Seasonal variation in biochemical and energy content of size-fractionated zooplankton in the Bay of Marseille (North-Western Mediterranean Sea)

Abstract Zooplankton plays a prominent role in marine pelagic food webs, but its contribution to organic matter transfer from phytoplankton to upper level consumers is modulated by size and specific composition. The biochemical composition (protein, carbohydrate and lipid concentrations) and energy content of size- and group-fractionated zooplankton were analyzed in the North-Western Mediterranean Sea (Marseille) over 18 months. Proteins were the most abundant biochemical compounds in all size classes and zooplankton groups, and provided the largest part of the energy content. The medium (200–1000 μm) size classes, mainly composed of copepods, crustacean larvae and eggs, presented a higher protein and energy content than both the smallest (80–200 μm) size class, mainly composed of phytoplankton, and the largest (>1000 μm) size classes, composed of siphonophores, salps and chaetognaths. Strong and similar seasonal variations in zooplankton biochemical composition were observed in all size classes, with higher energy content associated with cold oxygen- and nutrient-rich waters in spring 2017 and winter 2018. Thus, the biochemical composition of plankton size classes and groups differed and varied seasonally along with environmental parameters. This novel approach in the North-Western Mediterranean Sea may be further related with studies on the energy and nutrient transfer into pelagic food webs and contribute to understand how zooplanktivorous fish feeding preferences may be related to the quality of zooplankton prey.