The stable isotopic composition of <i>Daphnia</i> ephippia reflects changes in δ<sup>13</sup>C and δ<sup>18</sup>O values of food and water

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 (δ 13 C value) of the algae, stable oxygen isotopic composition (δ 18 O value) of the water and the water temperature, respectively. The stable nitrogen isotopic composition (δ 15 N value) of the algae was similar for all treatments. At 12 • C, differences in algal δ 13 C values and in δ 18 O values of water were reflected in those of Daphnia. The differences between ephippia and Daphnia stable isotope ratios were similar in the different treatments (δ 13 C: +0.2 ± 0.4 ‰ (standard deviation); δ 15 N: −1.6 ± 0.4 ‰; δ 18 O: −0.9 ± 0.4 ‰), indicating that changes in dietary δ 13 C values and in δ 18 O values of water are passed on to these fossilizing structures. A higher water temperature (20 • C) resulted in lower δ 13 C values in Daphnia and ephippia than in the other treatments with the same food source and in a minor change in the difference between δ 13 C 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 δ 18 O and δ 15 N values of ephippia and Daphnia between the 12 and 20 • C treatments, but the δ 18 O 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 δ 13 C, δ 15 N and δ 18 O values of Daphnia body tissue and ephippia. This aspect deserves attention in further controlled experiments.

Effects of temperature on food isotopic integrity and trophic fractionation in Chironomus riparius in laboratory experiments

“…Our findings of D 15 N estimates also confirmed that any temperature effects on D 15 N values for aquatic consumers are not straightforward. Indeed, findings from other aquatic organisms suggested a general trend of decreasing D 15 N values with increasing temperature (Power et al, 2003;Barnes et al, 2007), whereas other studies reported an opposite trend (Schilder et al, 2015;Masclaux & Richoux, 2017). In our study, temperature effects on D 13 C followed different patterns when calculated with fresh or altered isotopic baselines, and with or without larval lipid normalization.…”

“…5%, they also reported contradictory and unclear relationships with temperature. While Power et al (2003) and Masclaux & Richoux (2017) reported increase in D 13 C values with increasing temperature, Schilder et al (2015) found the opposite relationships, and they found similar contradicting observations for D 15 N. A range of confounding factors could explain this lack of consistency among studies (Masclaux & Richoux, 2017). For example, lipids exhibit lower d 13 C values than other constituents (protein) in animal tissues (DeNiro & Epstein, 1977), and a lipid normalization of larval d 13 C values could be required to get more reliable data interpretations (McConnaughey & McRoy, 1979;Smyntek et al, 2007).…”

“…Indeed, high fluctuations in environmental temperature could potentially induce substantial error in isotope-based approaches. Only a few experimental studies have quantified trophic fractionations for aquatic organisms across a wide range of temperatures (Power et al, 2003;Barnes et al, 2007;Schilder et al, 2015;Masclaux & Richoux, 2017). While these studies reported large uncertainties in trophic fractionation values with D 13 C values ranging from -1 to ?…”

The Incredible Lightness of Being Methane-Fuelled: Stable Isotopes Reveal Alternative Energy Pathways in Aquatic Ecosystems and Beyond

“…Further elegant experiments have shown 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 during formation. Schilder et al (2015b) demonstrated that there were only small offsets between Daphnia and ephippia relative to the range of variation in Daphnia stable isotopic composition reported from down-core studies. Interestingly however, their work also indicated that temperature may have a minor influence on the δ 13 C, δ 15 N, and δ 18 O values of Daphnia body tissue and ephippia which has implications for water temperature reconstruction work using oxygen isotopes, as well as highlighting the care with which controlled feeding experiments need to be conducted (sensu Perga and Grey, 2010).…”

Expanding the Isotopic Toolbox: Applications of Hydrogen and Oxygen Stable Isotope Ratios to Food Web Studies

“…Only 10% of the oxygen in chitin from aquatic crustaceans is derived from food (Nielson and Bowen, 2010), while the dietary contribution of O to human hair protein was estimated to be ∼8% , although that study may have underestimated the atomic routing of O from diet to hair keratin. This implies that changes in δ 18 O values of environmental and body water are the main drivers of the tissue O isotopic composition variability Wang et al, 2009;Nielson and Bowen, 2010;Soto et al, 2013c;Schilder et al, 2015; but see cautions for feline carnioves, Pietsch et al, 2011), and that little trophic information is obtained by using δ 18 O values.…”