The Humboldt Current Upwelling System (HUS) is the largest and most productive upwelling system in the world in terms of fish harvest. However, the base of the food chain, phytoplankton, does not seem to respond directly to the nutrients brought up by upwelling. During winter, when the upwelling intensity is highest and nutrient concentrations at the surface increase, phytoplankton biomass remains low; while in summer, when the upwelling intensity is reduced, the highest accumulations of photosynthetic biomass appear at the surface. There are several factors which might be responsible for this out-of-phase seasonal cycle: (1) reduced incoming irradiance during winter, (2) changes in mixed layer depth, (3) shifts in the nutrient availability (macro- vs micronutrients, such as iron), and (4) increased top-down control by grazers during winter.
 

A mechanistical understanding of the linkage between upwelling intensity (including all factors influenced by upwelling such as temperature and nutrient concentrations) and phytoplankton community composition and productivity is still lacking. Thus, by following an interdisciplinary approach in collaboration with physical oceanographers (IOW) and zooplankton ecologists (University of Bremen, Hamburg, Kiel and the IGB in Berlin), we aim to understand the factors regulating primary productivity, carbon transfer efficiency, and carbon export in the HUS. In addition, through modeling efforts led by our colleagues from Uni Hamburg and GEOMAR, we will try to improve current predictions of the fate of the HUS under changing upwelling conditions caused by different climate change scenarios. To achieve these goals, we combined a ship-based expedition (MSM80-Panama-Chile December 2018 - January 2019) and a field mesocosm experiment off the coast of Callao (Lima, Perú) in February-April 2020.