Abstract
The distribution of lotic fauna is widely acknowledged to be patchy reflecting the interaction between biotic and abiotic factors. In an in situ field study, the distribution of benthic and hyporheic invertebrates in the heads (downwelling) and tails (upwelling) of riffles were examined during stable baseflow conditions. Riffle heads were found to contain a greater proportion of interstitial fine sediment than riffle tails. Significant differences in the composition of benthic communities were associated with the amount of fine sediment. Riffle tail habitats supported a greater abundance and diversity of invertebrates sensitive to fine sediment such as EPT taxa. Shredder feeding taxa were more abundant in riffle heads suggesting greater availability of organic matter. In contrast, no significant differences in the hyporheic community were recorded between riffle heads and tails. We hypothesise that clogging of hyporheic interstices with fine sediments may have resulted in the homogenisation of the invertebrate community by limiting faunal movement into the hyporheic zone at both the riffle heads and tails. The results suggest that vertical hydrological exchange significantly influences the distribution of fine sediment and macroinvertebrate communities at the riffle scale.
Highlights
The distribution of lotic fauna is widely acknowledged to be patchy reflecting the interaction between biotic and abiotic factors
We addressed the following questions: (i) Do benthic and hyporheic invertebrate assemblages differ between riffle heads and tails under stable flow conditions?; (ii) Do patterns of fine sediment accumulation and vertical hydrological exchange differ between the heads and tails of riffles?; (ii) Can the patterns of fine sediment and vertical hydrological exchange help explain the microdistribution of invertebrate populations?
When differences in fine sediment content between the riffle head and tail were considered, greater quantities were recorded in riffle heads for all grain sizes examined (Fig. 2), this was only significant for the 2–1 mm fraction (F1,41 = 5.316, P = 0.026)
Summary
The distribution of lotic fauna is widely acknowledged to be patchy reflecting the interaction between biotic and abiotic factors. In an in situ field study, the distribution of benthic and hyporheic invertebrates in the heads (downwelling) and tails (upwelling) of riffles were examined during stable baseflow conditions. The results suggest that vertical hydrological exchange significantly influences the distribution of fine sediment and macroinvertebrate communities at the riffle scale. At the scale of riffle–pool sequences (bed-form driven), decreasing water depth at the end of a pool (riffle head) forces some water downwards into the sediments (downwelling) This water travels through the sediments in a downstream direction, where typically at the tail of a riffle, increasing water depth produces a zone of low pressure forcing upwelling water through the bed sediments and into the channel (Savant et al, 1987; Boulton et al, 1998; Franken et al, 2001). Flow paths are often more complex than conceptual models suggest, responding to a range of factors such as flooding (Wondzell & Swanson, 1996; Dudley-Southern & Binley, 2015), bed topography (Boano et al, 2013; Wildhaber et al, 2014), sediment composition and porosity (Packman & Bencala, 2003; GomezVelez et al, 2014) and other instream morphological units such as coarse wood accumulations (Sawyer & Cardenas, 2012; Krause et al, 2014)
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