Littoral benthic macroinvertebrate communities in lake reservoirs often are exposed to repeated fluctuations in water level, but little is known of the effects of drawdown on benthic community composition. We compared the taxonomic composition and spatial patterns of benthic macroinvertebrate communities in sediments of littoral areas in a reservoir with >30 y of seasonal drawdowns (Sooke Lake Reservoir [SLR]) and a natural lake with little seasonal change in water levels (Shawnigan Lake [SHL]). Contrary to our predictions, macroinvertebrate density and biomass usually were greater in SLR than in SHL. In SLR, densities and biomasses of macroinvertebrates, especially chironomids, were higher below the drawdown exposure zone than in the upper littoral area. Chironomids with r-selected survival strategies (i.e., smaller size) or desiccant-resistant stages appeared well suited to the fluctuating environment of littoral zones in reservoirs. Orthocladiinae, Chironomini, Tanytarsini, and Tanypodinae dominated at sampling sites immediately below the drawdown exposure zone in SLR, whereas only Orthocladiinae dominated at deeper sites. Warm water from an expanded epilimnion apparently extended the distribution of Diamesinae farther from shore in SLR than in SHL. Chironomini generally were the dominant macroinvertebrate taxon in SHL, and the relative biomass of Diamesinae increased with depth. Distributions of oligochaetes and nematodes extended farther from shore in SLR than in SHL. These contrasting benthic macroinvertebrate communities indicate that variable drawdown regimes could have significant impacts on benthic food webs and the transfer of energy and nutrients to the pelagic area.
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