The University of Bremen’s contribution to the CUSCO consortium (Coastal Upwelling System in a Changing Ocean) focused on the role of meso‑ and macrozooplankton in the pelagic food web of the Humboldt Current off Peru. From 1 October 2018 to 31 December 2022, researchers Prof. Dr. Wilhelm Hagen, PD Dr. Holger Auel and Dr. Anna Schukat carried out a series of field and laboratory studies aimed at quantifying how zooplankton mediate the transfer of energy and carbon from primary producers to higher trophic levels, thereby influencing the system’s trophic transfer efficiency (TTE). The Humboldt Current is known for producing fishery yields up to eight times higher than other upwelling systems despite comparable primary production, a phenomenon that can only be explained by a markedly higher TTE. The Bremen team sought to identify the mechanisms behind this efficiency by analysing zooplankton biomass, diversity, species composition, respiration, ingestion, and fecal pellet production, and by applying stable isotope (δ¹⁵N, δ¹³C) and fatty‑acid profiling to resolve trophic niches and feeding preferences.

A key element of the work was the detailed study of the dominant krill species Euphausia mucronata. During a 24‑hour station at depth T5, researchers recorded vertical migration patterns of larval, juvenile, and adult stages. Larvae (Calyptopis and Furcilia) remained within 0–50 m throughout the day and night, whereas juveniles and adults migrated from the surface at night to depths of 200–400 m during daylight. Weighted mean depths ranged from 13–37 m for larvae to 200–300 m for adults during the day and 25–110 m at night, with migration amplitudes reaching 275 m for mature individuals. These observations highlight the importance of large filter feeders in structuring the food web and enhancing TTE. Respiration rates of key zooplankton species were measured, although specific numerical values were not reported in the summary. The combination of isotopic and fatty‑acid data allowed the team to delineate trophic interactions and quantify the contribution of zooplankton to carbon fluxes within the upwelling system.

Fieldwork was coordinated through the FS Maria S. Merian expedition (MSM80), which took place from 20 December 2018 to 31 January 2019. Samples collected during this cruise were rapidly transported to Bremen for laboratory analysis. In early 2020, the team prepared mesocosm experiments (KOSMOS) in Lima, Peru, to investigate zooplankton responses to controlled environmental conditions. However, COVID‑19 restrictions in spring 2020 forced an early termination of the KOSMOS study and the relocation of most participants back to Germany, limiting the scope of experimental data. Despite these setbacks, the Bremen subproject produced a comprehensive dataset on zooplankton ecology that feeds into the broader CUSCO objectives of improving predictions of upwelling system responses to climate change and informing sustainable fisheries management.

The project was funded by the German Federal Ministry of Education and Research (BMBF) under the framework program “Research for Sustainable Development” (FONA3) with the grant code 03F0813C. Dr. Schukat served as a research assistant with a 35‑week annual contract from 1 December 2018 to 31 August 2022, supporting both field sampling and laboratory analyses. Dr. Auel led the expedition logistics and data collection, while Prof. Hagen coordinated the overall scientific strategy and integration of results into the CUSCO consortium. The collaboration among the University of Bremen, other CUSCO partners, and Peruvian institutions has yielded critical insights into the mechanisms driving the exceptional productivity of the Humboldt Current, thereby contributing to the development of climate‑resilient marine resource management strategies.