Modulation of zooplankton-mediated fluxes in oxygen minimum zones

Element fluxes associated with the diel vertical migration of zooplankton represent a major compartment of the biological pump. Zooplankton activity is strongly dependent on the availability of oxygen and low oxygen concentrations found in tropical oxygen minimum zones (OMZs) therefore impact zooplankton distribution and activity. In the Eastern Tropical South Pacific (ETSP), the endemic krill Euphausia mucronata and the squat lobster Pleuroncodes monodon migrate into the anoxic core of the OMZ. We here show that they drastically downregulate respiration and excretion, a fact that needs to be considered when calculating DVM-mediated fluxes in the ETSP. In the better ventilated Eastern Tropical North Atlantic (ETNA), copepods and euphausiids are only exposed to anoxic conditions in low-oxygen mesoscale eddies observed in the Cape Verde region. We show that they downregulate respiration and excretion rates already at oxygen concentrations of about 40 µmol/kg. Analysis of moored acoustic backscatter target strength (300 kHz ADCP) and oxygen concentration data from the Cape Verde Ocean Observatory mooring, shows that inhibition of diel vertical migrations start at an oxygen concentration of about 20 µmol/kg, which is consistent with our physiological experiments. In summary, the impacts of low oxygen levels on DVM-mediated fluxes differ in the two investigation areas. DVM-mediated fluxes into the OMZ in the ETSP are down-regulated via the extremely low oxygen levels, whereas extremely low oxygen levels in the ETNA are avoided. Such regional differences need to be included in global estimates of DVM-mediated fluxes and in model parameterizations to predict future changes of the biological pump.