Spatial differences in elemental stoichiometry and essential fatty acid content of food sources and consumers in a stream food web

Our study characterizes spatial differences in food webs of two reaches of a New York 3rd-order stream differing in light availability. Food web components were analyzed using marker fatty acids (FAs). Food source nutritional quality for consumers and predators (insect larvae) was measured through stoichiometry of C, N and P and essential FAs. There were strong imbalances between detrital food sources (low N, P and essential FAs) and insects in both shaded and open reaches, and food sources and invertebrate consumers were differentially affected by light with respect to their elemental stoichiometry and essential FA content. Biochemical patterns indicated that invertebrates fed selectively on higher-quality sources (algae, bryophytes, epiphytic bacteria, transported matter) and less on lower-quality periphyton and benthic matter. In addition to confirming the importance of autochthonous food sources for stream invertebrates, this study has highlighted that local ecological processes driven by changes in light availability and canopy-derived nutrient-rich matter can alter the micro- and macronutrient content of primary producers and detrital matter. These changes tracked differently within each consumer and varied with types of nutrient. Invertebrates exhibited a greater degree of homeostasis with respect to N and P than their essential FAs, and across trophic levels.