Assessing invertebrate assemblages in the subsurface zone of stream sediments (0-15 cm deep) using a hyporheic sampler

Abstract

[1] Quantitative comparisons between benthic and hyporheic invertebrate communities are crucial for understanding the biological functions of the hyporheic zone, such as storage, migrations, and exchanges of invertebrates with the surface stream. Such comparisons are still hampered by the use of different techniques adapted to each habitat (benthic versus hyporheic). This work combines two different techniques for sampling the upper layers of bed sediments (0-15 cm): the semiquantitative ''Bou-Rouch'' pump classically used to sample the hyporheic zone (>15 cm), and the quantitative Hess sampler commonly used to sample the benthic zone (15 cm), in order to evaluate the quantitative efficiency of the pump in this 0-15 cm zone. First, a Bou-Rouch sample (BR) was taken within the cylinder of a Hess inserted within the streambed, then a second sample (benthic complement, BC) was collected within the Hess after removing the pump, in order to catch all invertebrates not extracted with the pump. The BR samples collected on average 14.5% of the total abundance and about 50% of the actual richness. The large range of variation indicates that the combination of the two techniques is not valid for a quantitative evaluation of benthic communities. Contrary to expectations, the pump did not collect more interstitial and groundwater invertebrates and no differences in faunal composition between upstream and downstream riffle positions were observed. Our results do not question the use of the BR technique under standard conditions i.e., when sampling the hyporheic zone, but underline how it is crucial to know its quantitative limits.

Is the unsaturated sediment a neglected habitat for riparian arthropods? Evidence from a large gravel-bed river

“…These zone can provide habitat for (aquatic) invertebrates down to at least 30 cm depth below the stream bed (Boulton et al, 1991;Marchant, 1988Marchant, , 1995 and far out into the floodplain alluvium (Marmonier et al, 1992;Stanford and Ward, 1988), therewith connecting surface and groundwater vertically and horizontally. They substantially contribute to surface biodiversity and resilience through their multiple functions as a nursery, storage, and refuge zone, respectively (Dole-Olivier, 2011;Dole-Olivier et al, 2014).…”

Do benthic invertebrates use hyporheic refuges during streambed drying? A manipulative field experiment in nested hyporheic flowpaths

“…fusca, Baetis sp., Caenis sp., Orthocladiinae, Tanypodinae Figure 5a), which were also the most abundant in the benthic assemblage of this river site (at predisturbance conditions; Dole-Olivier et al, 2014, Figure 4). Taxa belonging to the SX group did not migrate into the sediments, despite their reported dominance in the surface stream (e.g., Heptageniidae, Dole-Olivier et al, 2014). …”

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.

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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).

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“…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.…”

“…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).…”