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 adult s), E. longirostris (adults) and Nematoscelis megalops (adults) were invest igated using samples collected in the vicinity of the Prince Edward archipe lago during austral autumn (April/May) of 1998 on voyage 87 of the MV 'SA A gulhas'. Gut content analysis and stable nitrogen isotope measurements (del ta N-15) were performed on the same individuals. For comparative purposes, nitrogen isotope ratios of the copepod Calanus simillimis, the hyperid amph ipod Themisto gaudichaudi and the fish Ceratoscopelus warmingi were also an alysed. The results of the gut content analysis for E. vallentini juveniles showed that there was a high contribution of phytoplankton to the diet. St able nitrogen isotope analysis supported this and indicated that E. vallent ini juveniles occupied a similar trophic position to that of the copepod (m ean 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 car nivory to the diet was difficult to determine. High phytoplankton and low m esozooplankton contributions to the diet from the gut content analysis sugg ested a predominantly herbivorous diet, However, the stable nitrogen isotop e results showed that E. vallentini adults were closely grouped with T. gau dichaudi (mean delta N-15 values of 3.66 and 4.13 parts per thousand respec tively), indicating a high degree of omnivory and a relatively large hetero trophic 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 l arge amounts of both phytoplankton and metazoan prey. However, the stable n itrogen isotope analysis indicated a high degree of carnivory (mean delta N -15 = 6.88 parts per thousand). Although the gut content analysis showed th at N. megalops had a lower contribution of mesozooplankton to the diet comp ared to that of E. longirostris, the Stable isotope analysis showed that th ese 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. meg alops adults are carnivorous. However, adults of E, vallentini and E. longi rostris appear to have higher contributions of heterotrophic carbon to thei r diets than has been assumed and may occupy higher trophic positions than initially predicted.