Krill of the northern Benguela Current and the Angola-Benguela frontal zone compared: physiological performance and short-term starvation in Euphausia hanseni

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 Q(10) 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.