Plankton organisms play a crucial role in the functioning of aquatic ecosystems as they serve as a basis in terms of primary production and the transfer of matter and energy from one trophic level to the next. Thus, the majority of benthic and pelagic flux processes are fuelled by energy derived from the plankton. From an anthropocentric point of view, top-predators (e.g. fish, birds, mammals) at the uppermost trophic levels might be considered as more relevant compared to those organisms at the base of aquatic food webs. However, from an ecosystems’ perspective, lower food web dynamics are pivotal, as the baseline is inevitably and constantly subject to changes as a result from external drivers. Accordingly, changes at the basis are directly transferred up the food web. In order to fully understand processes in the plankton, to enable accurate food web models and to generate more realistic energy budgets, it is essential to consider external drivers that act directly or indirectly at the base of food webs. The present habilitation provides a comprehensive study on how external drivers affect lower food web dynamics by including both, bottom-up and top-down control processes. CHAPTER 1 addresses the role of micro-zooplankton (MZP) as a trophic link between phytoplankton and mesozooplankton. It shows that MZP can be considered as a key component of the zooplankton throughout the year and points at its relevance particularly during diatom blooms in spring. MZP was proven to play a substantial role as phytoplankton grazers showing an even higher grazing efficiency when compared to copepods. Further MZP was shown to play a crucial role in modulating phytoplankton blooms and in providing a high quality food source for copepods. Commensalism was proven to be an efficient strategy when different MZP species compete for resources. Climate-change is considered as a major threat of aquatic ecosystems. The degree to which plankton communities will be affected strongly depends on the responses of plankton organisms to climate change related external drivers (e.g. global warming and ocean acidification). CHAPTER 2 addresses the responses of micro- and mesozooplankton communities to changes in external drivers under future greenhouse conditions. While the timing of phytoplankton was shown to be only slightly affected by warming, the growth of MZP and copepod nauplii showed a strong acceleration by temperature. Further, higher grazing rates and a strong dietary overlap between MZP and copepods could be observed in relation to warming as well as a strong suppression of MZP by overwintering copepod densities. Analyses across all experiments within a series of indoor mesocosm experiments demonstrated some general, warming-induced trends e.g. reduced time-lags between phytoplankton and MZP, earlier bloom timings of almost all functional groups in the plankton, changes in peak biomass and size structure of phytoplankton as well as enhanced grazing. Further, changes in the light climate proved to affect the timing, peak biomass and cell size of phytoplankton. The consideration of indirect, trophic cascading effects with warming resulted in e.g. enhanced copepod grazing on medium-sized phytoplankton. Investigations on the vulnerability of MZP to ocean acidification demonstrated a high tolerance of Arctic MZP communities to OA as neither direct nor indirect effects on MZP composition and diversity was observed. Nutrient-limitations and the consequences associated with nutritional imbalances in the plankton are presented in CHAPTER 3. Stable isotopes (SI) were used to trace nutrient limitations and relate these to the trophic position of consumers. A strong variation in SI was found between different algal species and in relation to different nutrient regimes. Further, the SI enrichment of consumers was significantly affected by their diets’ nutrient composition. Further, a strong contribution of ’new’ nitrogen to the pelagic food web in a meso-oligotrophic system was found which was associated with the utilization of aerosol nitrate by unicellular cyanobacteria. This signal was directly transferred to higher trophic thus providing new insights into the food webs and nitrogen budgets of sub-tropical and tropical oceans. A series of food chain experiments using 3- to 4-trophic levels to investigate the effects of nutrient limitations on different trophic levels showed a rather weak homeostatic ability of herbivores with regard to their nutrient content, a finding which enabled new views in the field of ecological stoichiometry. Further, it was shown that nutrient-limitations of primary producers propagate up the food web. Nutrient-limited diets affected herbivores by reducing growth and reproduction rates of consumers thus resulting in lower densities and lower condition of the latter. When a trophic intermediary (e.g. a protist grazer) was added to the food chain, nutritional imbalances were buffered thus improving the food quality for higher trophic levels. The studies presented here show that changes occurring at the base of the food web are directly transferred to higher trophic levels. Thus, changes in environmental parameters (nutrients, temperature and CO2) will have far-reaching consequences for aquatic ecosystems. The complexity of trophic interactions shows that the predictability of implications associated to e.g. eutrophication and climate change is still limited. Therefore, it is inevitable to increase efforts to combine experimental and modeling approaches thus enabling the simulation of complex interactions and setting existing and future data sets into a broader context.