Trophic positions of three euphausiid species from the Prince Edward Islands (Southern Ocean): implications for the pelagic food web structure

The relative trophic positions of Euphausia vallentini (juveniles and adults), E. longirostris (adults) and Nematoscelis megalops (adults) were investigated using samples collected in the vicinity of the Prince Edward archipelago during austral autumn (April/May) of 1998 on voyage 87 of the MV 'SA Agulhas'. Gut content analysis and stable nitrogen isotope measurements (delta N-15) were performed on the same individuals. For comparative purposes, nitrogen isotope ratios of the copepod Calanus simillimis, the hyperid amphipod Themisto gaudichaudi and the fish Ceratoscopelus warmingi were also analysed. The results of the gut content analysis for E. vallentini juveniles showed that there was a high contribution of phytoplankton to the diet. Stable nitrogen isotope analysis supported this and indicated that E. vallentini juveniles occupied a similar trophic position to that of the copepod (mean delta N-15 values of 2.38 and 1.72 parts per thousand respectively). E. vallentini adults were found to be omnivorous, but the contribution of carnivory to the diet was difficult to determine. High phytoplankton and low mesozooplankton contributions to the diet from the gut content analysis suggested a predominantly herbivorous diet, However, the stable nitrogen isotope results showed that E. vallentini adults were closely grouped with T. gaudichaudi (mean delta N-15 values of 3.66 and 4.13 parts per thousand respectively), indicating a high degree of omnivory and a relatively large heterotrophic contribution to the diet. Irrespective of the degree of carnivory, a dietary shift with an increase in size was evident for this species. The gut content analysis of E. longirostris showed that this species consumed large amounts of both phytoplankton and metazoan prey. However, the stable nitrogen isotope analysis indicated a high degree of carnivory (mean delta N-15 = 6.88 parts per thousand). Although the gut content analysis showed that N. megalops had a lower contribution of mesozooplankton to the diet compared to that of E. longirostris, the Stable isotope analysis showed that these 2 species occupied a similar position (N. megalops mean delta N-15 = 6.83 parts per thousand). The findings of this study support findings in the literature and show that E. vallentini juveniles are herbivorous and N. megalops adults are carnivorous. However, adults of E, vallentini and E. longirostris appear to have higher contributions of heterotrophic carbon to their diets than has been assumed and may occupy higher trophic positions than initially predicted.