Oral Presentation Society for Freshwater Science 2026 Annual Meeting

Whole-stream isotope additions reveal light and predator controls on nitrogen fluxes in Tropical stream food webs (136143)

Frank C Akamagwuna 1 , Steve Thomas 1 , Matthew Dunkle 2 , Matthieu Bruneaux 3 , Andrés López-Sepulcre 4 , Sarah Collins 5
  1. University of Alabama, Tuscaloosa, Alabama, United States
  2. Institute of Arctic Biology, University of Alaska, Fairbanks, Alaska, United States
  3. Jyväskylä, Findland
  4. Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, New York, United States
  5. Department of Zoology and Physiology, University of Wyoming, Laramie, Wyoming, United States

Whole-ecosystem isotope tracer additions provide a powerful tool for quantifying nitrogen cycling and food-web dynamics in streams. We conducted whole-stream 15N tracer additions in tropical headwater streams in Trinidad to test how predator introduction and canopy thinning shape nitrogen fluxes through stream food webs. We combined tracer addition with Bayesian hierarchical modeling to examine how guppy introduction (Poecilia reticulata) and canopy thinning affect nitrogen fluxes among ecosystem compartments, including inorganic nitrogen pools, basal resources, detrital compartments, invertebrate consumers, and fishes (P. reticulata and Anablepsoides hartii). Experimental methods consisted of a 10-day constant-rate addition of 15NH4+ (target enrichment ~20,000‰) in two reaches (control and + guppy) in two parallel streams (ambient and thinned-canopy). Identical experiments were conducted in 2010 and 2019.  All food web compartments were sampled before tracer addition, repeatedly during enrichment (days 1, 3, 5, 7, and 10), and five times over 30 days following the enrichment period. Tracer data were modeled using the IsoTracer Bayesian hidden Markov model to simultaneously estimate nitrogen fluxes between food web compartments and to compute posterior contrasts that account for propagated uncertainty. Our results demonstrated that both guppy introduction and canopy thinning altered nitrogen cycling, and that these effects persisted for nearly a decade as guppies adapted to novel predator conditions. However, the relative effects were largely context dependent, with both thinning and guppy presence increasing nitrogen flux from basal resources to invertebrate consumers. Posterior contrasts indicated that total nitrogen flux from basal resources to consumers increased in thinned canopy reaches (Pr(Δ > 0) = 0.62). The introduction of guppies also had a positive effect on N flux to consumers (Pr(Δ > 0) = 0.76).  Quantitative comparisons between 2010 and 2019 are ongoing, but early results suggest that the influence of canopy thinning and guppy introduction has remained consistent through time. By integrating ecosystem-scale manipulations with Bayesian tracer modeling, this study provides a mechanistic framework for understanding how light and consumer community traits regulate nutrient cycling in streams.