Condition of larval and juvenile Antarctic krill Euphausia superba under sea ice in late winter in the northern Weddell Sea, Antarctica

Antarctic krill, Euphausia superba, is a key species in the Southern Ocean. However, information on larval krill overwintering strategy and habitat requirement is limited during winter. Our study investigated the physiological condition of larval and juvenile krill during austral winter 2013 in the Northern Weddell Sea. Larval krill condition was compared in different habitats (open water, Marginal ice zone, pack ice). In all regions, condition was quantified by determining their body length (BL), dry weight (DW), elemental and biochemical composition and rates of growth. Chlorophyll a (chl a) concentration was measured as a proxy of food availability in the water column and sea ice. Additionally, concentrations of particulate organic carbon (POC) were measured in sea ice. Growth rates in juveniles were similar in open water (0.011 mm d-1) and in the pack ice (0.019 mm d-1). But highest values were in juveniles at MIZ-2 (0.169 mm d-1). BL and DW were similar in juveniles at open water, the Marginal ice zone and the pack ice. Elemental and biochemical analysis revealed two main feeding strategies: juveniles in the pack ice primarily used body protein for energy provision and had an autotrophic diet. At open water and the MIZ juveniles utilized lipid reserves for energy and ingested a protein-rich diet. This indicates a degree of flexibility in energy source and diet. The physiological condition of the juveniles differed between the studied regions, but no overall better condition was found at one region. Within the pack ice, larvae from Ice camp 1 were generally in a better condition. Equivalent larval stages were 5 mm larger and had more DW at Ice camp 1. Stage composition consisted mainly in fucilia 6 and juveniles at Ice camp 1, whereas younger stages (F3-F6) were dominant at Ice camp 2. The inter-molt period was 50 days longer in Ice camp 2. Thus, the Ice camp 2 larvae were late spawners. We conclude that the present state of Ice camp 2 larvae reflects poor food availability in summer and fall rather than suboptimal food conditions in winter. Recruitment success is positively correlated to early spawning because larval growth and development is possible over longer time