Leaf fall affects the isotopic niches of meiofauna and macrofauna in a stream food web

The dynamics of resource subsidies in small forested streams fluctuate seasonally and reflect donor-controlled linkage between terrestrial and aquatic ecosystems. Here we ask whether and how complex communities adjust their diet in response to the pulses of leaf litter reaching streams every year in autumn. Answers to these questions were sought in a seasonal comparison of the isotopic niches of a community of stream invertebrates (105 body size-range: ~ 0.2-20 000 μg ind.-1) using Bayesian ellipse fitting and mixing models. We measured manifest seasonal changes in the isotopic niches at the population and community-levels suggesting the existence of a general trophic switch towards the assimilation of leaf litter in autumn. This switch characterized by a reduction of the area of isotopic niches for populations of most consumers (except harpacticoid copepods) and their predators. In autumn, the isotopic signature of the invertebrate community corresponded with that of fine particulate organic matter and litter in terms of δ13C, while total δ15N range increased, suggesting an increase in food chain length. Interestingly, the smallest invertebrates (meiofauna) occupied various trophic levels irrespective of their size, demonstrating the need to consider those animals to better understand trophic dynamics in stream ecosystems.

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