Life in the sea of plenty: Seasonal and regional comparison of physiological performance of Euphausia hanseni in the northern Benguela upwelling system

a r t i c l e i n f o Variability in upwelling events may lead to periods of constrained food availability in the northern Benguela upwelling system (NBUS), thereby affecting the physiological state and metabolic activity of euphausiids. Most attention has so far been paid to seasonal effects but little is known about regional variability. Metabolic activity (expressed by respiration and excretion rates) and physiological state (expressed by reproductive effort and moult activity) in Euphausia hanseni were examined at different stations during austral summer (minimum upwelling) and austral winter (maximum upwelling). Overall, regional differences in physiological state, influencing metabolic activity, were greater than seasonal ones, indicating favourable conditions for growth and reproduction year-round. Higher respiration rates were found for females in more advanced stages of sexual development. Moult stage did not affect oxygen consumption rates, however. The physiological state of E. hanseni at the time of capture may serve as a meaningful indicator of the associated hydrographic conditions in the NBUS, to be further used in ecosystem analysis on seasonal or long-term time scales. A latitudinal comparison of species highlights the extraordinary physiological plasticity of euphausiids. One of the four major Eastern Boundary upwelling systems, the northern Benguela upwelling system (NBUS), is located off the west-coast of Namibia. It is bordered in the north by the Angola-Benguela front (~17°S) and in the south by the strong upwelling cell at Lüderitz (26°S). The NBUS is characterized by perennial upwelling, with a maximum during austral winter/spring and a minimum during austral summer/autumn. Influenced by several atmospheric and oceanographic processes, the NBUS is a complex and highly variable ecosystem (Shannon and Nelson, 1996). In addition, wind-driven coastal upwell-ing makes this area one of the most productive ecosystems in the world's oceans. Fluctuations in upwelling intensity correlate with variations in the magnitude and direction of winds. Timing and duration of upwelling events influence the physical and biological properties of coastal seas, including the population biology of krill (Dorman et al., 2005). In the NBUS the biomass and distribution of mesozooplankton are spatially and temporally highly variable and upwelling intensity shows a clear seasonal signal (Martin et al., 2014). Upwelling events favour phytoplankton growth through nutrient input and subsequent zooplankton blooms, thereby supporting omnivorous species like Euphausia hanseni, the dominant krill species in the NBUS. Consequently, E. hanseni may show physiological adaptations to these poly-pulsed, upwelling-mediated, plankton blooms. The adjustment of metabolic rates …