Description: Adult Euphausia hanseni, keystone and most abundant krill species in the northern Benguela Upwelling area, were sampled during late austral summer in two water masses (northern Benguela Current versus Angola-Benguela-Front waters) along the Atlantic coast off Namibia. This study investigates the species' physiological performance at different temperatures which it naturally experiences during diel vertical migration as well as its capacity to physiologically adapt to seven consecutive days of starvation. Moulting rates, metabolic rates, carbon demand, total lipid and protein contents, citrate synthase activity and kinetics, C:N ratios and stable isotope ratios were measured. These parameters were used to estimate the species' physiological condition and adaptive capability within the nutritionally poly-pulsed Benguela upwelling system to cope with short periods of food absence. Moulting rates correlated negatively with temperature. Metabolic rates followed the Q10 rule and declined significantly over the starvation period. Decreasing trends in the other parameters similarly suggest an adaptation to remain metabolically efficient. Ammonium excretion rates, oxygen to nitrogen ratios and stable isotopes showed strong distinctions between regions. Considerable differences were found between regions in the nutritional condition of E. hanseni. The total lipid content and the physiological reaction to starvation are different from euphausiids from other latitudes and help to define E. hanseni as a true upwelling organism.

Description: In the highly productive northern Benguela upwelling system, euphausiids can dominate the mesozooplankton community and may contribute substantially to the vertical flux of organic carbon. The diurnal vertical distribution of four euphausiid species was observed over three seasons from different years. The most abundant, Euphausia hanseni, showed pronounced diel vertical migration (DVM), regularly crossing the thermocline and retreating again to the oxygen minimum zone (OMZ). Nematoscelis megalops was a weak migrant, persisting in the OMZ throughout 24 h. Euphausia recurva showed vertical migration into the OMZ but may have avoided oxygen concentrations below 1 mLO2 L21, Euphausia americana remained in the upper water layers above the OMZ. Thus, euphausiids were divided into different ecological groups using or avoiding the OMZ and were vertically separated, thus avoiding interspecific competition. However, DVM behaviour was adjusted to seasonal variations in water temperature, oxygen and food availability. A conceptual model, combining DVM patterns, environmental parameters such as temperature and food availability and physiological constraints such as species-specific respiration rates, was used to assess the carbon demand of the seasonal DVM behaviours. Energetic considerations based on the DVM mode showed that temperature acted as the controlling and limiting factor with food abundance further modifying vertical positioning of euphausiid species.

Description: Calanoid copepods and euphausiids are key components of marine zooplankton communities worldwide. Most euphausiids and several copepod species perform diel vertical migrations (DVMs) that contribute to the export of particulate and dissolved matter to midwater depths. In vast areas of the global ocean, and in particular in the eastern tropical Atlantic and Pacific, the daytime distribution depth of many migrating organisms corresponds to the core of the oxygen minimum zone (OMZ). At depth, the animals experience reduced temperature and oxygen partial pressure (pO2) and an increased carbon dioxide partial pressure (pCO2) compared to their near-surface nighttime habitat. Although it is well known that low oxygen levels can inhibit respiratory activity, the respiration response of tropical copepods and euphausiids to relevant pCO2, pO2, and temperature conditions remains poorly parameterized. Further, the regulation of ammonium excretion at OMZ conditions is generally not well understood. It was recently estimated that DVM-mediated ammonium supply could fuel bacterial anaerobic ammonium oxidation – a major loss process for fixed nitrogen in the ocean considerably. These estimates were based on the implicit assumption that hypoxia or anoxia in combination with hypercapnia (elevated pCO2) does not result in a down-regulation of ammonium excretion. We exposed calanoid copepods from the Eastern Tropical North Atlantic (ETNA; Undinula vulgaris and Pleuromamma abdominalis) and euphausiids from the Eastern Tropical South Pacific (ETSP; Euphausia mucronata) andthe ETNA (Euphausia gibboides) to different temperatures, carbon dioxide and oxygen levels to study their survival, respiration and excretion rates at these conditions. An increase in temperature by 10°C led to an approximately 2-fold increase of the respiration and excretion rates of U.vulgaris (Q10, respiration=1.4; Q10,NH4-excretion=1.6), P. abdominalis (Q10, respiration=2.0; Q10,NH4-excretion=2.4) and E. gibboides (Q10, respiration=2.0; Q10,NH4-excretion=2.4; E. mucronata not tested). Exposure to differing carbon dioxide levels had no overall significant impact on the respiration or excretion rates. Species from the ETNA were less tolerant to low oxygen levels than E. mucronata from the ETSP, which survived exposure to anoxia at 13°C. Respiration and excretion rates were reduced upon exposure to low oxygen levels, albeit at different species-specific levels. Reduction of the excretion and respiration rates in ETNA species occurred at a pO2 of 0.6 (P. abdominalis) and 2.4 kPa (U. vulgaris and E.gibboides) at OMZ temperatures. Such low oxygen levels are normally not encountered by these species in the ETNA. E. mucronata however regularly migrates into the strongly hypoxic to anoxic core of the ETSP OMZ. Exposure to low oxygen levels led to a strong reduction of respiration and ammonium excretion in E. mucronata (pcrit respiration=0.6, pcrit NH4excretion=0.73). A drastic reduction of respiratory activity was also observed by other authors for euphausiids, squat lobsters and calanoid copepods, but was not yet accounted for when calculating DVM-mediated active fluxes into the ETSP OMZ. Current estimates of DVM-mediated active export of carbon and nitrogen into the ETSP OMZ are therefore likely too high and future efforts to calculate these export rates should take the physiological responses of migratory species to OMZ conditions into account.