Reliance on blue, green, and brown energy channels drives a shift in the trophic position of riparian spiders.

Understanding the mechanisms shaping food chain length (FCL) has long been central to food web ecology. FCL is a key determinant of stability, energy flow efficiency, and biodiversity maintenance, but there is an ongoing debate about its underlying drivers. It is particularly important in meta-ecosystems, where predator trophic position (TP) is influenced by multiple energy channels. In this study, we focused on spiders in riparian ecosystems, which rely on resources linked to distinct energy channels: blue (algal herbivory), green (terrestrial herbivory), and brown (terrestrial detritivory). We applied nitrogen isotope analysis of amino acids to estimate the TP of both spiders and their prey. This method is a powerful tool for determining TP from a single sample and even allows for capturing decomposer trophic steps. However, the TP estimate requires special care for riparian spiders, as spiders show a specific trophic discrimination factor (TDFGlx-Phe), and that energy channel use can confound the TP estimate. Our detailed food web resolution supports the use of specific parameters for spiders, particularly the low trophic discrimination factor (TDFGlx-Phe ~ 2‰), and raises caution about the importance of estimating resource use of predators to estimate their TP. We show that the primary factor driving variation in spider TP is the energy channel they utilize, from blue (TP ~ 2.9) to green (TP ~ 3.6) to brown (TP ~ 4.1). This increase was largely due to prey omnivory in green channels, and microbial and fungal decomposers serving as an initial trophic step between litter and invertebrate detritivores in brown channels. We propose that this pattern is likely influenced by differences in basal nutritional quality, which increases from brown (low) to green (medium) and to blue (high) sources. This suggests that shifts in energy channels within meta-ecosystems in the course of global change (e.g., climate warming, eutrophication and land-use change) may significantly impact FCL, with significant consequences for trophic interactions, nutrient fluxes, and biomagnification processes.