Characterization of the density and body size of a Gammarus pulex (Crustacea: Amphipoda) population in subsurface sediments reflects the sampling technique used

The hyporheic zone and its invertebrate fauna play vital roles in the functioning of lotic ecosystems. However, although sampling invertebrates from subsurface sediments is recognized as challenging, few studies have quantified the effectiveness of common sampling techniques. We conducted laboratory experiments to compare two common, semi-quantitative pump-sampling techniques—Bou–Rouch and vacuum-pump sampling. We determined the proportion of a Gammarus pulex (Crustacea: Amphipoda) population sampled by each method in five sediment treatments comprising coarse (16–32 mm), medium (8–16 mm), and fine (4–8 mm) gravel and combinations thereof. We compared the body size of individuals sampled and not sampled by each technique to the population mean. Density estimates obtained using both methods were low: 33 ± 5 and 5 ± 1% of the population present for Bou–Rouch and vacuum-pump samples, respectively. Density estimates were significantly higher for Bou–Rouch than for vacuum-pump samples in four sediment treatments, but were comparable in coarse gravel. The body size of organisms captured by the Bou–Rouch technique was comparable to the population mean, whereas those in vacuum-pump samples were smaller. With hyporheic invertebrates suggested as future biomonitors of ecosystem health, we recommend Bou–Rouch sampling as the more effective pump-sampling method for community characterization.