Description: Adaptive processes linked to overall metabolism were studied in terms of oxygen consumption andammonia excretion in each of three self-contained krill populations along a climatic gradient. In the Danish Kattegat,krill were exposed to temperatures which ranged from 4°C to 16°C between seasons and a vertical temperaturegradient of up to 10°C during summer. In the Scottish Clyde Sea, water temperatures varied less between seasonsand the vertical temperature gradient in summer was only 3°C. Temperatures in the Ligurian Sea, off Nice, wererelatively constant around 12-13°C throughout the year, with a thin surface layer (20-30 m) of warm waterdeveloping during summer. The trophic conditions were rich in the Kattegat and, particularly, in the Clyde, butcomparatively poor in the Ligurian Sea. Oxygen consumption increased exponentially with increasing experimentaltemperature, which ranged from 4°C to 16°C. Overall respiration rates were between 19.9 and 89.9 µmol O2 g-1dry wt h-1. Krill from the Kattegat, the Clyde Sea, and the Ligurian Sea all exhibited approximately the same level ofoxygen consumption (30-35 µmol O2 g-1 dry wt h-1) when incubated at the ambient temperatures found in theirrespective environments (9°C, 5°C, and 12°C). This indicates that krill adjust their overall metabolic rates to theprevailing thermal conditions. The exception to this were the respiration rates of Ligurian krill from winter/spring,which were about twice as high as the rates from summer krill despite the fact that the thermal conditions were thesame. This effect appears to result from enhanced somatic activity during a short period of increased food availabilityand reproduction. Accordingly, krill appears to be capable of adapting to both changing thermal and trophicconditions, especially when nutrition is a limiting factor in physiological processes.

Description: Activities and characteristics of two metabolic key enzymes, citrate synthase (CS) and pyruvate kinase(PK), were studied in the Northern krill, Meganyctiphanes norvegica, with respect to adaptive properties underdifferent thermal conditions. Krill were sampled during late winter/spring and summer from the constantly warmLigurian Sea (12-13°C below the thermocline), the colder but also comparatively constant Clyde Sea (7-8°C), andthe variable Kattegat (2-16°C). Both enzymes showed distinct tissue- and organ-specific activities, which werehighest in the pleopods - the principal locomotive organs. The fourth and fifth abdominal segments, however, wereused for routine investigation due to lowest variability. Specific activity of CS and PK did not differ between seasonsin krill from the Kattegat or the Clyde Sea. In the Ligurian Sea, in contrast, specific CS activities were significantlylower during summer. Analysis of individual data illustrated a decrease of CS activity with size and an increase of PKactivity with size. Taking these allometric effects into account, as emphasized by calculating the ratio between bothenzymes, variation of CS and PK activities turned out to be solely dependent on body size, which differed betweenlocations and seasons. Ligurian krill from the summer, however, were unique in that they showed a lower CS/PK ratiothan would be predicted by the scaling effect. Thermal characteristics of each enzyme were similar between locationsand seasons. During the winter, in Kattegat and Clyde Sea krill, Km values (Michaelis-Menten constant) of CStowards acetyl-coenzyme A exhibited an almost constant level over the experimental temperature range of 4-16°C.During summer, however, Km values were lower at 8°C in the Clyde Sea and at 12°C in the Kattegat. In Liguriankrill from the summer, Km values were consistently lower than those of winter krill over the entire experimentaltemperature range. In conclusion, Kattegat and Clyde Sea krill show only minor adaptations to their respectivethermal environments in terms of CS and PK characteristics. Ligurian krill, in contrast, exhibited decreased specificCS activity during summer, which might be compensated by elevated enzyme-substrate affinity as indicated by lowerKm values. Since temperature was constant during both seasons, this effect cannot be explained as a reaction tothermal conditions. Consequently, oligotrophic conditions in the Ligurian Sea during summer may entail a reduction inthe somatic performance of krill, which is reflected by lower CS activity.