Quantification of Fish Littoral Carbon Use and Trophic Position Using Stable Isotopes: An Empirical Comparison of Equations Using Freshwater Lakes

ABSTRACT Stable isotope analysis is widely used to quantify the flow of energy, nutrients, and biomass through aquatic food webs, where values of carbon (δ 13 C) and nitrogen (δ 15 N) have been used to estimate littoral carbon use (LCU) and trophic position (TP) in lentic ecosystems, respectively. Standardizing stable isotope values with these metrics allows for comparison across different systems and time scales; however, several equations to quantify LCU and TP have been introduced with little guidance on the bias and limitations of each equation, or how and when different equations should be used. Here, we provide recommendations on the appropriate use of LCU and TP equations in freshwater ecosystems based on our analysis of case studies using five fish species common to the study lakes. We address three common challenges of ecosystem variability, namely (1) temporal, (2) spatial, and (3) differing rates of tissue turnover between study and baseline organisms. We begin with a relatively simple case study (Parry Sound) then explore challenges around spatial (Lake Erie) and temporal (Canoe Lake) variation, and the use of multiple fish tissue types (Canoe Lake). We found LCU and TP estimates to be highly variable and dependent on the equation used. High percentages of individual fish exhibited unrealistic LCU values, and the equation of LCU used had a large effect on calculated TP values. We found relative littoral carbon use (LCU R ) produces the most consistent TP estimate, avoiding extreme values while reflecting the natural variability of the system. We propose a framework for calculating LCU and TP, allowing researchers to produce the least amount of bias relative to the known feeding ecology of study species while making estimates comparable across systems. Based on our case studies, we have developed a decision tree to guide researchers in estimating LCU and TP in freshwater ecosystems.