Linking bioenergetics of a top stream predator to ecosystem function in an arctic spring-fed stream

Arctic spring-streams undergo dramatic seasonal oscillations in light regime and associated primary productivity, while their thermal stability maintains many metabolic processes at constant rates. Primary production and ecosystem respiration are consequently decoupled during winter, potentially leading to seasonal carbon limitation and related impacts on consumer activity within spring-stream food webs. We used bioenergetic modeling to quantify prey consumption and subsequent nutrient recycling by juvenile Dolly Varden char at a representative arctic spring-stream (Ivishak Spring, North Slope of Alaska, USA), where these predators show sharply seasonal patterns of growth. Prey demand was high during summer (~3g dry mass [DM] invertebrates m^-2 mo^-1). Steep declines in prey demand during winter (~0.3 g DM m^-2 mo^-1), when char metabolic rates are maintained by stable temperatures, suggest that low growth rates result from reductions in winter prey production. Contributions of char excretion to nutrient fluxes were negligible even in summer (e.g., <5% of areal N uptake), precluding this potentially important feedback role of char consumption. Although reciprocal control of prey biomass by char may be seasonally important, our results suggest that char production in arctic spring-streams is subject to pervasive bottom-up control, with little scope for a significant role in the recycling of nutrients.