A study of the δ13C offset between chironomid larvae and their exuvial head capsules: implications for palaeoecology

We conducted a rearing experiment with the chironomid species Chironomus riparius to assess the relationship between the delta C-13 values of chironomid larvae and the delta C-13 values of their exuvial head capsules. Our experiment was also designed to study the extent of the trophic fractionation factor (Delta C-13) under different dietary conditions. Three food sources were used (Tetramin, oats and corn), covering a range in delta C-13 values of 14.55 aEuro degrees. For each of the four successive instars, carbon isotope ratios were measured in larval tissues and head capsules. This approach highlighted the variability in delta C-13 for both larvae and their head capsules during larval development. Once the larvae reached the 3rd instar, their delta C-13 values were stabilised and did not significantly differ from their food delta C-13 (Delta C-13 = 0 aEuro degrees). It is probable that the variability in the delta C-13 offset during larval development reflected a difference in the carbon turnover for the chironomid cuticle compared with the whole body. At the 4th instar, the delta C-13 offset did not significantly differ between the three food sources and was -0.9 +/- A 0.2 aEuro degrees. The proposed Delta C-13 and delta C-13 offset values can be considered as a first step for the reconstructions of the chironomid larvae paleo-diets with the aim of deciphering the different organic carbon sources supporting chironomid larvae productions. However, the influence of the environment (e.g. temperature, oxygen), other food sources (e.g. different nutritive values) as well as taxonomy (i.e. other chironomid species) should be assessed to strengthen the robustness of these results.