Oral Presentation Society for Freshwater Science 2026 Annual Meeting

Warming alters biofilm nutrient cycling recovery after drought (135726)

Jacqueline E Todd 1 , Arial J Shogren 1 , Nina K Keck 1 , Annabel R Schreiber 1 , Jonathan P Benstead 1 , Mia Berwick 2 , Wyatt F Cross 3 , Alexander D Huryn 1 , Kieran Khamis 2 , Mark E Ledger 2 , Steve A Thomas 1
  1. The University of Alabama, Tuscaloosa, AL, United States
  2. The University of Birmingham, Birmingham, United Kingdom
  3. Montana State University, Bozeman, MT, United States

In a changing climate, freshwater networks are experiencing more frequent and intense droughts that co-occur with rising temperatures. Intensifying drought events are thought to destabilize stream ecosystems and disrupt nutrient cycling, but emerging hypotheses suggest that warming may enhance ecosystem resilience to drought by triggering thermal compensatory responses in stream biofilms. However, to what degree warming influences biofilm-mediated nutrient cycling in response to drought remains poorly understood. We conducted an experiment in 8 ambient (~20°C) and 8 warmed (+5°C) outdoor stream flumes and applied a one-month experimental drought in all flumes after one year of biofilm establishment. Before and shortly after the drought, we conducted short-term nutrient additions of nitrate (NO3--N), ammonium (NH4+-N), and soluble reactive phosphorus (SRP) to quantify uptake velocity (vf; mm min-1) and volumetric nutrient uptake (Uvol; µg N or SRP cm-3 hr-1), as well as incubations to quantify net N2 flux rates (Fnet; µmol N m-2 hr-1) of the stream biofilms in chambers within the stream. We characterized biofilms using ash-free dry mass, chlorophyll-a, and community composition with FlowCam microscopy. To quantify recovery, we calculated recovery ratios of post- to pre-drought measurements, with a ratio greater than 1 indicating recovery. Preliminary results suggested variable responses between N and P vf and Uvol in warmed versus ambient flumes. Recovery ratios for NO3--N uptake metrics indicated that flumes at both temperature treatments did not exhibit full recovery after drought, while recovery ratios for NH4+-N uptake metrics indicated full recovery. Recovery ratios for SRP uptake metrics indicated full recovery in the ambient mesocosms but not in the warmed mesocosms. Early Fnet results revealed greater N2 uptake in the ambient mesocosms pre- and post-drought. Our study’s findings inform predictions about the response of nutrient cycling to the interactive effects of rising temperatures and drought in freshwater ecosystems and suggest that warming does not necessarily confer resilience of stream biofilm nutrient cycling to flow cessation and drying.