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.

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.