The Colorado River is home to two of the most notable dams in the world: Glen Canyon and Hoover Dams. These dams have benefits, including water storage, energy supply and recreation. However, they have also had unforeseen consequences, particularly on the river’s ecological functioning. Recent work in the Colorado River between these dams shows that high daily and low nightly flows, a consequence of Glen Canyon Dam’s operations termed hydropeaking, creates a pattern of highs and lows in aquatic insect emergence. Emergent insects, however, are not isolated to the aquatic ecosystem. Studies in other riparian systems show these insects are an integral part of terrestrial riparian food webs. Therefore, we sought to determine if hydropeaking, through its effects on emergence, also affects riparian consumer diets. To do this, tissue samples were collected from riparian consumers, including lizards, toads, rodents, and bats downstream of Glen Canyon Dam over three years in areas of high, low, and moderate insect emergence. Aquatic and terrestrial vegetation samples were also collected for baselines. Carbon and nitrogen isotopes were measured in each sample and compared across sites to determine the source (aquatic vs. terrestrial) of riparian consumer diets and their relationships to emergent insect abundance.
Results/Conclusions
With the exception of toads, carbon and nitrogen isotope ratios of riparian consumer tissues showed variation along this 400-km river segment, signifying a variation in diet by river location. For all other consumers, we found depleted 13C:12C ratios and enriched 15N:14N ratios in consumer tissue relative to the terrestrial baseline, indicating reliance on aquatic production. Lizard tissues showed a pattern of relatively depleted carbon isotope ratios with increasing emergence, though their δ15N values show variation unexplained by emergence alone. Bats and rodents, on the other hand, showed both depleted carbon isotope ratios and enriched nitrogen isotope ratios in response to increasing emergence. These results suggest that many riparian consumers are affected by artificial flow variation caused by dam operations through a dietary linkage to emergent insects. This highlights the significance of emergent insects in riparian habitats in an arid large-river system, which has yet to be recorded elsewhere. These findings are also relevant to the management of dams globally, as their operations can affect not just the aquatic food web, but also the surrounding terrestrial food web. Environmental flows that mitigate these effects could improve the health of riparian ecosystems worldwide.