Toxicity and Starvation Induce Major Trophic Isotope Variation in Daphnia Individuals: A Diet Switch Experiment Using Eight Phytoplankton Species of Differing Nutritional Quality

SIMPLE SUMMARY: The estimates of animal diets and trophic structures using stable isotope analyses are highly influenced by the diet–tissue discrimination and tissue turnover rates. However, these factors are often unknown because they must be measured using controlled feeding studies. Furthermore, these parameters may be influenced by the diet quality, quantity, toxic stress, and starvation or fasting, as well as other factors. We measured the effects of toxic stress, starvation, and diet quality on the turnover rate and diet–tissue discrimination in Daphnia individuals. We raised individuals with a common laboratory diet and switched them to eight different dietary sources with varying levels of nutritional quality, while one group experienced starvation. The isotopic values were assessed on a daily basis post-diet change. Overall, we showed that in addition to the nutritional quality, toxic stress and starvation are the main processes that affect the two key parameters of the stable isotope analysis. ABSTRACT: Stable isotope values can express resource usage by organisms, but their precise interpretation is predicated using a controlled experiment-based validation process. Here, we develop a stable isotope tracking approach towards exploring resource shifts in a key primary consumer species Daphnia magna. We used a diet switch experiment and model fitting to quantify the stable carbon (δ (13)C) and nitrogen (δ (15)N) isotope turnover rates and discrimination factors for eight dietary sources of the plankton species that differ in their cellular organization (unicellular or filamentous), pigment and nutrient compositions (sterols and polyunsaturated fatty acids), and secondary metabolite production rates. We also conduct a starvation experiment. We evaluate nine tissue turnover models using Akaike’s information criterion and estimate the repetitive trophic discrimination factors. Using the parameter estimates, we calculate the hourly stable isotope turnover rates. We report an exceedingly faster turnover value following dietary switching (72 to 96 h) and a measurable variation in trophic discrimination factors. The results show that toxic stress and the dietary quantity and quality induce trophic isotope variation in Daphnia individuals. This study provides insight into the physiological processes that underpin stable isotope patterns. We explicitly test multiple alternative dietary sources and fasting and discuss the parameters that are fundamental for field- and laboratory-based stable isotope studies.