Elevated Temperatures Constrain Microbial Metabolism in Benthic and Underlying Sediments of Intermittent Streams

Abstract The influence of elevated surface water temperature in disconnected pools of intermittent rivers on microbial metabolism within the benthic biolayer and underlying sediments remains unclear. This study investigates sediment metabolism under pool temperatures of 20°C, 30°C, and 40°C, focusing on biofilm growth, bioclogging, and reaction rates. Column‐scale tracer tests using sodium chloride and resazurin, coupled with subsurface transport modeling, quantified respiration rates over 77 days. At 20°C, pronounced bioclogging caused flow instability, fragmenting the resazurin plume into finger‐like channels and increasing its metabolic rate 12.7‐fold by day 29. Paradoxically, elevated temperatures reduced bioclogging, constrained microbial metabolism, and altered reaction‐transport dynamics. Instead of maintaining a fixed state, Damköhler numbers ( Da ) shifted dynamically, transitioning from reaction‐limited ( Da < 1) to transport‐limited ( Da > 1) conditions during biofilm development, with these shifts dampened at higher temperatures. These findings challenge assumptions that warmer conditions always enhance respiration and CO 2 fluxes in river ecosystems.