Assessing a standardized method to identify optimal baselines for trophic position estimation in stable isotope studies of stream ecosystems

Nitrogen stable isotope ratios (δ¹⁵N) are widely used to quantify trophic position in aquatic ecosystems. Comparing trophic position across space requires identifying baselines to account for variation in δ¹⁵N values of basal energy resources, but few standardized methods exist for identifying suitable baselines. We evaluated a standardized method for identifying optimal isotopic baselines in streams spanning the Rocky Mountains–Great Plains ecotone. We assessed candidate taxonomic groups and feeding groups following four criteria: (1) Organisms should be easy to collect and widely distributed, (2) Within-site δ¹⁵N variation should be low (representative of uniform feeding behavior), (3) δ¹⁵N values should be correlated with geographic variability in δ¹⁵N values, and (4) Trophic position of consumers calculated using the baseline should be independent of geographic δ¹⁵N variability when there is no change in diet. Simuliidae (obligate, sestonic filter feeders) met all four criteria for four fishes and produced trophic position estimates consistent with dietary changes for brown trout along a longitudinal stream gradient. The four-criteria screening method is suitable for temperate streams in North America and supports the recommendation to use Simuliidae or potentially grouped filter feeders as baseline organisms for stable isotope studies quantifying trophic position in higher-order consumers.