Complementary information from fatty acid and nutrient stoichiometry data improve stream food web analyses

Abstract

Our study characterized the food web of a shaded stream to investigate consumer food choices and food source quality. We measured fatty acids (FA) and elemental carbon (C), nitrogen (N), and phosphorus (P) of autochthonous and allochthonous resources and macroinvertebrates (Hydropsyche sp., Ephemerella sp., Rhyacophila sp., and crayfish-Cambaridae). We examined trophic links using FAs and identified food source quality based on essential fatty acids (EFAs) and elemental nutrients. Autochthonous food sources had greater EFA content (20:5ω3 in periphyton, 20:4ω6 in bryophyte Hygrohypnum) than terrestrial matter. FAs confirmed the grazing nature of Ephemerella. Periphyton may release this invertebrate from N and P limitation, and FA biochemical constraints. Limitation by elemental nutrients, but not FAs, may exist for Hydropsyche if feeding solely on transported matter. Crayfish FA signature suggests consumption of the bryophyte Hygrohypnum as well as terrestrial matter. Our data demonstrate that autochthonous sources are crucial for many invertebrates in shaded streams, despite limited light availability. However, detrital food sources can also be important and their contribution to stream food webs should not be overlooked. Our study highlights the importance of measuring nutrient and biochemical constraints in order to understand factors driving secondary production in streams.