The stable isotopic composition of <i>Daphnia</i> ephippia reflects changes in δ¹³C and δ¹⁸O values of food and water

• Main Authors: J. Schilder, C. Tellenbach, M. Möst, P. Spaak, M. van Hardenbroek, M. J. Wooller, O. Heiri
• Format: Article
• Language: English
• Published: Copernicus Publications 2015-06-01
• Series: Biogeosciences
• Online Access: http://www.biogeosciences.net/12/3819/2015/bg-12-3819-2015.pdf
• ISSN: 1726-4170; 1726-4189

The stable isotopic composition of fossil resting eggs (ephippia) of <i>Daphnia</i> 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 <i>Daphnia</i>, of their diet and of the environmental water have yet to be confirmed in a controlled experimental setting. We performed experiments with <i>Daphnia pulicaria</i> 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 (&delta;¹⁵N value) of the algae was similar for all treatments. At 12 °C, differences in algal δ¹³C values and in &delta;¹⁸O values of water were reflected in those of <i>Daphnia</i>. The differences between ephippia and <i>Daphnia</i> stable isotope ratios were similar in the different treatments (δ¹³C: +0.2 ± 0.4 &permil; (standard deviation); δ¹⁵N: −1.6 ± 0.4 &permil;; δ¹⁸O: −0.9 ± 0.4 &permil;), 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 <i>Daphnia</i> 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 <i>Daphnia</i> (to −1.3 ± 0.3 &permil;). This may have been due to microbial processes or increased algal respiration rates in the experimental containers, which may not affect <i>Daphnia</i> in natural environments. There was no significant difference in the offset between δ¹⁸O and &delta;¹⁵N values of ephippia and <i>Daphnia</i> between the 12 and 20 °C treatments, but the δ¹⁸O values of <i>Daphnia</i> and ephippia were on average 1.2 &permil; lower at 20 °C than at 12 °C. We conclude that the stable isotopic composition of <i>Daphnia</i> ephippia provides information on that of the parent <i>Daphnia</i> and of the food and water they were exposed to, with small offsets between <i>Daphnia</i> and ephippia relative to variations in <i>Daphnia</i> 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 <i>Daphnia</i> body tissue and ephippia. This aspect deserves attention in further controlled experiments.