LAUSR

Food quality is recognized as a key parameter of food web functioning in which zooplankton plays a crucial role not only in linking lower to upper trophic levels but also in transforming the quality of the organic matter available to predators. The influence of size and taxonomic group composition of zooplankton in these processes was assessed in eastern Kerguelen waters (Southern Ocean) at the onset of the spring bloom in 2011. Biochemical (lipids, proteins and carbohydrates) and elemental (carbon and nitrogen) composition were measured in five size—fractions of bulk zooplankton ranging from 80 µm to > 2000 µm and in large copepods, euphausiids, annelids and salps, and energy content was derived from biochemical contents. Proteins were the dominant component of zooplankton dry weight (21.5% dw), followed by lipids (8.9% dw), soluble carbohydrates (2.2% dw) and insoluble carbohydrates (1.0% dw). A concentration increase with zooplankton size for all biochemical components was observed, particularly stronger for proteins and lipids. Copepods and salps provided, respectively, the highest and the lowest amount of lipids and energy. A four-fold increase in energy content was observed from the smallest to the largest fraction inducing a significant increase (> 10 kJ g−1 dw) in the quality of zooplankton matter. This may explain why large zooplankton represent a major food resource for numerous fish, seabirds and marine mammals in the Southern Ocean. Such unique results are required to better quantify energy dynamics in polar food webs.