Generalisations drawn from various studies on the assemblage composition of the hyporheic zone sampled using a Bou-Rouch pump are hampered by uncertainty about the comparability of different sample volumes and the uneven taxonomic resolution across studies. To asses the effect of sample volume on perceived assemblage composition, we compared results from 10 consecutive one-L samples collected using a Bou-Rouch pump from each of nine wells inserted in a 9m² area in the down-welling hyporheic zone of the Rhône River, France, and in a weakly-upwelling zone of a slowly-flowing anabranch. At the upwelling site, assemblage composition, numbers of taxa, and density of individuals per L in the first few one-L samples were significantly different from those of subsequent samples. Although numbers of taxa and individuals levelled off after the third to fifth L, perceived assemblage composition continued to change with increasing sample volume. In contrast, consecutive samples from most of the wells at the downwelling site collected progressively fewer individuals. However, assemblage composition was also strongly associate with sample volume, largely due to differences in relative abundance. Despite these effects of sample volume at both sites, differences in perceived assemblage composition among wells were consistent at volumes of 3-5 L in most cases. There was evidence for a 'filtering effect' with small-bodied taxa such as nematodes, ostracods and cladocerans being preferentially collected in the early samples. However, larger taxa (e.g., isopods) tended to occur equally often in earlier and later samples. Taxonomic resolution of specimens to order instead of species level did not significantly change the general ordination patterns of assemblage composition among wells at both sites. Overall, our results imply that a sample volume of 5L and broad-level taxonomic resolution would suffice to reveal spatial variation in assemblage composition between these two sites and, at a finer scale, among wells.

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) and 'mesosaline' (>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.

Silt and fine sediments from anthropogenic activities frequently clog river bed sediments, impairing vertical exchanged between stream and subsurface water. River managers need a simple technique to detect the extent of interstitial clogging and monitor the effectiveness of measures to reduce siltation. We evaluated the use of 30-cm long pine-wood stakes, inserted for 3-6 weeks in the sediments of four French rivers varying in interstitial clogging, to determine the association between changes in the colour of the wood and the adjacent interstitial conditions. There was a general association between depth to interstitial hypoxia and location of the colour change of the wooden stakes from brown to pale grey or black after 3-4 weeks. This change in colour also broadly matched interstitial contents of fine sediment, ammonium, and nitrate although the method could not reliably detect microscale zones of anoxia or short-term changes in dissolved oxygen. thus, its effectiveness lies in its use as a cheap, simple, and broad-scale indicator for collecting long-term integrated data of interstitial oxygenation in stream sediments with minimal disruption of the gravel bed, and appears an ideal tool for river managers and salmonoid fish biologists.

To determine the optimal combination of sample volume and replicationrequired to assess hyporheic taxa richness and abundance, 10 consecutive one-L Bou-Rouch pump samples were collected from each of 9 wells inserted in a 3x3m grid at two sites on the Rhône River, France. Variable and non-linear relationships between sample volume and numbers of taxa and individuals demonstrate that hyporheic densities cannot simply be expressed per L for comparison among studies that have collected different sample volumes. Our results imply that an optimal sampling design for rapid assessment of hyporheic biodiversity and abundance would need at least five samples of 3–5 L to provide reasonable levels of precision (20%). At both sites, 6–7 replicates of 5L sufficed to be able to detect a 50% change in mean taxa richness with a power of 0.90 and α = 0.10. Although this sampling exercise should be repeated whenever accurate population estimates are needed, it appears that the spatial variance of the hyporheos at a fine scale (< 9m²) is great enough to obscure detection of potentially large differences when replication or sample volume is small.