Trophic pathways of phytoplankton size classes through the zooplankton food web over the spring transition period in the north-west Mediterranean Sea

International audienceKnowledge of the relative contributions of phytoplankton size classes to zooplankton biomass is necessary to understand food-web functioning and response to climate change. During the Deep Water formation Experiment (DEWEX), conducted in the northwest Mediterranean Sea in winter (February) and spring (April) of 2013, we investigated phytoplankton-zooplankton trophic links in contrasting oligotrophic and eutrophic conditions. Size fractionated particulate matter (pico-POM, nano-POM, and micro-POM) and zooplankton (64 to >4000 lm) composition and carbon and nitrogen stable isotope ratios were measured inside and outside the nutrient-rich deep convection zone in the central Liguro-Provencal basin. In winter, phytoplankton biomass was low (0.28 mg m 23) and evenly spread among picophytoplankton, nanophyto-plankton, and microphytoplankton. Using an isotope mixing model, we estimated average contributions to zooplankton biomass by pico-POM, nano-POM, and micro-POM of 28, 59, and 15%, respectively. In spring, the nutrient poor region outside the convection zone had low phytoplankton biomass (0.58 mg m 23) and was dominated by pico/nanophytoplankton. Estimated average contributions to zooplankton biomass by pico-POM, nano-POM, and micro-POM were 64, 28 and 10%, respectively, although the model did not differentiate well between pico-POM and nano-POM in this region. In the deep convection zone, spring phyto-plankton biomass was high (1.34 mg m 23) and dominated by micro/nano phytoplankton. Estimated average contributions to zooplankton biomass by pico-POM, nano-POM, and micro-POM were 42, 42, and 20%, respectively, indicating that a large part of the microphytoplankton biomass may have remained ungrazed. Plain Language Summary The grazing of zooplankton on algal phytoplankton is a critical step in the transfer of energy through all ocean food webs. Although microscopic, phytoplankton span an enormous size range. The smallest picophytoplankton are generally thought to be too small to be directly grazed by zooplankton, resulting in less efficient energy transfer through the food web. This has implications for our future oceans where warming and lower nutrient supply are predicted to favor picophytoplankton over the larger nanosize and microsize classes. We tested the importance of phytoplankton size classes in the transfer of energy to zooplankton in the northwest Mediterranean Sea, where conditions naturally result in contrasting regions of small and large phytoplankton dominance. Contrary to expectation, biochemical tracers showed that microphytoplankton never contributed more than 20% to zooplankton biomass, even in regions where microphytoplankton were plentiful. On the other hand, picophytoplankton contributed 25-65% to zooplankton biomass. This finding indicates that there are well-established food-web pathways from picophytoplankton to zooplankton, and that these pathways play an important role even in ocean regions where microphytoplankton dominate. Accordingly, a decline in phytoplankton size classes may have a greater effect on carbon sequestration than on food-web productivity