Diel vertical migration behaviour in Euphausiids of the northern Benguela current: seasonal adaptations to food availability and strong gradients of temperature and oxygen

Abstract

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.