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Shifting baselines: Integrating ecological and isotopic time-lags improves trophic position estimates in aquatic consumers

Bianca Possamai*, David J. Hoeinghaus, Alexandre M. Garcia

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ABSTRACT: Nitrogen stable isotope ratios (δ15N) are routinely used to estimate consumer trophic positions (TP). This method is very sensitive to the chosen isotopic baseline, which should incorporate all temporal isotopic variability occurring in the base of the food web sustaining the consumers. However, there is still no consensus on the different methods to represent time intervals for isotopic assimilation. Thus, it is uncertain if researchers should compute a single baseline obtained by averaging δ15N source values across all time periods, multiple baselines based on δ15N source values collected concomitantly with consumers or considering a time lag between sources and consumers. Aiming to provide empirical evidence to help select the best option, we designed 4 methods considering different temporal lags between consumers and producers based on a 9-year time series of carbon and nitrogen stable isotopes in an estuary. The tested baseline methods considered the δ15N average of producers sampled in all the years (global); the same season as consumers (concomitantly); the season before the collection of consumers (delayed); and the previous and in the same season of consumers sampling (interpolation). The comparison of the results with an expected model (based on stomach content data) showed that the delayed method is the most appropriate. We conclude that time-lags in the incorporation of stable isotope values between sources and consumers must be considered in baseline δ15N calculations to improve TP estimates and provide more reliable modeling results. Additionally, we provide other recommendations to improve sampling designs when using stable isotopes in TP estimations.