International audienceAntarctic krill Euphausia superba (hereafter `krill') occur in regions undergoing rapid environmental change, particularly loss of winter sea ice. During recent years, harvesting of krill has in creased, possibly enhancing stress on krill and Antarctic ecosystems. Here we review the overall impact of climate change on krill and Antarctic ecosystems, discuss implications for an ecosystem-based fisheries management approach and identify critical knowledge gaps. Sea ice decline, ocean warming and other environmental stressors act in concert to modify the abundance, distribution and life cycle of krill. Although some of these changes can have positive effects on krill, their cumulative impact is most likely negative. Recruitment, driven largely by the winter survival of larval krill, is probably the population parameter most susceptible to climate change. Predicting changes to krill populations is urgent, because they will seriously impact Antarctic ecosystems. Such predictions, however, are complicated by an intense inter-annual variability in recruitment success and krill abundance. To improve the responsiveness of the ecosystem-based management approach adopted by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR), critical knowledge gaps need to be filled. In addition to a better understanding of the factors influencing recruitment, management will require a better understanding of the resilience and the genetic plasticity of krill life stages, and a quantitative understanding of under-ice and benthic habitat use. Current precautionary management measures of CCAMLR should be maintained until a better understanding of these processes has been achieved. [GRAPHICS]

In the past, two euphausiid species prevailed in the high Arctic Kongsfjorden, the arcto-boreal Thysanoessa inermis (Kroeyer, 1846) and Thysanoessa raschii (Sars. 1964). Both were considered expatriates from the Barents Sea or Norwegian Sea and non-reproductive due to low temperatures. The macro-zooplankton of the fjord has been studied as a component in an ecosystem context since 2006, including baseline investigation of distribution and functional performance of key species. In recent years, three additional krill species were regularly detected in the fjord and are the focus of an intensive long-term study. Of these species, Thysanoessa longicaudata (Kroeyer, 1846) and Meganyctiphanes norvegica (Sars, 1857) are typical for the boreal Atlantic whereas Nematoscelis megalops (Sars, 1883) has a broad distribution in temperate to subtropical provinces. Their occurrence in the Kongsfjorden clearly indicates increasing Atlantic influence. During the 2011 campaign, T. raschii was observed spawning in the field for the first time and showed development up to the naupliar stage in the laboratory. Should more evidence of reproduction be encountered in any of the five krill species in the Kongsfjorden in the future, it will be taken as an indication of a changing environment concerning temperature and food web composition.

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 mL O 2 L 21 , 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 model

Rates of respiration and ammonia excretion of Euphausia hanseni and Nematoscelis megalops were determined experimentally at four temperatures representative of conditions encountered by these euphausiid species in the northern Benguela upwelling environment. The respiration rate increased from 7.7 μmol O 2 h -1 g ww -1 at 5 °C to 18.1 μmol O 2 h -1 g ww -1 at 20 °C in E. hanseni and from 7.0 μmol O 2 h -1 g ww -1 (5 °C) to 23.4 μmol O 2 h -1 g ww -1 (20 °C) in N. megalops. The impact of temperature on oxygen uptake of the two species differed significantly. Nematoscelis megalops showed thermal adaptations to temperatures between 5 °C and 10 °C (Q 10 = 1.9) and metabolic constraint was evident at higher temperatures (Q 10 = 2.6). In contrast, E. hanseni showed adaptations to temperatures of 10-20 °C (Q 10 = 1.5) and experienced metabolic depression below 10 °C (Q 10 = 2.6). Proteins were predominantly metabolised by E. hanseni in contrast to lipids by N. megalops. Carbon demand of N. megalops between 5 and 15 °C was lower than in E. hanseni versus equal food requirements at 20 °C. It is concluded that the two species display different physiological adaptations, based on their respective temperature adaptations, which are mirrored in their differential vertical positioning in the water column.

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