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    Seasonal and interannual variability of krill, salp and other zooplankton populations in the northwest Antarctic Peninsula region: summer 2001 in relation to the Long-Term AMLR Data Set

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    V. Loeb (USA)
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    The US AMLR Program conducted large area surveys in the Elephant Island-South Shetland Island region during 18-30 January 2001 (Survey A, 101 stations) and 12 February-12 March 2001 (Survey D, 96 stations). Krill, salps and other zooplankton were collected at each station using a 180 cm Isaacs Kidd Midwater Trawl fitted with 505um mesh nets. Data obtained from these surveys were compared to assess seasonal variations in distribution, abundance and demography of Antarctic krill (Euphausia superba) and salps (Salpa thompsoni), and distribution and abundance of biomass dominant copepod species and other common zooplankton taxa. Comparisons were also made with data obtained during 1992-2000 AMLR field seasons to assess interannual variations. Mean and median krill abundance values in the Elephant Island area during the 2000/2001 field season were intermediate to highs in 1996 and lows in 1999. Increased abundance relative to 1999 resulted in part from recruitment of the 1998/99 and 1999/2000 year classes as indicated by modest proportions of juvenile and immature stages. Large proportions of advanced female maturity stages, substantial larval krill concentrations and more developed larval stages during February-March reflected normal seasonal spawning in 2000/2001. This is the third year in a row that spawning conditions have been favorable for krill recruitment success. Both large area surveys were characterized by wide spread distribution of abundant salps but extremely large concentrations like those of the 1993 and 1998 salp years were not encountered. Length frequency distribution of the dominant aggregate stage indicated a curtailed production season. A dramatic 60% abundance decrease between the two surveys was apparently due to loss of large aggregates from the upper water column. Within the 1993-2001 Elephant Island data set prolonged salp budding periods with pulses of late-season aggregate production preceded years with enhanced salp population size while curtailed budding periods preceded years with diminished salp populations. Assuming that these trends continue, reduced salp population size can be expected during the 2002 field season. Favorable krill spawning conditions in conjunction with reduced salp abundance improve the prospects of larval production and survival. Should winter sea ice development and spring bloom conditions also be favorable we may expect strong recruitment success of the 2000/2001 year class. Copepod abundance values in the Elephant Island area were among the highest observed over the past 20 years. This resulted from large concentrations of Calanoides acutus, Metridia gerlachei and Calanus propinquus and indicate enrichment in oceanic and coastal waters relative to other years.