A current challenge to ecologists is to predict how compositional shifts at the base of the aquatic food web propagate up to higher trophic levels. Energy or resource availability for upper trophic levels is governed by food-chain length and by factors that regulate the rate of energy transfer from primary producers to consumers. The conceptual ideas are however based on limited empirical data and few studies have tested these concepts at ecosystem level. The goal of this project is to test the overall prediction that phytoplankton community structure and species composition are key regulators of energy and food quality transfer to mesozooplankton. The objectives are to (i) test the stability of fatty acid-specific δ13C isotope between the phyto-zooplankton interface, which will be an important requirement as biomarker for trophic interactions, and (ii) manipulate natural microbial species composition and bacterial activity in mesocosms and measure carbon and nutritional transfer efficiency to copepods and their interactive effects with appendicularians. Copepods are a key link for carbon and nutritional flux and appendicularians are abundant filter-feeding organisms that are expected to increase with climate warming. The microbial food web is an important bridge between primary producers and upper trophic levels, and supports provided from the IRG will concentrate mainly on analysis of the microbial community. Trophic efficiency will be estimated from production rates, dietary sources and nutritional transfer using biomarkers. An improved understanding on how shifts in food-web structure affects carbon flux through the plankton food web will be important because these processes have profound influence on fisheries and climate.This research project will contribute to European excellence and competitiveness by recruiting a scientist with a high-qualified research record, global experience, and Europe-oriented career prospects into the European research network.