River Tees is given with additional data on the life‐histories of some of the more common species, during the period May 1971 to May 1972. Of the seventy‐two taxa found, fourteen made up 95% of the total numbers. Hydra, Polycentropodidae, and the molluscs Limnaea peregra and Ancylus fluviatilis were the most abundant animals in the slower‐flowing water (10–26 cm s −1 ) and Simuliidae, Orthocladiinae, and Baetis rhodani dominate the riffle (50–75 cm s −1 ) fauna numerically. Some organisms, Nais spp., Caenis rivulorum and Baetis scambus favoured intermediate flows of 20–60 cm s −1 . The slowest‐flowing water contained the largest number of taxa and the site with the fastest flow supported the least. A mean monthly weight per square metre of 14·56 g (95% limits from 7 to 31) was calculated using data from all four sites. The frequency distribution of nymphal size‐classes is given for B. scambus, C. rivulorum, Ephemerella ignita, and Brachycentrus subnubilus, and the association of certain nymphal size‐classes with a particular site is analysed. The composition and distribution of the fauna is discussed. It is suggested that the relative richness of the fauna in terms of biomass is attributable to the organic enrichment of the river following the building of the Cow Green dam. The regulation of the flow has allowed the dense growth of algae and mosses, and the development of large molluscan populations, and the reservoir itself provides a rich source of food, particularly zooplankton and phytoplankton, for the river benthos.