Strong temporal variation of consumer δ13C value in an oligotrophic reservoir is related to water level fluctuation

Using stable carbon and nitrogen isotope analysis (delta C-13 and delta N-15) to assess trophic interactions in freshwater ecosystems is a well established method, providing insight into ecosystem functioning. However, the spatial and temporal variability of isotope values, driven by environmental fluctuation is poorly understood and can complicate interpretations. We investigated how the temporal variation of stable isotopes in consumers (fish, crayfish and macrozoobenthos) of a canyon-shaped oligotrophic reservoir is associated with environmental factors such as water temperature, transparency, flooded area, and water quality measures. Consumers and their putative food sources were sampled and analyzed for carbon and nitrogen stable isotopes annually, and environmental parameters were measured monthly from 2014 to 2016. Results revealed significant differences in delta C-13 and delta N-15 values in each consumer among studied years. Over the years, fish and crayfish expressed differences in delta C-13 between 3 and 5 parts per thousand, whereas in zoobenthos differences were 12 parts per thousand. Variability in delta N-15 was similar across all consumers (2-4 parts per thousand). Moreover, results suggest that the flooded area of the reservoir was a major driver of delta C-13 stable isotope values variation in consumers, while variation in delta N-15 was not linked to any of the studied environmental factors. Bayesian mixing models further showed significant changes in the origin of detritivorous zoobenthos carbon sources (reversal shift from terrestrial detritus to algae origin) between years with low water level to years with the standard water level. Other species showed only slight differences in food source utilization among years. Our study highlights the importance of environmental factors as sources of variation in consumer's stable isotope values which should be considered especially when studied ecosystem strongly fluctuate in some environmental factor.