Comment on “Assessing invertebrate assemblages in the subsurface zone of stream sediments (0–15 cm deep) using a hyporheic sampler” by Dole‐Olivier et al.

Abstract

Dole-Olivier et al. [2014] performed field tests to evaluate the quantitative efficiency of the Bou-Rouch (hereafter B-R) pump for sampling invertebrates in the benthic zone (0-15 cm deep) of streams. The B-R pump is more commonly used to sample invertebrates from the hyporheic zone (>15 cm deep). It has also been considered as a semiquantitative measurement, as explained by Dole-Olivier et al. [2014]. The testing was based on a comparison of invertebrates sampled using the B-R pump and using a Hess sampler, the latter being more commonly applied for benthic sampling and considered a quantitative technique. For this purpose, they used a B-R pump to obtain samples within a volume of sediments bounded by a Hess cylinder, which had specifically designed dimensions for this test. The B-R pump was found to significantly underestimate the total abundance and actual richness of benthic invertebrate species. However, we assert that these two methods are not sampling the same sediment volume (shape and magnitude), as we will explain and illustrate below, meaning that the B-R pump may be unfairly judged in this work.We appreciate the authors interest in identifying a single invertebrate sampling technique for application to both benthic and hyporheic zones. We certainly agree with their statement:Knowledge of the efficiency of pumping techniques for evaluating the absolute number of individuals in a given volume, the actual number of species and the ecological characteristics of these species is valuable data for a more quantitative use of pumping methods, especially in the 0-15 cm subsurface zone.However, we have identified two main issues, both related to assumptions of pumping-induced flow in this system made by Dole-Olivier et al. [2014] that are problematic in their assessment and quantification of the efficiency of sampling with the B-R pump in the benthic zone:Issue 1: the volume of sediment flushed while pumping with the B-R pump was smaller and had different dimensions than that within the confines of the Hess sampler.Issue 2: spatial variation in groundwater velocity during pumping will lead to spatial variation in invertebrate extraction ability, reducing the effective sampled volume.These issues are apparent from general hydraulic theory; however, to help illustrate them, we developed a numerical model in MODFLOW [Harbaugh, 2005] to simulate the three-dimensional flow system in the shallow sediments inside as well as in the proximity of the Hess sampler during pumping with the B-R sampler, following the description of the sampling procedure presented by Dole-Olivier et al. [2014]. Where uncertainty in the actual site conditions exists, we chose parameters or conditions that would produce the most conservative results (i.e., those that would favor the purpose of Dole-Olivier et al.). The dimensions and boundary conditions of the modeled system are illustrated in Figure 1. All boundaries, including the top boundary, which represents the sediment-stream interface, are treated as constant head at 1.05 m (i.e., 5...

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”

“…They show that the pump takes in a large amount of surface water from the stream flow (from the open top of the cylinder) and in a lesser way from outside, through the bottom of the cylinder, which is open during pumping. Although not familiar with MODFLOW, we agree from experience with Roy and Danielescu [] on the choice of the values of the parameters used in the model (K s and pumping rate). However, several points may be considered: 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 comment by Roy and Danielescu [] 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 , ; Bou , ], that so formal and precise hydrologic representations are given.…”

“…The first comment by Roy and Danielescu [] demonstrates, through a modeling approach, that the pumped volume (5 L) corresponds only partially to the volume of voids theoretically isolated by the Hess cylinder. They show that the pump takes in a large amount of surface water from the stream flow (from the open top of the cylinder) and in a lesser way from outside, through the bottom of the cylinder, which is open during pumping.…”

“…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 + BC = 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 [] suggests. As pointed out by many authors, the Bou‐Rouch pump takes in water along with sands and organic matter (diameter of screen holes = 5 mm). This removal likely alters water permeability and circulation patterns during pumping and also modifies the area of influence of the pump.…”

“…The second comment by Roy and Danielescu [] relates to the spatial variation of pore water velocity during pumping. The authors provide a vectored representation of such spatial variation and indicate that it might lead to a spatial variation in the extraction capacity of invertebrates by the pump, which is sensible.…”