Freshwater Biology (1983) 13,521-544 Community structure in some southern English streams: the influence of physicochemical factors COLIN R. TOWNSEND. ALAN G. HILDREW* and JEANETTE FRANCIS* School of Biological Sciences. University of East Anglia. Norwich NR4 7TJ, and *Schoolof Biological Sciences, Queen Mary College, London El 4NS SUMMARY. 1. Invertebrates and fish were surveyed during October 1976 in thirty-four stony riffle stream sites in Ashdown Forest, Sussex. 2. A variety of physicochemical factors were also measured in an tittempt to assess the importance of each in determining the distribution i)f species and the structure of communities. 3. Three analytical techniques—stepwise multiple regression analysis, ordination and community classification—revealed that the structure of these communities was strongly related to variation in stream pH. Aeid sites had low numbers of individuals, low species richness and low equitabilities. Summer temperature and stream discharge also appeared to play significant roles. The pattern of catchment land use was shown to have an important bearing on stream pH. 4. In the most acid sites only collectors, shredders and predators occurred. In more basic sites the number of speeies in collector and predator categories increased and these were joined by grazer/scrapers and filter feeders. 5. A theoretical basis for explaining stream eommunity structure is discussed. Introduction Much of the impetus for the study of running water commutiitics has derived from practical considerations. For example, a wide variety of indices of invertebrate diversity have been derived empirically and used in the assessment of water pollution (e.g. Hellawell, 1978; Mason, 1981). There seems to be little theoretical basis for such work and, with a few exceptions (Patrick. iy75). river studies have not been prominent in discussions of recent advances in community ecology (May, 1981) nor. Correspondence; Dr C. R. Townsend, School of Biological Sciences, University of East Angiia. Nor- wich NR4 7TJ. apparently, have theoretical advances heen recognized as relevant by the majority of river ecologists. It seems time, therefore, for a more fundamental appraisal of changes in eommunity structure resulting from variations in a numher of environmental factors, both natural and those influenced by man. Low pH and high iron concentration are factors of growing concern to aquatic ecolog- ists. Acidification of surface waters by polluted rainfall is widespread and acute (Drabl0s &. Tollan. 1980; Fowler et al., 1982), and acid waters always seem impoverished in compari- son with systems which are more basic but otherwise similar (Patrick, Roberts & Davis, 1968: Sutcliffe & Carrick, 1973; Ziemann, 1975; 0046-5070/83/1200-0521 $02.00 ©1983 Blaekwell Scientific Publications 521