FOOD WEB OF A TROPICAL HIGH MOUNTAIN STREAM: EFFECTS OF NUTRIENT ADDITION

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Using a nutrient enrichment experiment in an Andean mountain stream, we used stable isotope ratios (¹⁵N and ¹³C) to analyze different trophic compartments: 1) basal level: CPOM and biofilm; 2) primary consumers – macroinvertebrates: collector-gatherers(Heterelmissp, Thraulodessp andTrichorythodessp), and collector-filterers (Simuliumsp); 3) predators – fish (Oncorhynchusmykiss and Trichomycterusbogotensis).The average fractionation of nitrogen among the primary consumers with respect to CPOM was 4.7‰ and 1.7‰ with respect to biofilm. Predators incrementedtheir¹⁵N signalby 5.9% with respect to primary consumers.A depletion of¹⁵N was observed in Impact with respect tocontrol reach after fertilization in different compartments (biofilm, Heterelmissp., Simuliumsp. andTricorythodessp.), while depletion was not significant for top predators. In most cases, the ¹³C signal of biofilm overlapped with that of primary consumers, but a clear enrichment was observed with respect to CPOM.The macroinvertebrates referred to were selected to analyze their gut content and the results showed us that fine detritus is the most abundant food in invertebrates, and onlyHeterelmis sp. showed significant differences in fine detritus and vegetal matter between control and impact reaches after the nutrient addition.