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

Abstract

AbstractStable-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 ...