The Distribution of Benthic Invertebrates on Substrata in Fast-Flowing Streams

The highland and moorland reaches of rivers and streams can usually be divided into riffles, with shallow fast-flowing water, and pools, with deeper slow-flowing water. The invertebrate communities living in such distinct habitats are very different and the biological reasons for this have received considerable attention. Only a limited amount of work, however, has been carried out on the spatial distributions of populations of invertebrates and their relationships to environmental factors within these, or any other relatively more homogeneous stream habitat. Scott (1958) and Ambiuhl (1959) have related the distributions of several species to current speeds, and Egglishaw (1964) showed that, even in what was apparently a fairly uniform stretch of riffle, the quantities of several benthic species at different sites varied greatly and were correlated with the distribution of disintegrating plant detritus. Several studies of the effects of certain factors on the distribution of selected species in experimental conditions have, however, been carried out (see Macan 1963). The sampling difficulties imposed by the non-random distribution of most animals apply to stream riffles in the same way as they do to other habitats. The present paper sets out to explore the spatial distributions of benthic organisms (of length greater than 0 5 mm) and their relationships to certain environmental factors in riffles, as riffles usually make up by far the larger proportion of the areas of fast-flowing streams. Within riffles the greatest weight of invertebrates is found among and under stones, and so most of the observations were made on that habitat. Other habitats in riffles in which invertebrates are found, and on which observations were made, include the algal coverings on the upper surface of stones and clumps of moss.