Trophic ecology of zooplankton at a frontal transition zone: fatty acid signatures at the subtropical convergence, Southern Ocean

Abstract

Regional variations in fatty acid composition were assessed in zooplankton communities situated immediately north and south of the subtropical convergence (STC), where warm (maximum 218C) nutrient-poor waters of the southwest Indian Ocean converge with cool (minimum 118C) nutrient-rich subantarctic waters of the Southern Ocean. I hypothesized that food web structures differ from north to south based on average spatial differences in primary productivity, and that zooplankton in the more productive region show enhanced herbivorous feeding. Several taxonomic groups including euphausiids showed differences in their fatty acid signatures from north to south, indicating the existence of measurable within- and among-species differences in trophic relationships between the two communities, presumably stemming from variations in food quality and availability. The indices 22:6v3/20:5v3, 18:1v9/18:1v7 and Sv3/Sv6 indicated that carnivory was the dominant feeding mode in the north, whereas herbivory was more prevalent in the south, a pattern that was not detected in the same region using stable isotope ratios. Such conflicting results substantiate the importance of utilizing more than one method to investigate feeding relationships in aquatic systems. Fatty acid profiles of the amphipod Themisto gaudichaudii revealed some distinct herbivory markers, indicating more complex feeding habits by this important pelagic species than previously reported. My findings shed new light on the regional variations in zooplankton diet and food web dynamics in a poorly understood but critically important frontal zone bordering the Southern Ocean.