Sediment texture mediates high-flow effects on lotic macroinvertebrates

Abstract

We conducted 2 experiments to examine how sorting, shape, and size of sediment grains affected drift and mortality of the mayfly Deleatidium spp., the snail Potamopyrgus antipodarum, and the stone-cased caddisfly Pycnocentrodes aeris in ramped flows in a large, outdoor, recirculating flume. These macroinvertebrates are common to many New Zealand streams and have different flow habitat preferences, body morphologies, and motilities. In the 1st experiment, we quantified taxon drift and mortality and sediment erosion in well-sorted gravel (i.e., high grain-size homogeneity), moderately sorted cobble, and a poorly sorted mixture of gravel and cobble. In the 2nd experiment, we examined the same response variables in angular (rough), oblate (flat, elliptical), and spherical gravel or cobble. Grain sorting significantly affected drift, and this effect was taxon-specific. Drift was highest from moderately sorted cobble, largely because of the relatively high number of dislodged snails. Overall, caddisflies had the highest propensity to drift, regardless of sorting treatment, whereas mayflies had the lowest. Caddisflies generally remained on tops of rocks in ramped flows, and their dislodgment was linked to velocity increases rather than sediment erosion. In contrast, dorsoventrally flattened Deleatidium clung to rock undersides. The result of clinging to sediment undersides to reduce dislodgment was significantly higher mortality compared to the other 2 taxa, particularly in well-sorted gravel with relatively low transport thresholds. The addition of cobble to gravel (i.e., poorly sorted sediments) decreased sediment erosion and significantly reduced mayfly mortality. Grain shape also significantly affected sediment transport thresholds and hence taxon responses. Erosion of angular gravel equaled that of spherical cobble, and no angular cobble eroded in ramped flows. Drift by dislodgment was relatively low in angular gravel and cobble, even for caddisflies. Mortality was again greater for mayflies than for snails and caddisflies, particularly in spherical and oblate gravel that tended to erode. Our results suggest that the interplay between sediment textural properties that affect transport thresholds, and species behaviors and morphologies, influence the overall effectiveness of sedimentary flow-refugia.