Optimizing a sampling strategy for assessing hyporheic invertebrate biodiversity using the Bou-Rouch method: Within-site replication and sample volume

To determine the optimal combination of sample volume and replication required 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 3 x 3 m grid at two sites on the Rhone 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 5 L sufficed to be able to detect a 50% change in mean taxa richness with a power of 0.90 and alpha = 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 (<9 m(2)) is great enough to obscure detection of potentially large differences when replication or sample volume is small.