The Effect of Temperature on Trophic Discrimination of Stable Isotopes ( ¹³ C and ¹⁵ N) and Biokinetics in Common Carp ( <i>Cyprinus carpio</i> , L. 1758)

Stable isotope analysis (SIA) provides essential information toward a better understanding of trophic ecology. However, the interpretation of SIA results relies on assumptions about the trophic discrimination factor (TDF), which aims to improve the accuracy but may lead to bias. In aquatic ecosystems, most biota are poikilothermic organisms, thus temperature is one of the most important parameters affecting all biological processes, including the trophic discrimination of stable isotopes. Therefore, we conducted an experiment to establish TDF for freshwater fish under different temperature regimes (15°C, 25°C, and natural ambient pond temperatures from July to September). We used common carp (Cyprinus carpio, L., 1758) as a model organism. In the first phase of the experiment (weeks 1-6), all fish were fed a defined fish feed to establish a baseline isotopic signal for all individuals. In the second phase (weeks 7-18), fish were randomly divided into three temperature groups and were all fed different types of fish feed. Results indicated temperature-dependent TDF, with nitrogen showing higher temperature dependency, where highest TDF were observed in the 15°C treatment and lowest in the 25°C treatment. In contrast, for carbon, an equilibrium of muscle isotopic values was reached only in the natural ambient temperature regime. Presumably, discrimination of nitrogen isotopes was related to metabolic turnover rates, where all values were significant, while for carbon, only significant values were found under natural ambient temperature conditions. These findings highlight the importance of considering environmental thermal conditions in evaluating stable isotope signatures in trophic studies.