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Stable-isotope analysis is widely used in aquatic ecosystem studies to evaluate trophic structure and resource dynamics. Because δ15N values vary in freshwater systems, e.g., reflecting variations in land use, suitable baseline indicators must be specified. Few investigators have identified specific baseline organisms based on thorough and methodical screening. We screened for baseline organisms in temperate lotic waters based on 4 criteria: 1) baseline organisms should be easy to collect, 2) within-site variation in δ15N levels should be low, 3) δ15N should reflect land use, and 4) trophic position (TP) of consumers calculated from the baseline should be independent of system-specific δ15N variability as long as no systematic change in food consumption occurred. We investigated individual taxa and bulked groups representing different feeding modes as baselines. We found that Simuliidae, a sestonic filter feeder, fulfilled all criteria. Furthermore, TP estimates of 2 common fishes that were based on the Simuliidae or grouped filterers as baselines were the only estimates in our study that were independent of landuse changes. In addition, the diet of these fishes did not change across land use as based on stable-isotope mixing-model analysis. Simuliidae also had the lowest within-site variation, i.e., the lowest trophic level range, probably a result of uniform feeding behavior. Therefore, Simuliidae and grouped filterers could be suitable baseline indicators in future studies. We recommend minimizing δ15N variability in and among systems because the precise, complex choice, timing, or proportions of food sources consumed cannot be mimicked. We also promote combining TP estimation and mixing-model analyses as a strong tool in studies of stream food webs.

Baseline identification in stable-isotope studies of temperate lotic systems and implications for calculated trophic positions