Oral Presentation Society for Freshwater Science 2025 Annual Meeting

Stream warming disrupts linkages between biofilm biomass, metabolism, and nutrient uptake in experimental streams (118883)

Mitchell J Liddick 1 , Jennifer L Tank 1 , Abagael N Pruitt 1 , Emma M Thrift-Cahall 1 , Ryan T Meyer 1 , Grace Krane 1
  1. University of Notre Dame, South Bend, IN, United States

Stream temperatures in the Midwest are projected to rise by 1-6oC by century’s end. Previous research has shown that stream functional metrics are sensitive to changes in temperature, but rarely are multiple metrics explored simultaneously in a controlled setting. Using a reach-scale warming platform at the Notre Dame Linked Experimental Ecosystem Facility, we examined the effects of changing stream temperatures on biofilms, exploring the relationship between biomass, metabolism, and nutrient removal. We hypothesized that increasing water temperatures would stimulate biofilms, enhance gross primary production (GPP) and ecosystem respiration (ER), and increase nutrient removal from the water column. We used a heat exchanger to simultaneously cool/heat 50-meter experimental streams: n=2 cooled (-2.5oC ambient) and n=2 warmed (+2.5oC ambient) over 28 days (August-September 2024). We quantified algal biomass as chlorophyll-a (Chla), reach-scale GPP and ER, and conducted n=112 short-term nutrient additions of soluble reactive phosphorus (SRP), ammonium (NH4+-N), and nitrate (NO3--N). Warming significantly increased Chla over time (Linear Mixed Model;LMM,p<0.05); diatom and cyanobacterial biomass was significantly higher in warmed streams (LMM,p<0.05), while green algae were more variable. This increase in algal biomass did not result in a significant change in GPP (LMM,p=0.20). However, ER was significantly higher in warmed streams (LMM,p<0.05). On Day1, warming did not significantly affect nutrient uptake lengths (Sw) for SRP (warm Sw=78±32m, cool Sw=78±18m), NH4+-N (warm Sw=24±2m, cool Sw=20±1m), or NO3--N (warm Sw=89±20m, cool Sw=56±11m)(ANCOVA,p>0.05). Surprisingly, by Day28, warming significantly increased Sw for NH4+-N (warm Sw=44±2m, cool Sw=26±2m) and NO3--N (warm Sw=69±2m, cool Sw=54±9m)(ANCOVA,p<0.05), but not SRP (warm Sw=41±8m, cool Sw=33±9m)(ANCOVA,p>0.05). Our results suggest that warming differentially influenced autotrophic and heterotrophic activity, and may have decoupled linkages between biofilm biomass, metabolism, and nutrient removal.