Feeding and energy budgets of larval Antarctic krill Euphausia superba in summer

The physiological condition and feeding activity of the dominant larval stages of Euphausia superba (calyptopis stage III, furcilia stages I and II) were investigated from February to March 2000 at the Rothera Time Series monitoring station (67degrees34'S, 68degrees07'W, Adelaide Island, Western Antarctic Peninsula). A dense phytoplankton bloom (5 to 25 mug chl a l(-1)) occupied the mixed layer throughout the study period. The feeding of larvae was measured by incubating the animals in natural seawater. Food concentrations ranged from 102 to 518 mug C l(-1) across experiments, and the mean daily C rations were 28% body C for calyptosis stage Ill (CIII), 25% for furcilia stage I (FI) and 15% for FII. The phytoplankton, dominated by diatoms and motile prey taxa, ranged from 8 to 79 pm in size. Across this size spectrum of diatoms, CIII cleared small cells most efficiently, as did FI to a lesser degree. FII, however, showed no clear tendency for a specific cell size. Across the measured size spectrum of the motile taxa, all larvae stages showed a clear preference towards the larger cells. Estimated C assimilation efficiencies were high, from 70 to 92% (mean 84%). Respiration rates of freshly caught larvae were 0.7 to 1.1 mul O-2 mg DM-1 h(-1). The calculated respiratory C loss showed a significant increase with increasing food concentration in all larval stages, ranging from 0.9 to 2.4% body C d(-1). These respiratory losses, combined with the high assimilation efficiencies, thus give the larvae ample capacity for growth at these food concentrations. Critical concentrations of food to achieve maximum daily rations were in the range of 100 to 200 mug C l(-1) (similar to2 to 4 mug chl a l(-1)). Thus productive shelf sites along the Antarctic Peninsula, such as Rothera, may act as good 'nursery' areas for krill larvae.

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