Variability in delta C-13 values between individual Daphnia ephippia: Implications for palaeo-studies

The stable carbon isotope ratio (delta C-13 value) of Daphnia spp. resting egg shells (ephippia) provides information on past changes in Daphnia diet. Measurements are typically performed on samples of >= 20 ephippia, which obscures the range of values associated with individual ephippia. Using a recently developed laser ablation-based technique, we perform multiple delta C-13 analyses on individual ephippia, which show a high degree of reproducibility (standard deviations 0.1-0.5 parts per thousand). We further measured delta C-13 values of 13 ephippia from surface sediments of three Swiss lakes. In the well-oxygenated lake with low methane concentrations, delta C-13 values are close to values typical for algae (-31.4 parts per thousand) and the range in values is relatively small (5.8 parts per thousand). This variability is likely driven by seasonal (or inter-annual) variability in algae delta C-13 values. In two seasonally anoxic lakes with higher methane concentrations, average values were lower (-41.4 and 43.9 parts per thousand, respectively) and the ranges much larger (10.7 and 20.0 parts per thousand). We attribute this variability to seasonal variation in incorporation of methane-derived carbon. In one lake we identify two statistically distinct isotopic populations, which may reflect separate production peaks. The potentially large within-sample variability should be considered when interpreting small-amplitude, short-lived isotope excursions based on samples consisting of few ephippia. We show that measurements on single ephippia can be performed using laser ablation, which allows for refined assessments of past Daphnia diet and carbon cycling in lake food webs. Furthermore, our study provides a basis for similar measurements on other chitinous remains (e.g. from chironomids, bryozoans). (C) 2018 Elsevier Ltd. All rights reserved.