Abstract
The hyporheic zone of river ecosystems provides a habitat for a diverse macroinvertebrate community that makes a vital contribution to ecosystem functioning and biodiversity. However, effective methods for sampling this community have proved difficult to establish, due to the inaccessibility of subsurface sediments. The aim of this study was to compare the two most common semi-quantitative macroinvertebrate pump-sampling techniques: Bou-Rouch and vacuum-pump sampling. We used both techniques to collect replicate samples in three contrasting temperate-zone streams, in each of two biogeographical regions (Atlantic region, central England, UK; Continental region, southeast France). Results were typically consistent across streams in both regions: Bou-Rouch samples provided significantly higher estimates of taxa richness, macroinvertebrate abundance, and the abundance of all UK and eight of 10 French common taxa. Seven and nine taxa which were rare in Bou-Rouch samples were absent from vacuum-pump samples in the UK and France, respectively; no taxon was repeatedly sampled exclusively by the vacuum pump. Rarefaction curves (rescaled to the number of incidences) and non-parametric richness estimators indicated no significant difference in richness between techniques, highlighting the capture of more individuals as crucial to Bou-Rouch sampling performance. Compared to assemblages in replicate vacuum-pump samples, multivariate analyses indicated greater distinction among Bou-Rouch assemblages from different streams, as well as significantly greater consistency in assemblage composition among replicate Bou-Rouch samples collected in one stream. We recommend Bou-Rouch sampling for most study types, including rapid biomonitoring surveys and studies requiring acquisition of comprehensive taxon lists that include rare taxa. Despite collecting fewer macroinvertebrates, vacuum-pump sampling remains an important option for inexpensive and rapid sample collection.
Highlights
The hyporheic zone comprises the subsurface sediments beneath a streambed that exchange water with the surface stream and the underlying aquifer [1]
During the Würm / Devensian glacial maxima, greater coverage of the UK by ice resulted in long-term reductions in biodiversity, whereas tundra vegetation dominated in France and more taxa were able to persist; these regions have overlapping but distinct faunas [39,40,41]
VP sampling remains an important option for inexpensive and rapid sample collection, and we recommend that results of VP studies be interpreted in light of limitations highlighted here
Summary
The hyporheic zone comprises the subsurface sediments beneath a streambed that exchange water with the surface stream and the underlying aquifer [1]. Alongside a permanent hyporheic community, many predominantly benthic macroinvertebrates migrate vertically to exploit the subsurface sediments as a nursery that protects juveniles from predation [6, 7] and as a refuge that promotes persistence during disturbance events in surface streams [8,9,10]. In addition to research-focussed interest in hyporheic invertebrates and the ecosystem functions they perform [11, 12], this fauna has been proposed as a biomonitor of ecosystem health, for example in temporary streams [13, 14] and in response to metal pollution [15] and eutrophication [16]. Identifying suitable biomonitors of groundwater-dependent ecosystems including the hyporheic zone may become a priority for regulatory agencies, in particular due to legal drivers including the EU Water Framework Directive [20,21,22]
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