The nutritional quality of zooplankton in the sub-antarctic Southern Ocean during late austral summer

Knowledge of the trophic ecology of zooplankton is essential for evaluating their functional roles in marine food webs and nutrient cycling since they represent the link between primary producers and higher trophic levels. Here we investigated the fatty acid (FA) composition of different zooplankton size classes and selected species collected in the vicinity of the sub-Antarctic Kerguelen Islands in late austral summer 2018 as part of the MOBYDICK research project. The analysis revealed that zooplankton FA composition varied significantly across size classes and species but not among stations. Larger zooplankton (>1000 μm) generally had higher total FA (TFA) amounts per dry weight than smaller classes (22.1 ± 3.0 vs. 61.9 ± 11.8 mg g −1 ). Essential FAs (EFA) accounted for 40.5 ± 0.8 % of TFA, with 22:6n-3 (DHA) and 20:5n-3 (EPA) being the most prominent. Diatom trophic markers (TM) were abundant in larger zooplankton size classes, while non-diatom TM were more prevalent in smaller size classes. The FA-based nutritional quality index (NQI) of zooplankton was positively correlated with EFA and DHA, and it was higher than the NQI of phytoplankton concurrently collected, indicating that zooplankton has a better nutritional quality than primary producers. This study highlights the importance of size and species-specific dietary preferences in determining zooplankton FA profiles and the high nutritional quality of this group collected during late austral summer, which significantly contributes to our understanding of zooplankton's ecological role in sub-Antarctic pelagic food webs. • The fatty acid (FA) composition of zooplankton at the Kerguelen Islands was assessed. • Zooplankton FA composition varied across size classes and species investigated. • Larger zooplankton had high total FA amounts with essential FA accounting for 45 %. • Small classes were caractherized by non-diatom trophic marker and the FA 22:6n-3. • We highlight the high nutritional quality of zooplankton during late austral summer.