Influence of hydraulic conditions over dunes on the distribution of the benthic macroinvertebrates in a large sand bed river

Abstract

[1] This study aims to relate the flow structure over mobile dunes recorded on the channel bed of the Paraná River (Argentina) with the spatial distribution of the benthic macroinvertebrates. The following main conclusions have been obtained: (1) the dunes found in the active channel could be considered as hydraulic biotopes at a mesohabitat scale: the bed forms in the thalweg region are subjected to higher shear stresses with low benthic densities; (2) differences in benthic densities were also recorded at within-dunes microhabitat scales: the largest densities were found in the dune troughs where small bed shear stresses occur and minimum densities on the low stoss side of dunes where turbulent agitation near the bottom strongly disturb the bed particles; (3) superimposed dunes on larger dunes may be considered as another microhabitat of still smaller dimensions. Summarizing, multiscale approaches are needed if a comprehensive understanding linking hydrodynamics and morphodynamics processes with benthic ecology is intended.

Hydraulic factors controlling the benthic invertebrate distribution within and among dunes of the Middle Paraná River (Argentina) and sampling techniques

“…By applying the log Pearson analysis, a significant but poor correlation between sand and benthic density was obtained only on the trough of the dune 1 (r ¼ 0.81; p ¼ 0.01; Table 4). Although this result is inconclusive, it suggest that further studies of this kind would be advisable to decide if the sand amount of each sample should be consider as an additional parameter in invertebrate Amsler et al (2009) is the first dealing with the relationships between bed hydraulic variables measured on dunes in large rivers and the benthic fauna distribution. These authors described successfully this relationship on three areas (stoss-side, crest and trough) of dunes in the Middle Paraná River.…”

“…That is because they describe the gross of the flow characteristics at a given cross section or vertical and, consequently, they say little about the local flow conditions close to the bed at a given point (see Amsler et al, 2009). …”

“…Thus this number gives a measure of the transport intensity of the bottom sediment grains (Yalin, 1977). Taking into account that the median of the bed sediment size distribution (d 50 ) and s 0 are included into s * (Table 1), this variable should be an important parameter to know the incidence of the bed sediment transport on the benthic community as was shown by Amsler et al (2009), provided that its value be representative of the flow conditions surrounding the sampling point.…”

“…Regarding the specific incidence of alluvial dunes on the benthic distribution it is a subject insufficiently studied. One of the first contributions is due to Amsler et al (2009) in the Middle Paraná River. Those authors concluded that dunes could be viewed as hydraulic biotopes (Wadeson, 1994) considering the benthic community distribution.…”

“…thalweg and outside the thalweg) two dunes with the spatial distribution of the macroinvertebrates. This last purpose relies on the results of Amsler et al (2009) which report that dunes influence the invertebrates' distribution primarily by differences in the bed hydraulic conditions. The authors have also discussed the possibility of considering the dunes as true hydraulic biotopes, following the definition introduced by Wadeson (1994).…”