Chemical composition of phytoplankton as the determinant of food quality

The trophic transfer across phytoplankton-zooplankton interface is crucially important in aquatic food webs. The factors regulating the trophic transfer efficiency have been widely studied. There is an increasing awareness of food quality in terms of chemical composition of phytoplankton as the major control of the phytoplankton-zooplankton interaction via bottom-up processes. Nutrient availability has broad effects on chemical composition of phytoplankton. Other factors, e.g., dilution rate in continuous and semicontinuous cultures, can interact with nutrient supply and affect phytoplankton food quality, hence affecting the performance of zooplankton. This study aimed to investigate the effects of nitrogen (N):phosphorus (P) supply ratios and growth rates (dilution rates) on elemental and biochemical composition of marine phytoplankton, as well as the effects of food quantity and quality on the trophic transfer of essential chemicals and the performance of copepods. For this purpose, laboratory experiments were firstly conducted with three species of marine phytoplankton in semicontinuous cultures to test the interactive effect of five N:P supply ratios and four growth rates. Subsequent copepod experiments were performed with one species of marine copepods (Acartia tonsa) feeding on one phytoplankton species (Rhodomonas sp.), where the factors of food quantity and quality were crossed. The results in this thesis highlight the importance of simultaneous consideration of elemental and biochemical food quality for understanding the trophic transfer of energy and matter in food webs. Evaluating responses of this mutual regulation to multiple ambient factors is a necessary step towards the phytoplankton-zooplankton relationship in more realistic scenarios that will allow in the future the prediction of zooplankton’s performances in changing aquatic environments.