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: 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: Abundance and community structure of calanoid copepods of one day (stn. 16; bottom depth 5,433 m) and one night station (stn. 15; bottom depth 5,462 m) were analyzed (Fig. 1). Stratified vertical hauls were carried out within 24 h with a HydroBios Multinet Maxi (0.5 m2 net opening, 9 nets, 150 µm mesh size) from 800 m depth to the surface (strata: 800-700-600-500-400-300-200-100-50-0 m). The filtered water volume was measured with a flowmeter attached to the net opening. After retrieval, samples were preserved in a 4% borax-buffered formaldehyde in seawater solution. Calanoid copepods were sorted according to their developmental stages (copepodids C1-3 and C4/5, adult females and males), counted and identified to genus or, if possible, to species level under a dissecting microscope (Leica MZ12). Rare species (〈100 individuals per sample) were counted from the entire sample. Total length (TL) of up to 100 calanoid individuals per taxonomic category (i.e. family/genus/species) and stage was measured (~6,600 specimens in total). Dry mass (DM) of calanoids was calculated based on the median TL of each taxonomic category. Individual respiration rates were calculated from individual DM and in situ temperatures, which were then converted to carbon units and used to calculate ingestion and egestion rates.

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 majority of global ocean production and total export production is attributed to oligotrophic oceanic regions due to their vast regional expanse. However, energy transfers, food-web structures and trophic relationships in these areas remain largely unknown. Regional and vertical inter- and intra-specific differences in trophic interactions and dietary preferences of calanoid copepods were investigated in four different regions in the open eastern Atlantic Ocean (38°N to 21°S) in October/November 2012 using a combination of fatty acid (FA) and stable isotope (SI) analyses. Mean carnivory indices (CI) based on FA trophic markers generally agreed with trophic positions (TP) derived from d15N analysis. Most copepods were classified as omnivorous (CI ~0.5, TP 1.8 to ~2.5) or carnivorous (CI 〉=0.7, TP 〉=2.9). Herbivorous copepods showed typical CIs of 〈=0.3. Geographical differences in d15N values of epi- (200-0 m) to mesopelagic (1000-200 m) copepods reflected corresponding spatial differences in baseline d15N of particulate organic matter from the upper 100 m. In contrast, species restricted to lower meso- and bathypelagic (2000-1000 m) layers did not show this regional trend. FA compositions were species-specific without distinct intra-specific vertical or spatial variations. Differences were only observed in the southernmost region influenced by the highly productive Benguela Current. Apparently, food availability and dietary composition were widely homogeneous throughout the oligotrophic oceanic regions of the tropical and subtropical Atlantic. Four major species clusters were identified by principal component analysis based on FA compositions. Vertically migrating species clustered with epi- to mesopelagic, non-migrating species, of which only Neocalanus gracilis was moderately enriched in lipids with 16% of dry mass (DM) and stored wax esters (WE) with 37% of total lipid (TL). All other species of this cluster had low lipid contents (〈 10% DM) without WE. Of these, the tropical epipelagic Undinula vulgaris showed highest portions of bacterial markers. Rhincalanus cornutus, R. nasutus and Calanoides carinatus formed three separate clusters with species-specific lipid profiles, high lipid contents (〉=41% DM), mainly accumulated as WE (〉=79% TL). C. carinatus and R. nasutus were primarily herbivorous with almost no bacterial input. Despite deviating feeding strategies, R. nasutus clustered with deep-dwelling, carnivorous species, which had high amounts of lipids (〉=37% DM) and WE (〉=54% TL). Tropical and subtropical calanoid copepods exhibited a wide variety of life strategies, characterized by specialized feeding. This allows them, together with vertical habitat partitioning, to maintain high abundance and diversity in tropical oligotrophic open oceans, where they play an essential role in the energy flux and carbon cycling.