Resource Composition and Macroinvertebrate Resource Consumption in the Colorado River Below Glen Canyon Dam

Physical and biological changes to rivers induced by large dams can significantly alter downstream communities, decreasing the biotic integrity of these rivers. For example, completion of Glen Canyon Dam on the Colorado River in 1963 has altered the downstream ecosystem and contributed to the decline of native fish populations and dramatic changes in the macroinvertebrate communities. Physical changes associated with the dam may also influence the food resources supporting macroinvertebrate production, but this has not been previously measured. For example, autochthonous production is high in the clear tailwaters of the dam, but downstream tributary allochthonous carbon inputs are substantial and may provide an important food resource. I predict that macroinvertebrate diets will mirror these longitudinal changes in resource availability and may indicate how the dam has altered the macroinvertebrate food webs of this large river. I also predict that monsoon tributary flooding in the autumn and lower light availability in the winter, will amplify the longitudinal change in resource use by macroinvertebrates. I examined the diets of the most common macroinvertebrates (Simulium arcticum, Gammarus lacustris,Potamopyrgus antipodarum, and chironomids) at six sites below Glen Canyon Dam during all seasons. Macroinvertebrate diet composition was compared to the composition of the epilithon (rock faces), epicremnon (cliff faces) communities, and the suspended organic seston. Using these data, I calculated the relative contribution of autochthonous and allochthonous resources to macroinvertebrate production. To estimate the extent that tailwater primary production provides a subsidy to downstream consumers, during one season I identified algae to species in the various biofilms, seston and in macroinvertebrate diets to find species-specific tracers of tailwater production. In general, I found that macroinvertebrate diets tracked downstream changes in resource availability, and autochthonous resources were consumed in greater proportions in the tailwaters while more allochthonous resources were consumed downstream. The extent of diet shifts depended on consumer identity and season. I observed similar patterns in the resources that support macroinvertebrate production, despite the greater assimilation of autochthonous carbon. Allochthonous resources were most important during the monsoon season (July-September), when tributaries can contribute significant amounts of organic matter to the mainstem. Finally, using the species-specific indicators of tailwater production I estimated that less than seven percent of downstream macroinvertebrate production is attributable to tailwater algal production. This suggests that the tailwaters may not provide a significant subsidy to downstream food webs. These findings demonstrate that the influence of the Glen Canyon Dam is most evident in the tailwaters and as distance from the dam increases, tributary allochthonous carbon fuels food webs, which may be similar to pre-dam food webs