Abstract
Abstract. 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 (δ13C value) of the algae, stable oxygen isotopic composition (δ18O value) of the water and the water temperature, respectively. The stable nitrogen isotopic composition (δ15N value) of the algae was similar for all treatments. At 12 °C, differences in algal δ13C values and in δ18O values of water were reflected in those of Daphnia. The differences between ephippia and Daphnia stable isotope ratios were similar in the different treatments (δ13C: +0.2 ± 0.4 ‰ (standard deviation); δ15N: −1.6 ± 0.4 ‰; δ18O: −0.9 ± 0.4 ‰), indicating that changes in dietary δ13C values and in δ18O values of water are passed on to these fossilizing structures. A higher water temperature (20 °C) resulted in lower δ13C values in Daphnia and ephippia than in the other treatments with the same food source and in a minor change in the difference between δ13C 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 δ18O and δ15N values of ephippia and Daphnia between the 12 and 20 °C treatments, but the δ18O 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 δ13C, δ15N and δ18O values of Daphnia body tissue and ephippia. This aspect deserves attention in further controlled experiments.
Highlights
Past variations in lake water δ18O values have been reconstructed by analyzing the δ18O values of fossil chironomid head capsules (Wooller et al, 2004; Verbruggen et al, 2010b), and a correspondence has been found between δ18O values of lake water and of chironomid head capsules and Daphnia ephippia buried in surface sediments (Verbruggen et al, 2011)
We investigated whether the stable isotopic compositions of Daphnia and their ephippia are influenced by temperature by performing the experiment at two different temperatures
Significant differences were found between most treatments for all investigated Daphnia stable isotope ratios, even in cases where we expected no differences based on the manipulations
Summary
J. Schilder et al.: The stable isotopic composition of Daphnia ephippia autotrophic sources of organic matter at the base of a food web (DeNiro and Epstein, 1978; Vander Zanden and Rasmussen, 1999; McCutchan et al, 2003). The δ13C values of the fossil head capsules of benthic larvae of non-biting midges (Chironomidae) and δ13C values of the remains of water fleas of the genus Daphnia (Cladocera) have been used to investigate past changes in carbon cycling and energy pathways in lake food webs (Perga, 2011; Wooller et al, 2012; van Hardenbroek et al, 2013; Belle et al, 2014; Frossard et al, 2014). Past variations in lake water δ18O values have been reconstructed by analyzing the δ18O values of fossil chironomid head capsules (Wooller et al, 2004; Verbruggen et al, 2010b), and a correspondence has been found between δ18O values of lake water and of chironomid head capsules and Daphnia ephippia buried in surface sediments (Verbruggen et al, 2011)
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