Interannual variability of krill, salp and other zooplankton populations in the South Shetland Island area during Austral summer 1993–1998

The summer 1998 AMLR field season followed late-forming but spatially extensive winter sea ice in the Antarctic Peninsula region. Overall krill length/maturity composition reflected poor recruitment from the past two years. Unusually small juvenile krill in 1998 suggested that survival success was limited to late-spawned eggs and larvae in 1997. Delayed spawning and few larval krill in 1998 were associated with extremely high salp abundance. Sa/pa thompsoni abundance was similar to levels during the 1993 "salp year". The magnitude of this salp bloom may have resulted from a large overwintering "seed" population developed the preceding autumn. Presence of large numbers of another salp species, Ihlea racovitzai, was unique to AMLR surveys and indicated faunal input from the east.
Comparisons of January and February 1998 survey data with data from previous AMLR field seasons showed recurring patterns of species abundance relationships that allowed definition of different ecological regimes identified here as "cope pod years", "salp years" and "transition periods". The 4-5 year periodicity of "salp years" over the past fifteen years is discussed with respect to the Antarctic Circumpolar Wave (ACW).