Variability in δ15N natural abundance of basal resources in fluvial ecosystems: a meta-analysis

Variation in N stable isotope (δ15N) signatures of basal resources can influence interpretation of trophic relationships in ecosystems, and significant variation in δ15N signatures has been reported in streams and rivers. However, a comprehensive understanding of the main factors driving δ15N variability is lacking, and this variability confounds the consumer's trophic-level position during δ15N analysis. We conducted a meta-analysis to examine the variability in δ15N natural abundance of basal resources and dissolved inorganic N (DIN) in streams and rivers in relation to the environmental factors that may drive this variability. The meta-analysis was based on a literature review over the last 20 y (1989–2009) and contained signatures of δ15N-DIN (δ15N-NO3 and δ15N-NH4) and δ15N-basal resources (δ15N-detrital compartments, δ15N-biofilm, δ15N-algae, and δ15N-macrophytes) from >100 rivers or streams. Signatures of δ15N-DIN varied widely (−8.4–19.4‰), and we found fewer values for δ15N-NH4 than δ15N-NO3, even though NH4+ is assimilated rapidly by basal resources. The range of δ15N-basal resources was also broad (−4–16‰) within and among compartments. Human land use was the most significant factor explaining variability in δ15N-DIN and δ15N-basal resource signatures. We found significant differences between δ15N signatures of photoautotrophic (i.e., autochthonous) and detrital (i.e., allochthonous) basal resources. Our results point out the difficulty in defining a baseline δ15N signature of the food web, and provide a basis to explain confounding results in studies using δ15N analysis to identify trophic linkages in fluvial food webs.