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Habitat variation due to seasonal flooding of the lower Fraser river and the influence on the macroinvertebrate community Rempel, Laura L.


Most large rivers of the world are of exceptional importance to many human uses but the ecosystems they support are often neglected in ecological research. The Fraser River, British Columbia, is a large river characterized by a regularly recurring flood cycle driven by snowmelt which seasonally inundates an extensive lateral shore zone. This study investigated the importance of the shore zone as a flow refugium to the macroinvertebrate community of the lower Fraser River during flooding, and the physical variations to the benthic habitat associated with flooding. Measures of hydraulic forces and substrate mobility were significantly higher at water depths of 1.5 and 3.0 m in the active channel during flooding of the Fraser River in 1995, than depths of 0.2 and 0.5 m within the shore zone where the substrate remained immobile. This variation produced a lateral gradient of increasing hydraulic stress from shallow to deep water depths along which the benthic community of the Fraser River appeared to be organized. The spatial distribution of invertebrate taxa generally reflected an organism's morphological and trophic suitability to particular hydraulic conditions which varied laterally between the shore zone and active channel. Shallow water depths of the shore zone satisfied the physical criteria of flow refugia by maintaining substrate stability and low hydraulic stress during flooding. Demonstrating the provision of flow refugia in the shore zone was the first of two steps to determining the ecological importance of the lateral shore zone for the persistence of the macroinvertebrate community during flooding in the Fraser River. The extent to which organisms used the shore zone during high flows was also necessary to confirm its refuge potential. On the rising limb of the flood hydrograph, total density and species richness remained stable while the wetted area of the channel increased significantly. The persistence of the invertebrate community appears to have been facilitated by a shift in the distribution of a major proportion of organisms from depths of 1.5 and 3.0 m in the active channel to the lateral shore zone during months of peak discharge. The ecological importance of this zone was demonstrated by the broad diversity of taxa with varying feeding behaviours and morphologies which concentrated in the shore zone.

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