B.Sc. (Calgary 1995), M.Sc. (UBC 1997)
Project: Flow refugia for benthic macroinvertebrates during flooding of a large river
Rempel, L.L., J.S. Richardson, and M.C. Healey. 1999. Flow refugia for benthic macroinvertebrates during flooding of a large river. Journal of the North American Benthological Society 18:34-48.
Abstract. Benthic macroinvertebrates shifted from deep water to shallow water of the shore zone during annual flooding of a large, gravel-bed river. The shore zone is seasonally inundated during flooding for up to 4 mo of the year, and is a geomorphic feature of many large rivers in northern latitudes with limited floodplain habitat. We collected samples of invertebrates along with measures of several hydraulic and sedimentary variables 5 times through the flood cycle from 4 fixed water depths; stations shifted laterally over a distance of 30 m between baseflow and peak flow. In deep water (1.5 and 3.0 m), shear velocity and substrate mobility increased as a result of flooding. At shallow depths (0.2 and 0.5 m) within the shore zone, stable substrate and low hydraulic stress remained throughout the flood cycle, providing flow refugia to benthic invertebrates. Invertebrate density and species richness were similar during months on the rising limb of the hydrograph while the wetted area of the channel markedly increased. Persistence of the invertebrate community appears to have been facilitated by a shift in a large proportion of organisms from deep water to shallow depths of the shore zone during flooding. Total density and the density of most collector-gatherers such as Rhithrogena and Baetis was highest at 1.5 m prior to flooding but shifted to depths of 0.5 and 0.2 m at peak flow. The filter-feeding caddisfly Hydropsyche was most abundant at 1.5 m in all months of the study while the location of 1.5 m samples shifted laterally over a distance of 30 m through the flood cycle. The ecological importance of the shore zone as a flow refugium was demonstrated by the broad diversity of species with varying feeding behaviors and morphologies that concentrated in this zone during flooding.
Project: Macroinvertebrate community structure along gradients of hydraulic and sedimentary conditions in a large, gravel-bed river.
Rempel, L.L., J.S. Richardson, and M.C. Healey. Macroinvertebrate community structure along gradients of hydraulic and sedimentary conditions in a large, gravel-bed river.
(revised and resubmitted to Freshwater Biology November 23, 1999)
The spatial distribution of macroinvertebrate species was examined in relation to hydraulic and sedimentary conditions in a large gravel-bed river, the Fraser River, Canada. Mean annual discharge in the Fraser River is 2 900 m3s?1 and annual flood discharge averages 8 760 m3s?1 due to snowmelt in May and June.
Invertebrates were sampled from four water depths (0.2, 0.5, 1.5, 3.0 m) at various levels of flow discharge that together captured the spatial and temporal variability of the physical habitat. Several hydraulic (near-bed shear velocity, Boundary Reynolds number, turbulence intensity, depth-averaged velocity, Froude number, Reynolds number) and substrate variables (mean grain size, Trask’s sorting coefficient, Nikuradse’s roughness, percentage of fine sediment, and Shields entrainment function) were measured for each sample of macroinvertebrates. Concentrations of fine and coarse particulate organic matter were also assessed.
The physical habitat was characterized by a major gradient of hydraulic conditions that accounted for 52% of the total variation in the habitat data and corresponded positively with increasing water depth. Substrate conditions and the concentration of organic matter explained 24% of the total variation in the benthic habitat.
The distribution of invertebrate species was correlated significantly with hydraulic conditions and suggests that flow patterns represent a major physical gradient along which the benthic community is organized. The distribution of organic matter and substrate texture were also important physical factors for some species. The spatial distribution of most species reflected an organism’s morphological and trophic suitability to particular habitat conditions.
Hydraulic stress associated with foraging and maintaining position, as well as organic matter retention in coarse substrate are likely mechanisms affecting the spatial distribution of macroinvertebrates.