Gone With the Flow: Flow Reduction Coincides With Reduced Metabolism and Standing Stock of Benthic Algae and Macroinvertebrates in Desert Rivers

River metabolism, the measure of energy flow in a river, is driven by hydrological and physicochemical regimes (e.g., temperature, turbidity, and nutrients). However, these regimes are increasingly altered by human water demand and climate change-driven drought. These stressors alter the magnitude, timing, and duration of critical spring-snowmelt floods with poorly understood consequences on ecosystem function, particularly in desert rivers. To investigate the linkages between flow reduction, river metabolism, and basal food resource productivity (i.e., algae and aquatic invertebrates), we measured metabolism and basal resource availability in seven desert tributaries with varying degrees of flow reduction during low-flow conditions in 2023 and 2024. Flow reduction was calculated as the percent change in median annual flow from a historic 20-year period to a contemporary 20-year period. We used models to evaluate which environmental variables correlated with river metabolism and standing stock (amount) of algae and aquatic invertebrates. We found that all sites were seasonally heterotrophic (often subsidized by terrestrial carbon and CO2 producing), except for the largest river which was seasonally autotrophic (produces and stores organic carbon). Primary production increased with light availability and was reduced in rivers with more altered flow regimes. Ecosystem respiration, the amount of carbon fixed or energy consumed, did not correlate with any environmental variables. Algae and macroinvertebrate standing stock also decreased with flow reduction. These findings challenge the current understanding of how river ecosystems and food webs change in response to human alteration. As water demand increases in arid regions, understanding the linkages between flow reduction and ecosystem function will enhance our ability to conserve and protect unique desert river ecosystems.