The stable isotopic composition of Daphnia ephippia reflects changes in δC and δO values of food and water

The stable isotopic composition of fossil resting eggs (ephippia) of Daphnia spp. is being used to reconstruct past environmental conditions in lake ecosystems. However, the underlying assumption that the stable isotopic composition of the ephippia reflects the stable isotopic composition of the parent Daphnia, of their diet and of the environmental water have yet to be confirmed in a controlled experimental setting. We performed experiments with Daphnia pulicaria cultures, which included a control treatment conducted at 12 C in filtered lake water and with a diet of fresh algae and three treatments in which we manipulated the stable carbon isotopic composition (δC value) of the algae, stable oxygen isotopic composition (δO value) of the water and the water temperature, respectively. The stable nitrogen isotopic composition (δN value) of the algae was similar for all treatments. At 12 C, differences in algal δC values and in δO values of water were reflected in those of Daphnia. The differences between ephippia and Daphnia stable isotope ratios were similar in the different treatments (δC: +0.2± 0.4 ‰ (standard deviation); δN: −1.6± 0.4 ‰; δO: −0.9± 0.4 ‰), indicating that changes in dietary δC values and in δO values of water are passed on to these fossilizing structures. A higher water temperature (20 C) resulted in lower δC values in Daphnia and ephippia than in the other treatments with the same food source and in a minor change in the difference between δC values of ephippia and Daphnia (to −1.3± 0.3 ‰). This may have been due to microbial processes or increased algal respiration rates in the experimental containers, which may not affect Daphnia in natural environments. There was no significant difference in the offset between δO and δN values of ephippia and Daphnia between the 12 and 20 C treatments, but the δO values of Daphnia and ephippia were on average 1.2 ‰ lower at 20 C than at 12 C. We conclude that the stable isotopic composition of Daphnia ephippia provides information on that of the parent Daphnia and of the food and water they were exposed to, with small offsets between Daphnia and ephippia relative to variations in Daphnia stable isotopic composition reported from downcore studies. However, our experiments also indicate that temperature may have a minor influence on the δC, δN and δO values of Daphnia body tissue and ephippia. This aspect deserves attention in further controlled experiments.