Effects of catchment disturbance on stream invertebrates: comparison of different habitats (vegetation, benthic and interstitial) using bio-ecological groups.

The intensification of agriculture has drastically modified the structure of rural landscapes, changing field size, destructing hedgerow networks, and increasing nutrients and fine sediments fluxes to rivers. To evaluate the effects of these disturbances at the catchment scale, we studied the surface and interstitial water chemistry and the invertebrate assemblages in three different habitats: the vegetation, the benthic, and the interstitial. Six headwater streams ranging from undisturbed to disturbed forests (wind-fallen area) and from traditional agriculture to intensive farming areas were studied. Differences between forested and agricultural streams lie in changes in water chemistry, in habitat quality, and in the composition of the assemblages with little reduction in species richness. The forested streams were only disturbed by large-scale modifications of their catchments (i.e. wind-fallen area along the stream), which made the invertebrate assemblages similar to those of agricultural streams, even if water characteristics were not modified. In contrast, large scale modifications of landscape structure near the agricultural streams (hedgerow removal) had little effect on the fauna, but the destruction of the riparian strip (with direct access of the cattle to the river) profoundly affected the system with drastic modifications of in-stream habitat quality and invertebrate assemblage composition. The effects of these disturbances differed according to the habitat considered, with increasing differences between assemblages from the interstitial, to the benthic, and to the vegetation habitats. Finally, groups of organisms based on bioecological traits appeared as efficient tools for the evaluation of catchment disturbances, at least for the benthic and the vegetation fauna

Hyporheic inverterbrate community composition in streams of varying salinity in southwestern Australia: Diversity peaks at intermediate thresholds

Many streams of southwestern Australia have become secondarily saline through land clearance and other human activities in their catchments. Elevated salinities impact on aquatic biota and ecological processes of surface streams but little is known of the effects on the diversity and community composition of hyporheic (subsurface) invertebrates occupying the saturated sediments where surface and groundwaters exchange. We hypothesized that biodiversity of hyporheic invertebrates would decline with increasing salinity, especially where saline groundwater upwelled into the surface stream. We also predicted changes in community composition associated with salinity and direction of vertical hydrological exchange. Water and hyporheic invertebrates were sampled from downwelling and upwelling zones of 13 streams in southwestern Australia ranging in median surface water salinity from 0.27 to 17.86 g L⁻¹. Overall, taxa richness of hyporheic invertebrates was uncorrelated with salinity but, surprisingly, correlated positively with the salinity of upwelling water. However, when the sites were divided into 'fresh' (3 g L⁻¹) groups, this relationship became non-significant. Instead, taxa richness and total abundance were correlated positively with salinity of downwelling water in fresh sites and negatively in mesosaline sites, resulting in a peak in richness at intermediate salinities. Community composition was unrelated to direction of hydrological exchange but was strongly associated with hyporheic salinity. Hyporheic assemblages of 'fresh' rivers were typified by harpacticoid copepods and candoniid ostracods, whereas the amphipod 'Austrochiltonia' and several dipteran groups were more common below 'mesosaline' rivers. Although many hyporheic taxa collected in this study apparently have broad tolerances to salinity, secondary salinization due to human activities potentially changed community composition, possibly altering rates of ecological processes such as organic matter breakdown occurring within the sediments of streams undergoing salinization