The contribution of subsurface invertebrates to benthic density and biomass in a gravel stream

The interstitial sediments of streams and rivers, the 'hyporheic zone', have a diverse and numerous fauna though the relative contribution of the subsurface and surface assemblages to total productivity remains poorly understood. Our main aim was to determine the contribution of hyporheic invertebrates to overall, reach-scale estimates of density and biomass in an acidic, first order stream in south-east England. We estimated metazoan density across a wide range of body sizes (meiofauna [>42mum to <500mum] and macrofauna [>500mum]) over time. A comparison with the assemblage from the streambed surface indicated that surface samples (0 - 5 cm) alone potentially underestimate annual, reach-scale estimates of density and biomass by factors of 2.2 and 2.8, respectively. An invertebrate assemblage of relatively few, large individuals near the surface was replaced by a suite of numerous but small-bodied organisms in the deeper sediments. Although abundances peaked at different depths in the sediments, most taxa were found throughout the profile. The assemblage of numerous, small-bodied invertebrates in the interstices at this stream may contribute substantially towards overall benthic production.