Investigating the impact of moving sand during summer spates on the spatial distribution of stream periphyton biomass in a gravel-cobble bed boreal river

The abrasive effects on periphyton biomass of saltating sand, that was transported over a stable gravel-cobble riverbed during summer flow events (spates), was studied on 35 riffles of an oligo-mesotrophic river in Quebec. A periphyton saltation abrasion model (PSAM) was developed from an existing bedrock erosion mechanistic model (Sklar and Dietrich 2004). The empirical PSAM models explained 56-59% of the variance in post-spate biomass on individual host-rocks following a low-magnitude, high-frequency spate.;An analysis of spatial patterns of periphyton biomass across riffles confirmed the tendency for a spate-related refuge habitat between the edge of the varial zone (zone of frequent wetting and drying) and the thalweg (deepest point) of the channel.;A hierarchical model (HPPM) that simulates how periphyton biomass is regulated by the flow of water and the flux of sand down a sedimentary link (segment of river along which grainsize and slope decreases systematically downstream) was developed and validated. Sedimentary links contained 3 channel types (hydrogeomorphic reaches, HGRs), each possessing significantly different morphological traits, ranging from straight-steep, cobble-bed reaches (High-HGR) at the upstream end, to low-gradient, sinuous gravel-bed reaches (Low-HGR) downstream. For low-magnitude spates (e.g. 0.85QMean_Annual), the most refuge (i.e. highest post spate biomass) was provided on riffles in Low-HGRs because sand transport rates were below the perturbation threshold. HPPM scenarios varying sand supply showed that link scale periphyton refuge area is very sensitive to normal variability in sand supply (e.g. a increase in sand on bed from 5% to 15% produced an 8 fold decrease in post-spate biomass). Further, biomass was universally low at higher levels of supply, characteristic of anthropogenically disturbed systems. Considering that these large scale losses occurred during spates with peak discharges that are exceeded 48% of the time annually, our results suggest that sand loading to river systems could severely limit biomass accumulation, and thus carbon and nutrient stores.;Periphyton biomass was curtailed on riffle rocks when sand transport rates exceeded 64-180 g m-1event-1 during spates. This perturbation threshold was confirmed by an in situ experiment that documented periphyton losses resulting from increasing rates of sand transport. Low profile substrates (2 cm) were scoured more thoroughly than high protruding substrates (6 cm), confirming the PSAM hypothesis that refuge potential increases with elevation above the stream bed relative to the mean sand saltation height.