Relations between invertebrate drift and flow velocity in sand-bed and riffle habitats and the limits imposed by substrate stability and benthic density

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Downstream drift of benthic invertebrates is one of the defining characteristics of lotic environments. We collected 90 drift samples from adjacent sand-bed and riffle sections in a Mediterranean river during low flow conditions. Our goal was to compare drift fluxes (densities and instantaneous drift) and model the drift response across the velocity gradient in each habitat. Velocity distributions did not differ significantly between the habitats, but bedload transport rates and suspended sediment concentrations were significantly higher in the sand-bed habitat, where substrate was finer and less stable than in the riffle habitat. Benthic invertebrate densities differed by an order of magnitude between the habitats, with low and relatively homogeneous densities in the sand-bed. However, mean drift density did not differ significantly between the habitats. Quantile regression indicated that the upper limits of the drift response across the velocity gradient differed between habitats. Sample values below these upper limits suggest that substrate stability and benthic density act to constrain drift in some locations. Instantaneous drift was much higher in the sand-bed than in the riffle habitat, with up to 42% of the benthos present in the drift at any given moment. The taxonomic composition of benthic and drift samples suggested that animals from the riffle contributed to drift in the sand-bed habitat. The high loss rates in the sand-bed habitat suggest a rapid turnover of animals, supported by invertebrates arriving from the upstream riffle.