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.
During the cruise we sampled physical oceanographic parameters, biogeochemical parameters, phytoplankton, and zooplankton communities through six transects perpendicular to the Peruvian coast. Our aim was to characterize the upwelling history of each transect and link it to the phyto- and zooplankton communities present at each station.
During the mesocosm experiment we tested the effect of light and nutrient limitation by recreating winter and summer conditions under different upwelling scenarios by shading half of the mesocosms and adding different percentages of nutrient-rich deep water. To maximise our knowledge yield, this study was also joined by international scientist. These were supported by the EU-project AQUACOSM, which is aiming to facilitate and connect mesocosm studies all over Europe.
Some of the research questions that we are addressing within the Coastal Upwelling System in a Changing Ocean (CUSCO) project are:
- What is the functional relationship between upwelling intensity and phytoplankton productivity and the factors controlling it in the HUS?
- What is the functional relationship between upwelling intensity and export potential and the factors controlling it?
- Is the mesocosm-derived functional relationship valid in the natural environment?
- Can we make reliable projections on future developments of the HUS productivity under climate change with the new understanding of the processes regulating the relationship between upwelling intensity and productivity?
Project: Coastal Upwelling Systems in a Changing Ocean (CUSCO)
Scientists from our group involved:
Dr. Mar Fernández-Méndez, Dr. Michael Meyerhöfer, and Prof. Dr. Ulf Riebesell (WP2: Scaling upwelling intensity with plankton community production)