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: Zooplankton metabolic processes play an important role in carbon budgets and fluxes of pelagic ecosystems. Respiration rates of several copepod species were determined to reveal their energy requirements and assess their significance in the carbon cycle. Respiration rates were measured by optode respirometry and allometrically based on body dry mass (DM). For the on-board measurements, a 10-channel optode respirometer (PreSens Precision Sensing Oxy-10 Mini) was used and experiments were run in gas-tight glass bottles (13-14 ml) filled with filtered seawater to reduce bias by microbial respiration. In addition, respiration rates for all dominant copepod species during MSM80 including copepodite stages C4 to C6 were determined based on individual DM and respective ambient temperatures after Bode et al. (2018). For that, individual DM, if not available from frozen specimens, was determined from formalin/Steedman-preserved samples by weighing the dried samples on a microbalance. Losses in body DM due to formalin/Steedman preservation were considered after Schukat et al. (2021). Respiration rates were calculated separately for the copepod family Eucalanidae (a) as they are rather sluggish while all other copepods exhibited normal activity (b). (a) lnRTF = -2.180 + 0.787 ln(DM) + 0.131T and (b) lnRAC = -0.890 + 0.646 ln(DM) + 0.094T, where R (μl O2 ind-1 h-1) is the individual respiration rate for eucalanid (RTF) and active (RAC) copepods, DM represents dry mass in mg and T the average temperature (°C) of the sampling interval. Respiration rates of the medium- to larger-sized copepods (female prosome length (PL) of 1.2-6.0 mm) were compared to those of "small copepods" (all copepods with female PL 〈1.1 mm and young stages). Medium- to larger-sized species ingested on average 13-212 mg C m-2 d-1 in coastal regions while "small copepods" on average consumed 118-328 mg C m-2 d-1. The potential egestion varied on average from 5-64 mg C m-2 d-1 for medium to larger-sized copepods and 35-98 mg C m-2 d-1 for "small copepods". Data of energy demands, consumption and egestion rates of copepod species differing in size are essential to improve carbon budgets and food-web models in the Humboldt Current System.