Ontogenetic differences in isotopic signatures and crop contents of Chaoborus

Intra-specific changes in trophic interactions due to ontogenetic transformations in macro-invertebrates can add to complexity of freshwater food webs. The objectives of this study are therefore to examine isotopic signatures and crop contents of different Chaoborus species and life stages, and to determine whether ontogenetic changes in diet are reflected in their isotopic signatures. Different Chaoborus species and life stages were collected from 15 Precambrian Shield lakes for stable isotope and crop contents analyses. In general, early instar Chaoborus δ13C and δ13CLC (lipid corrected δ13C) signatures were enriched in 13C, while their δ15NDC (Daphnia corrected δ15N) signatures were lower compared to those of late instars and pupae. Larval Chaoborus size was significantly related to their δ13C, δ13CLC&DC (lipid and Daphnia corrected δ13C), and δ15NDC isotopic signatures. Chaoborus crop contents varied among species and larval instars. Generally, rotifers were numerically predominant in crops, with declining abundance in late instars. Larger quantities of copepods, nauplii and cladocerans were found in crops of late instars compared with early instars. Overall, early instars consumed more rotifers and phytoplankton and had lower δ15NDC, whereas late instars consumed comparatively larger quantities of copepods, nauplii, cladocerans and early instars, and had higher δ15NDC. Together, our results show that there are differences among larval Chaoborus life stages. Hence, different instars of these important aquatic predators cannot be grouped together, but should be separated by species and life stage when examining trophic interactions in freshwater food webs.