Reply to comment by James W. Roy and Serban Danielescu on “Assessing invertebrate assemblages in the subsurface zone of stream sediments (0–15 cm deep) using a hyporheic sampler”

Abstract

Among the techniques available for sampling invertebrates in the hyporheic zone, the methods based on pumping (Bilge or Bou-Rouch types) are still by far the most practical and most frequently used. Thus, it is important to understand as much as possible and to visualize more clearly what happens during water uptake in the alluvium, from both physical and biological points of view. However, despite the extensive method testing, many issues and methodological uncertainties still remain. Consequently, it is crucial to discuss these issues, but inevitably with an approach supported by hydrological sciences.The comment by Roy and Danielescu [2014] is very interesting because it provides a graphical and modeled representation of hydrological processes and water flow circulation patterns occurring within the sediments of a gravel bed stream during a Bou-Rouch pumping. To our knowledge, this is the first time since 1967, the date of publication of this method [Bou and Rouch, 1967;Bou, 1974], that so formal and precise hydrologic representations are given. As pointed out in our article, obtaining hyporheic quantitative samples remains a difficult task. This problem increases when sampling living organisms, among other things, because of an additional uncontrolled dimension related to organism behavior, including intra and interspecific variability. 1. It is true that a small amount of surface water gets trapped during the insertion of the cylinder into the sediments, but it was not possible to quantify this volume of water; at some stations where the stream water level was very low, it was very small and likely negligible. The authors also suggest that there is also some chance of pulling in invertebrates from the sediment surface. We reiterate that the pumping test described in our paper included the sediment vertical zone between 0 and 15 cm below the sediment-water interface, and is therefore also supposed to collect a fraction of the benthic invertebrate assemblage. This is also why we compare our pumping with a Hess sample containing a large fraction of strictly benthic organisms.2. The bottom end of the cylinder is actually open (the cylinder teeth reaching the depth of 20 cm). In fact, we were not able to set up a locking system to prevent possible contamination from deep interstitial water. Nevertheless, we attempted to control this bias by collecting a set of parallel samples of complete Hess, so as to compare the sum of each B-R sample and its benthic complement with a standard Hess sample of the same size (''full Hess''). As mentioned in the text (e.g., p7, § 4-1), we did not observe significant differences between the sum of the two partial samples and the standard Hess (i.e., BR 1 BC 5 full Hess), which means that if the pump had taken in water, it had hardly included organisms outside of the cylinder. Again, if there is a bias by attracting ''extrainvertebrates,'' it should not be as important as the representation that Roy and Danielescu [2014] suggests.3. As pointed out by many authors, the Bou-Rouch pum...

Characterization of Macroinvertebrate Communities in the Hyporheic Zone of River Ecosystems Reflects the Pump-Sampling Technique Used

“…Pump-sampling techniques share the limitation that the sediments sampled are unknown: collection points are fixed, but water may enter them from any direction [ 34 ]. The two techniques are therefore unlikely to have sampled the same sediment volume or shape, this source of variability being consistent across samples.…”

“…Pump-sampling techniques have several advantages over other methods: disturbance to the sediments is limited and repeated sampling from the same points is therefore possible; no recovery period is required, and both rapid surveys and long-term studies can therefore be conducted; both spatial and temporal variability in community composition can be characterized [ 31 , 32 ]; sample collection is rapid (seconds to minutes); and the price of equipment is modest, with minimal ongoing sampling costs. Equally, pump-sampling techniques share limitations, including a bias towards the collection of smaller, less tenacious invertebrates [ 27 , 33 ]; collection of only a proportion of the taxa and individuals present [ 12 ]; and the unknown size and position of the sampled sediments [ 34 ]. However, although BR suction forces are often informally described as greater than those of the VP, no study has compared assemblages sampled by these two techniques.…”