Effects of nutrient enrichment on C and N stable isotope ratios of invertebrates, fish and their food resources in boreal streams

Carbon and nitrogen stable isotopes are frequently used to study energy sources and food web structure in ecosystems, and more recently, to study the effects of anthropogenic stress on aquatic ecosystems. We investigated the effect of nutrient enrichment on delta(13)C and delta(15)N in fine (FPOM), coarse (CPOM) particulate organic matter, periphyton, invertebrates and fish in nine boreal streams in south-central Sweden. In addition, we analysed the diet of benthic consumers using stable isotope data. Increases in delta(15)N of periphyton (R (2) = 0.88), CPOM (0.78), invertebrates (0.92) and fish (0.89) were related to nutrient enrichment. In contrast, delta(13)C signatures did not change along the nutrient gradient. Our results show that delta(15)N has potential as a sensitive indicator of nutrient enrichment in boreal streams. Carbon and nitrogen isotopes failed to elucidate putative diets of selected aquatic consumers. Indeed, comparison of low- and high-impact sites showed that delta(13)C of many consumers were found outside the ranges of basal resource delta(13)C. Moreover, ranges of basal resource delta(13)C and delta(15)N overlapped at both low and high sites, making discrimination between the importance of allochthonous and autochthonous production difficult. Our findings show that a fractionation rate of 3.4aEuro degrees is not always be appropriate to assess trophic interactions, suggesting that more studies are needed on fractionation rates along gradients of impairment.