Trophic importance of the hyperiid amphipod, Themisto gaudichaudi, in the Prince Edward Archipelago (Southern Ocean) ecosystem

The feeding dynamics and predation impact of the hyperiid amphipod, Themisto gaudichaudi, on the zooplankton community in the waters surrounding the Prince Edward Archipelago were investigated at 30 stations in late austral summer (April/May) 1998. Ingestion rates of T. gaudichaudi were estimated using two approaches, the gut fullness index and in vitro incubations. Throughout the investigation mesozooplankton, comprising copepods, pteropods and chaetognaths, numerically and by biomass dominated Bongo samples. Zooplankton abundances and biomass ranged from 8 to 271 ind. m(-3) and between 1.01 and 7.47 mg dwt m(-3), respectively. Densities of T. gaudichaudi during the study were low, never exceeding 0.4 ind. m(-3). Gut content analysis (n = 61) indicates that T. gaudichaudi is a non-selective, opportunistic carnivore generally feeding on the most abundant copepod and chaetognath species. Peaks in feeding activity were recorded at sunrise and sunset, corresponding to their diel vertical migration patterns. Daily rations estimated from in vitro incubations and gut fullness index were equivalent to 1.2-8.7% and between 11.5 and 19.8% of body dry weight, respectively. The predation impact of T. gaudichaudi averaged over the upper 300 m of the water column was low, accounting for <0.4% of the mesozooplankton biomass or <3% of the mesozooplankton secondary production. Indeed, the predation impact is likely to be lower as the contribution of the smaller copepods (e.g. Oithona spp.) to total zooplankton was underestimated due to the sampling gear employed. The low predation impact recorded during this study can be related to low abundances of T. gaudichaudi. It is likely that the importance of T. gaudichaudi as a secondary production consumer in the waters surrounding the Prince Edward Archipelago demonstrates a high degree of spatio-temporal variability.