Description: Increasing upwelling intensity and shoaling of the oxygen minimum zone (OMZ) is projected for Eastern Boundary Upwelling Systems (EBUSs) under ocean warming which may have severe consequences for mesopelagic food webs, trophic transfer, and fish production also in the Humboldt Current Upwelling System (HUS). To improve our mechanistic understanding, from February 23, 2017 until April 14, 2017 we performed a 50 days mesocosm experiment in the northern HUS (off Callao Bay, Peru) and monitored the zooplankton development prior to and following a simulated upwelling event through the addition of deeper water of two different OMZ-influenced subsurface waters to four of in total eight mesocosms. To elucidate plankton dynamics and trophic relationships, we followed the temporal development of the mesozooplankton community in relation to that of phytoplankton, analyzed the fatty acid composition and gut fluorescence of dominant copepods, and determined the stable isotope (SI) and elemental composition (C:N) of dominant zooplankton taxa. Zooplankton samples were collected from the mesocosms over the entire experiment duration using an Apstein net (17 cm diameter, 100 µm mesh) to determine abundance and taxonomic composition of the zooplankton community, and to analyze fatty acid composition, gut fluorescence and elemental composition of dominant zooplankton. Furthermore, abundance and biomass of zooplankton groups was estimated from scanned ZooScan images.

Description: Biomass of zooplankton taxa in µg DM per liter as determined by ZooScan, using published area to dry weight relationships (Lehette & Hernandez-Leon 2009). Each data point is one sampling day (date) in one mesocosm (MK). For details on experimental treatments and sampling, refer to Bach et al. 2021 (https://doi.org/10.5194/bg-17-4831-2020) and Ayon et al. 2022 (https://doi.org/10.5194/bg-2022-157). Raw images are stored in https://ecotaxa.obs-vlfr.fr/prj/3784. All taxonomic categories are self-expanatory.

Description: Zooplankton species/groups abundance table per mesocosm and sampling day. Abundances are given as individual per m-3 and individuals per liter.

Description: Fatty acid composition data for the two dominant copepods in the mesocosms (Paracalanus sp. and Hemicyclops sp.).

Description: Gut fluorescence and C/N ratio of Paracalanus sp. determined during two occassions during the mesocosm experiment (Sampling Day 21/22 and 34/35).

Description: This dataset shows abundance of zooplankton taxa in individuals per liter as determined by ZooScan. Each data point is one sampling day (date) in one mesocosm (MK). For details on experimental treatments and sampling, refer to Bach et al. 2021 (https://doi.org/10.5194/bg-17-4831-2020) and Ayon et al. 2022 (https://doi.org/10.5194/bg-2022-157). Raw images are stored in https://ecotaxa.obs-vlfr.fr/prj/3784. All taxonomic categories are self-explanatory.

Description: The northern Humboldt Current upwelling system (HCS) belongs to the most productive marine ecosystems, providing five to eight times higher fisheries landings per unit area than other coastal upwelling systems. To solve this “Peruvian puzzle”, to elucidate the pelagic food-web structure and to better understand trophic interactions in the HCS, a combined stable isotope and fatty acid trophic biomarker approach was adopted for key zooplankton taxa and higher trophic positions with an extensive spatial coverage from 8.5 to 16°S and a vertical range down to 1,000 m depth. A pronounced regional shift by up to ∼5‰ in the δ15N baseline of the food web occurred from North to South. Besides regional shifts, δ15N ratios of particulate organic matter (POM) also tended to increase with depth, with differences of up to 3.8‰ between surface waters and the oxygen minimum zone. In consequence, suspension-feeding zooplankton permanently residing at depth had up to ∼6‰ higher δ15N signals than surface-living species or diel vertical migrants. The comprehensive data set covered over 20 zooplankton taxa and indicated that three crustacean species usually are key in the zooplankton community, i.e., the copepods Calanus chilensis at the surface and Eucalanus inermis in the pronounced OMZ and the krill Euphausia mucronata, resulting in an overall low number of major trophic pathways toward anchovies. In addition, the semi-pelagic squat lobster Pleuroncodes monodon appears to play a key role in the benthic-pelagic coupling, as indicated by highest δ13C’ ratios of −14.7‰. If feeding on benthic resources and by diel vertical migration, they provide a unique pathway for returning carbon and energy from the seafloor to the epipelagic layer, increasing the food supply for pelagic fish. Overall, these mechanisms result in a very efficient food chain, channeling energy toward higher trophic positions and partially explaining the “Peruvian puzzle” of enormous fish production in the HCS.

Description: Highlights: • Environmental conditions cause specific zooplankton life strategies. • No ontogenetic or diel vertical migration in the life cycle of Calanus chilensis. • Spatial expansion of Calanus chilensis secondary production far offshore. • Compacted surface biomass of Calanus chilensis allows easy foraging by anchovy. Abstract: Calanid copepods of the genera Calanus and Calanoides are key components of zooplankton communities in upwelling systems. Here, we compare the life-history traits of Calanus chilensis from the Humboldt Current Systems (HCS) off northern Peru and its counterpart Calanoides natalis from the northern Benguela Current System (BCS) off Namibia. A comprehensive data set of the distribution and abundance patterns of these species along extensive horizontal and vertical scales is presented. C. chilensis from the HCS was almost exclusively restricted to the surface layer (50–0 m) above the oxygen minimum zone (OMZ), whereas C. natalis from the BCS inhabited the entire water column down to 800 m performing ontogenetic vertical migration (OVM) through the OMZ. Resting stages of C. natalis at depth accumulated high amounts of lipid (30–60% of dry mass, DM), whereas C. chilensis did not rely on lipid reserves. These findings confirm that the life cycle of C. chilensis does not include OVM with diapause at depth. Surprisingly, the regional distribution of C. chilensis secondary production extended much further offshore (〉200 km from the coast) than is typical of other coastal upwelling systems. Deviating environmental conditions forced the two key calanid species to develop specific, but different life strategies for HCS and BCS. Compacted biomass concentrations of C. chilensis in the surface layer from the shelf (≤3 g DM m−2) to offshore waters (≤1.5 g DM m−2) facilitate easy and efficient foraging by predators such as juvenile Peruvian anchovies. In contrast, a large fraction of the C. natalis biomass occurs within the OMZ and is thus out of reach for hypoxia-sensitive predators. Calanoid copepods (e.g. C. chilensis) play a crucial role as important prey for growth and recruitment of small pelagic fish. Thus, the compacted biomass and high productivity of C. chilensis at the surface derived from its adaptive life-history traits (no OVM) may explain the superior trophic transfer efficiency and hence enormous fisheries yield of the HCS compared to the BCS.