Can consumer stoichiometric regulation control nutrient spiraling in streams?

Homeostatic organisms regulate their elemental composition by retaining nutrients that are limiting and eliminating those in excess. We argue that this type of homeostatic regulation by consumers might decouple the downstream movement of limiting and nonlimiting nutrients in streams. To illustrate the influence of consumers on nutrient spiraling, we developed a longitudinal model of stream nutrient dynamics that explicitly incorporates stoichiometry and recycling. First, we simulated N- and P-tracer addition experiments in a P-limited stream with a particle-tracking algorithm that allowed us to follow the pathways of N and P atoms in the model ecosystem. Then, we varied the biomass, N:P ratio, and strength of stoichiometric regulation of consumers to quantify how these parameters affected modeled spiraling metrics. The particle-tracking simulation showed that the average time for a nutrient atom to complete a spiral increased with the fraction of atoms that entered the consumer compartment in each spiral, which in turn, increased with increasing consumer biomass. Increasing the consumer N:P ratio to exacerbate consumer stoichiometric imbalance with food resources changed the residence times of nutrients in the food web by increasing the downstream velocity of the nonlimiting nutrient and delaying downstream transport of the limiting nutrient. Decreasing the strength of stoichiometric regulation of consumers dampened the observed effects of increased consumer biomass and N:P ratios on nutrient spiraling. Our model results illustrate that consumers have the potential to influence stream nutrient dynamics through differential excretion of limiting and nonlimiting nutrients, but only at relatively high biomass. Stream biogeochemistry has largely focused on factors controlling the uptake of dissolved nutrients, but understanding how these nutrients are retained and recycled once they enter the stream food web will lead to a more complete understanding of nutrient dynamics in streams.