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Abstract. 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.

Abstract. The Humboldt Current Upwelling System (HUS) is the most productive eastern boundary upwelling system (EBU) in terms of fisheries yield on the planet. EBUs are considered hotspots of climate change with predicted expansion of mesopelagic oxygen minimum zones (OMZs) and related changes in frequency and intensity of upwelling of nutrient-rich/oxygen-low deep-water. To increase our mechanistic understanding of how upwelling impacts on plankton communities and trophic links, we investigated mesozooplankton community succession as well as gut fluorescence, fatty acid and elemental compositions (C, N, O, P), and stable isotope (δ13C, δ15N) ratios of dominant meso- and microzooplankton representatives in a mesocosm setup off Callao (Peru) after simulated upwelling with OMZ water from two different locations and N:P signatures. An oxycline between 5 and 15 m with hypoxic conditions below ~10 m (<50 μL-1) persisted in the mesocosms throughout the experiment. No treatment effects became apparent in the mesozooplankton community composition, but differences in nutrient concentrations established through OMZ water additions were negligible. Copepods and polychaete larvae dominated in terms of abundance and biomass. Development and reproduction of the dominant copepod genera Paracalanus spp., Hemicyclops sp., Acartia sp., and Oncaea sp. was 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 explains the retarded copepod development. Correlation analysis revealed no particular trophic relations between dominant copepods and phytoplankton groups. Possibly particulate organic matter with relatively high C:N ratio was a major diet of copepods. C:N ratios of copepods (pooled and species-specific for Paracalanus spp., Hemicyclops sp.) and polychaetes ranged between 4.8–5.8 and 4.2–4.3, respectively. Stable isotope signatures of copepods varied over time but not between treatments. 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 intensification and increased frequency of upwelling and expanding/shoaling OMZs could severely impact population dynamics and reproductive success of zooplankton communities and, thus, the pelagic food web of the HUS.

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