Fine sediment deposition and interstitial flow effects on macroinvertebrate community composition within riffle heads and tails

Abstract

The distribution of macroinvertebrates in the heads and tails of riffles were examined in an in situ field experiment under stable baseflow conditions. Paired colonisation cylinders were used to examine the influence of vertical hydraulic exchange (upwelling and downwelling) and horizontal interstitial flow on the patterns of sedimentation and invertebrate colonisation. Sedimentation rates were greatest in cylinders permitting vertical and horizontal flow (VHE cylinders), and were significantly lower (29%) in cylinders where only vertical flow and ingress of fine sediment were possible (VE cylinders). The results demonstrate that horizontal interstitial flows represent an important pathway for fine sediment transport. Differences in fine sediment accumulation were also observed between riffle heads and tails. Significantly higher sedimentation rates were recorded in riffle tails, with the macroinvertebrate communities characterised by larger proportions of fine sediment tolerant taxa. In contrast, riffle head communities were characterised by greater proportions of sediment sensitive taxa, and in the case of VHE cylinders, shredders and EPT taxa. The results demonstrate that spatial differences in fine sediment deposition are evident at the riffle scale as a function of vertical and horizontal subsurface flows and that these factors play a key role in the distribution of macroinvertebrate fauna.

Benthic and hyporheic macroinvertebrate distribution within the heads and tails of riffles during baseflow conditions

“…The quantities of fine sediment in benthic sediments within this study stream are unlikely to have been great enough to limit or prevent movement of organisms within the interstices of the benthic zone (Mathers & Wood, 2016), although it was clearly strong enough to have influenced the observed riffle-scale faunal distribution patterns. Lotic invertebrate communities typically constitute highly dynamic rather than sessile assemblages (Downes et al, 1993), and as a consequence, in response to abiotic and biotic cues, the varying dispersal abilities of taxa can lead to smallscale spatial variations in macroinvertebrate communities (Wilson & McTammany, 2016).…”

“…Sampling of five riffles within a 1,200 m reach was undertaken during spring (8th April-4th May 2013). Hydrological data from a local gauging station on the River Soar (Kegworth, 52°82 0 N, -1°27 0 E) indicated stable but gradually declining baseflow discharge values over the study period (average 6.5 m 3 /s, range 5.07-8.2 m 3 /s; see Mathers & Wood, 2016).…”

“…For each sampling point, open ended PVC pipes (19 mm internal diameter) were driven into the substrate using a stainless steel T-bar to a depth of 200 mm (Boulton & Stanley, 1995;Wood et al, 2010). This depth was selected based on a number of previous studies which have investigated fine sedimentation and macroinvertebrate relationships (Pacioglu et al, 2012;Descloux et al, 2013;Mathers and Wood, 2016). These pipes functioned as permanent sampling wells and were positioned C0.5 m apart to minimise any influence from sampling in adjacent wells (Stubbington et al, 2011).…”

“…The second pipe (stage well) had solid walls and was held in the water column facilitating the measurement of the river stage level. The direction of vertical hydraulic exchange was obtained through comparison of the two water levels (see Mathers & Wood, 2016). The relative volume of interstitial sedimentation was examined by retaining fine sediment extracted in the bilge pump samples (Mauclaire et al, 1998).…”

“…Dole-Olivier, 1998;Davy-Bowker et al, 2006;Mathers & Wood, 2016), the consequences of flow dynamics at small spatial scales and the associated sedimentary characteristics upon benthic and hyporheic invertebrate assemblages remain poorly quantified (Marmonier et al, 2010(Marmonier et al, , 2012Descloux et al, 2014;Jones et al, 2015). Despite several seminal papers over multiple decades which identified the potential importance of vertical hydraulic connectivity (Orghidan, 1959;Tilzer, 1968;Hynes, 1983), linkages between surface and groundwater and its influence on instream communities remain poorly studied (Boulton et al, 1999;Krause et al, 2011a).…”