Invertebrate communities provide sensitive indicators of physical stress, such as that imposed by moisture regime. With reference to two floodplain sectors - one non-regulated and one regulated - on the River Trent, UK, this paper uses spider communities to characterize the dominant floodplain units (riparian, wetland, wet woodland, pasture and arable) and compares the information derived with a parallel data set from the groundbeetle community (F. Carabidae). The study yielded 77 spider species, of which 43 were common to the two floodplain sectors. Hypomma bituberculatum (wetland), Bathyphantes niginus (wet woodland) and Pachygnatha degeeri, Pardosa pullata and Trochosa ruricola (arable) showed restricted distributions and are regarded as habitat specialists. Species richness was highest in the wetland patches and lowest in the pasture. Differences between the faunal communities of the different habitats were greatest in the regulated sector. Spiders and beetles collected from the same traps are shown to provide different and complementary information on floodplain functioning, which may prove valuable in any ecological assessment.