TITLE: PALMER LTER:  INTERANNUAL VARIABILITY IN GROWTH RATES OF YOUNG ANTARCTIC
KRILL IN RELATION TO THEIR FOOD ENVIRONMENT
AUTHOR: R.M.Ross, L.B.Quetin, K.S.Baker, R.C.Smith, M.Vernet
DATE: 31 Aug - 4 Sep 1998
PLACE:  SCAR VII International Biology Symposium in New Zealand 


Robin M. Ross1, L. B. Quetin1, K. S. Baker2, R. C. Smith3 and M. Vernet2
1Marine Science Institute, University of California, Santa Barbara, CA
93106 USA; 2Scripps Institution of Oceanography, University of California,
La Jolla CA 92093 USA; 3Institute of Computational and Earth System
Science, University of California, Santa Barbara, CA 93106 USA


As part of the seasonal time series conducted by the Palmer LTER (Long-Term
Ecological Research) program, growth rates of Antarctic krill (Euphausia
superba Dana) were measured at 7-10 d intervals during the spring and early
summer (mid-November to mid-January) of four years.  The standing stock and
composition of the phytoplankton community were measured twice a week
during the same period in the area where krill was collected.  A total of
26 short-term experiments (4 d) were conducted with freshly collected
1-yr-old krill to determine the molting frequency and intermolt period
(IMP, the inverse of the molting frequency), and to measure the length
increment of each molting individual.  Increase in length was normalized to
length, and expressed as % growth per IMP.  Growth in mm d-1 for each
individual was calculated by multiplying % growth per IMP, total length,
and the average IMP.  Since the IMP was not significantly different either
within or between years, the average IMP of 18.3 d was used in these
calculations.  The % growth per IMP varied significantly during the season
in all years, but only in 1991-1992 and 1995-1996 were patterns apparent.
Timing of highest growth rates varied, from late November - early December
in 9596 to several weeks later in late December - mid-January in 9192.
Over the range of temperatures and food concentrations found in the
environment, temperature did not appear to affect % growth per IMP but food
quantity and quality did.  Plots of % growth per IMP versus food quantity
(chlorophyll a concentration) showed a non-linear functional response
curve, with maximum growth reached at concentrations between 1 and 2 mg
m-3.  However, maximum growth rates varied among years, with the highest in
1995-1996 (9.61% per IMP) and the lowest in 1993-1994 (6.2% per IMP), and
were correlated with food quality as indexed by chemotaxonomic pigments.
Thus interannual differences in the seasonal pattern of growth appeared to
be a function of both food quantity and food quality, with the presence of
diatoms enhancing growth.