Characterizing river metabolism and resource availability across a gradient of flow alteration in desert rivers to inform native fish management

Authors: Chloe Lyles, Phaedra Budy, Charles Yackulic, Casey Pennock

Year: 2024

Abstract

River metabolism and aquatic communities are driven by hydrological and physiochemical regimes in rivers, but these regimes are often altered due to a suite of stressors including anthropogenic water demand and climate change-driven drought. Despite widespread river alteration, the effects of flow regime alteration on bottom-up processes and aquatic food webs remain understudied. To better understand the linkages between river alteration, metabolism, and food webs, we deployed oxygen and temperature sondes and measured benthic algae and macroinvertebrate standing stock in seven tributaries of the upper Colorado River Basin with varying degrees of flow alteration. We defined flow alteration as the percent difference between 21st and 20th-century spring flows. In 2023, we maintained sondes, measured benthic Chlorophyll-a, and collected benthic macroinvertebrates monthly from July to December during baseflow conditions. We found that rivers with less altered flow retained a greater range of gross primary production (GPP) and ecosystem respiration (ER) than rivers with more altered flow. Rivers with greater flow alteration also experienced a decoupling of the typically linear GPP/ER relationship. Whereas data and sample processing are ongoing, preliminary results for benthic algae suggest a negative correlation (r = -0.79; p = 0.023) between Chlorophyll-a and flow alteration. We also expect a negative relationship between flow alteration and macroinvertebrate biomass. To date, much research on limiting factors for higher trophic levels, such as fishes, has focused on habitat alteration and invasive species introductions, largely ignoring bottom-up effects. Improving our understanding of linkages between river alteration, metabolic regimes, and dynamics of higher trophic levels will enhance our ability to predict how riverine ecosystem structure and function might change in the future.