Overwintering Growth and Development of Larval Euphausia superba: An Interannual Comparison Under

Varying Environmental Conditions West of the Antarctic Peninsula

 

Kendra L. Daly

 

 

Growth, molting and development of larval Antarctic krill (Euphausia superba Dana) were investigated in the vicinity of Marguerite Bay during four cruises in austral fall and winter 2001 and 2002 as part of the U.S. Southern Ocean GLOBEC program. Overwintering survival of larvae has been linked to annual sea ice formation and extent, as sea ice biota may provide food when other sources are scarce in the water column. During fall 2001, larval stages C2 - F6 were very abundant (1 - 19 individuals m-3), with F4-F6 stages dominant at all stations over the shelf. During fall 2002, C2 - F4 larvae were abundant offshelf (0.01 - 110 m-3), while all stages were scarce on the shelf. Despite the presence of declining diatom and radiolarian blooms during fall of both years, average chlorophyll concentrations were low (0.10 vs.0.22 g l-1). Carbon content of larvae in fall suggested that lipid storage was moderate (41 vs. 38 % of DW). The median fall larval growth rate (0.027 mm d-1) was lower and the intermolt period (19 d) longer than summer values. The following results suggest that larvae were food limited in winter of both years: (1) the median growth rate decreased (0.00 mm d-1) and the intermolt period increased (40 d), (2) dry weight and body carbon and nitrogen decreased (ca. 0.4% d-1), and (3) 88% of F6 individuals did not develop to the juvenile stage after molting. In addition, an experiment showed that some larvae could survive starvation for a month by combusting body reserves (ca. 1% decrease in DW and body C and N d-1), implying that a portion of the population was resilient to the suboptimal food supply. In winter 2001, furcilia were commonly observed near the under-surface of sea ice, but only rarely in 2002 until mid-September. Even though sea ice formed up to two months earlier in 2002, ice algae accessible to krill at the ice-water interface was not an abundant food source in either year (0.05 vs. 0.07 g chl l-1). Low gut fluorescence values also indicate that little nutrition was derived from autotrophs in winter. Instead, larvae were opportunistic scavengers and likely exploited all available food sources, including microzooplankton, detritus, and sea ice biota. In summary, larval krill exhibited several overwintering behaviors: (1) flexible feeding, (2) flexible morphology (i.e., delayed development), (3) flexible physiology (i.e., increased intermolt period, reduced growth), (4) some lipid storage, and (6) ability to withstand starvation by combusting body C and N. Because most larvae did not shrink in length, this measurement may not be a good indicator of the body combustion strategy. At these high latitudes, sea ice biota may be a more important food in spring when irradiance levels increase than in winter. Winter survivors during 2001 resulted in a significant recruitment to the juvenile size class during spring.

 

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