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.

Description: The Humboldt Current Upwelling System (HCS) is the most productive eastern boundary upwelling system (EBUS) in terms of fishery yield on the planet. EBUSs are considered hotspots of climate change with predicted expansion of mesopelagic oxygen minimum zones (OMZs) and related changes in the frequency and intensity of upwelling of nutrient-rich, low-oxygen deep water. To increase our mechanistic understanding of how upwelling impacts plankton communities and trophic links, we investigated mesozooplankton community succession and gut fluorescence, fatty acid and elemental compositions (C, N, O, P), and stable isotope (δ13C, δ15N) ratios of dominant mesozooplankton and microzooplankton representatives in a mesocosm setup off Callao (Peru) after simulated upwelling with OMZ water from two different locations and different N:P signatures (moderate and extreme treatments). An oxycline between 5 and 15 m with hypoxic conditions (〈50 µmol L−1) below ∼10 m persisted in the mesocosms throughout the experiment. No treatment effects were determined for the measured parameters, but differences in nutrient concentrations established through OMZ water additions were only minor. Copepods and polychaete larvae dominated in terms of abundance and biomass. Development and reproduction of the dominant copepod genera Paracalanus sp., Hemicyclops sp., Acartia sp., and Oncaea sp. were hindered as evident from accumulation of adult copepodids but largely missing nauplii. Failed hatching of nauplii in the hypoxic bottom layer of the mesocosms and poor nutritional condition of copepods suggested from very low gut fluorescence and fatty acid compositions most likely explain the retarded copepod development. Correlation analysis revealed no particular trophic relations between dominant copepods and phytoplankton groups. Possibly, particulate organic matter with a relatively high C:N ratio was a major diet of copepods. C:N ratios of copepods and polychaetes ranged 4.8–5.8 and 4.2–4.3, respectively. δ15N was comparatively high (∼13 ‰–17 ‰), potentially because the injected OMZ source water was enriched in δ15N as a result of anoxic conditions. Elemental ratios of dinoflagellates deviated strongly from the Redfield ratio. We conclude that opportunistic feeding of copepods may have played an important role in the pelagic food web. Overall, projected changes in the frequency and intensity of upwelling hypoxic waters may make a huge difference for copepod reproduction and may be further enhanced by varying N:P ratios of upwelled OMZ water masses.

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: A combined stable isotope and fatty acid trophic biomarker approach was adopted for key zooplankton taxa and higher trophic positions of the northern Humboldt Current System to elucidate the pelagic food-web structure and to better understand trophic interactions. Samples covered an extensive spatial range from 8.5°S to 16°S and a vertical range down to 1,000 m depth. Immediately after each haul, specimens were sorted alive in the lab and apparently live and healthy individuals were stored in vials and deep-frozen at -80°C until further lipid and stable isotope analyses. The comprehensive data set covered over 20 zooplankton taxa and indicated that three biomass-rich crustacean species usually dominated the zooplankton community, i.e., the copepods Calanus chilensis at the surface and Eucalanus inermis in the pronounced oxygen minimum zone 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. By partly feeding on benthic resources and by diel vertical migration, P. monodon provides a unique pathway for returning carbon and energy from the sea floor to the epipelagic layer, increasing the food supply for pelagic fish.

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: Stable isotopes of carbon and nitrogen (δ13C, δ15N) were analyzed using a mass spectrometer with IAEA-VPDB (IAEA-C1) and atmospheric air (IAEA-N1) as standards, respectively. Two different approaches were used to determine a δ15N baseline for the food web of the northern Humboldt Current System. i) Particulate organic matter (POM) samples representing trophic position 1 and ii) Filter-feeding salps (Iasis cylindrica) as primary consumers representing trophic position 2. In general, low isotope ratios at the surface occurred in Thaliacea (I. cylindrica and Pyrosoma sp.). δ15N values of copepods inhabiting waters 〈50 m depth ranged from 3.9 ± 0.2‰ in Nannocalanus minor to 6.7 ± 0.6‰ in Calanus chilensis. Deeper-living copepods (〉200 m depth) within the oxygen minimum zone had about 4 to 8‰ higher δ15N values (~11-15‰) than surface living ones. For the diel vertical migrating euphausiids, mean δ15N and δ13C' values were in a similar range as for copepods from the upper water layers. The squat lobster Pleuroncodes monodon had extraordinarily high δ13C'. Among the key fish species, the eastern Pacific bonito Sarda chilensis had a slightly higher δ15N value compared to the anchovy Engraulis ringens. In some species a strong increase of the δ15N ratio of up to ~5‰ was determined from North (8.5°S) to South (16°S). These pronounced differences in d15N values highlight the importance of adequate baseline values to avoid misinterpretations of a species' regional feeding behaviour. In order to compensate for the reginal shift, using a filter-feeding salp as reference for herbivorous consumers seemed advantageous. In fact, calculated trophic positions for C. chilensis and other species did not differ between regions, when shifts in the baseline were taken into account.