Community structure and functioning below the streambed across contrasting geologies.

There is little understanding of the variability in the structure and function of metazoan hyporheic communities across streams draining geologies that weather to produce different pore sizes and, by extension, different hydrological conditions. In this study we selected two catchments in each of three geologies that had differing values of hydraulic conductivity and porosity, and sampled four riffles in each catchment at high and low water levels and at two depths. We found clear differences in the physical template of streams draining different geologies and in the composition and abundance of communities inhabiting the hyporheic zones of streams draining chalk/sandstone and limestone geologies. However, we did not detect any significant differences in body size, biomass or functional measures (diversity, richness, redundancy) between the geologies. Our findings imply that ecosystem functioning in streams draining geologies that produce fine grain sediments may be similar to those draining geologies that produce coarse grained sediments irrespective of differences in the physical template and community structure.