PLECOPTERA AND EPHEMEROPTERA SPECIES ASSEMBLAGES IN THE USK RIVER SYSTEM, WALES

The distribution and abundance of Plecoptera and Ephemeroptera species in the Usk river system, South Wales, were studied in relation to the physico-chemical environment. Faunal samples were collected from riff les during summer 1989 at 47 sites distributed throughout the river sy stem. Twenty-four species were collected; 10 Plecoptera and 14 Ephemer optera. The sites were classified by two-way indicator species analysi s (TWINSPAN) on the basis of faunal similarity, and the 'cohesiveness' of the resulting site groups was examined using detrended corresponde nce analysis. Relationships between eight site groups identified using TWINSPAN and various environmental variables were explored using prod uct-moment correlation analysis and multiple discriminant analysis (MD A). Membership of the TWINSPAN site groups could be determined with li ttle error using sub-sets of environmental variables; using MDA 72 % o f the sites were assigned correctly to their TWINSPAN Level 3 groups o n the basis of six variables. This implies that on a catchment/sub-cat chment scale the structuring of the Plecoptera/Ephemeroptera fauna in the Usk system is driven largely by the abiotic environment. TWINSPAN was also used to classify the fauna into species associations. Six ass ociations were defined which were characteristic of different regions of the main river and its tributaries.