Streams are intimately connected with the terrestrial landscape and often depend upon terrestrial subsidies to fuel food webs. Reciprocally, streams provide energy and nutrients in the form of emergent aquatic insects. However, the terrestrial resources that enter streams and the resources that streams provide to the surrounding terrestrial landscape differ in quality, especially in terms of their fatty acid composition. Highly unsaturated omega-3 fatty acids (HUFA) are scarce at the base of terrestrial food webs, but abundant throughout aquatic food webs. Our previous empirical research suggests that HUFA increase performance in and have the potential to subsidize riparian avian insectivores as essential nutrients. Here, we asked if emergent aquatic insects serve as a HUFA subsidy to Eastern Phoebe (Sayornis phoebe), a common avian insectivore, in natural riparian food webs. We assessed the quality of resources by analyzing the fatty acid composition of terrestrial and aquatic basal primary producers and insects from stream and riparian areas around central New York. We used bulk stable isotope analyses to determine the degree to which Eastern Phoebes consumed aquatic or terrestrial insects. We then used compound-specific carbon stable isotope analyses to determine where Eastern Phoebes derived their HUFA.
Results/Conclusions
We found that terrestrial basal resources contained only ALA, (18:3n-3) the precursor to HUFA, while aquatic basal resources were rich in both ALA and the HUFA EPA (20:5n-3). These differences in the quality of basal resources trickled up to the next trophic level: we found that emergent aquatic insects consistently contained much more HUFA than terrestrial insects. In addition, aquatic insects generally also contained more ALA than terrestrial insects. Bulk stable isotope analyses suggested that Eastern Phoebes fed on a mix of high quality aquatic insects and lower quality terrestrial insects, but compound-specific stable isotope analyses suggested that Eastern Phoebes obtained the majority of their HUFA from aquatic sources. Our research suggests that emergent aquatic insects provide a subsidy of high quality fatty acids to riparian avian insectivores and highlights the importance of maintaining healthy aquatic and riparian habitats for avian insectivores.