Quantitative aspects of the distribution of invertebrates in the benthos of a small stream system in western Ireland

The invertebrate community of a small stream system in western Ireland is described from the results of two sampling programmes. The investigation was conducted at four sites which occurred within the same range of altitude (between 60 and 130 m O.D.) and the greatest differences between them were rock size, current intensity and the amount and type of detritus in the substratum. Approximately 60,000 invertebrates were identified and almost 100 taxa were listed. Attempts which were made to relate the occurrence of certain taxa to three factors (rock size, current intensity and detrital content) showed that detrital content was the most significant. Using empirical clustering techniques efforts were made to demonstrate zonation of the invertebrates taken during one of the two sampling programmes and the groups which were so obtained were further amalgamated subjectively. The resemblance of the resulting pattern to lotic zonation is briefly discussed.