HYDRAULIC HABITAT AND THE ASSEMBLAGE STRUCTURE OF STREAM BENTHIC MICROCRUSTACEA

The relationship between microcrustacean assemblage structure and flow habitat was investigated in 9 streams with differing hydraulic charac teristics. A series of partial canonical correspondence analyses was u sed to relate species abundances of microcrustacea to geographical che mical hydraulic, and seasonal variables. Once the influence of geograp hy (latitude) on species distributions was removed, the environmental variables accounted for almost 38% of the variation in the combined (a ll seasons) data set. Circumneutral streams had greater species richne ss and higher abundance of cosmopolitan species than more acidic strea ms. The microcrustacean assemblages were distinguished further on the basis of stream hydraulic character Previous research had categorised the 9 streams into Type I, II, or III depending on the fraction of str eam bed retaining low shear stress at high discharge (declining from T ypes I to III). Taxa that are largely epibenthic in habit tended to be most abundant in Type I streams. These taxa may use as habitat the ex tensive low shear stress areas present in these streams, enabling thei r survival even at high discharge Conversely, taxa that are largely in terstitial in habit tended to be most abundant in Type II and III stre ams. The distribution of interstitial taxa may reflect conditions with in the substratum rather than at the surface The differences in microc rustacean assemblage structure were largely species-specific; the Harp acticoida were the only group to show a consistent pattern in relation to stream hydraulics.