The influence of dynamic resources and stable isotope incorporation rates on aquatic consumer trophic position estimation

Abstract

A key assumption in trophic position (TP) estimation using stable isotope analysis is that consumers are in isotopic equilibrium with their resources. Here, we assess the degree to which time-varying resource dynamics and isotope incorporation rates of consumers influence consumer TP estimates across multiple trophic levels and aquatic ecosystems. We constructed a first-order kinetics model to explore consumer stable isotope incorporation rates and modeled the effect on TP calculations using bulk and compound-specific stable isotope data from previous experimental and observational studies. We found TP estimates of higher trophic level consumers are less accurate than lower trophic level consumers when applying bulk stable isotope analysis (BSIA) and using particulate organic matter as the stable isotope baseline. The accuracy of TP estimates depended on the time-varying dynamics of the stable isotope baseline. Tertiary consumers had the highest TP estimation error, and this error was not eliminated by sampling tissues with fast incorporation rates (i.e., blood) in the tertiary consumer. Compound-specific stable isotope analysis (CSIA) of individual amino acids was more accurate in estimating TP for all consumers and ecosystems compared to BSIA. Our analysis confirms that consideration for the dynamic nature of stable isotope ratios is necessary for accurate TP estimates. Finally, we show how first-order kinetics models can provide a useful framework for integrating prey and consumer incorporation rates in stable isotope studies to improve TP estimates.