Physical forcing in the southwest Atlantic: ecosystem control

In the southwest Atlantic sector of the Southern Ocean, temporal variability in the physical environment has been recorded since the early part of the last century. For example, ocean temperatures are known to vary both between seasons and between years. Time series analysis of sea surface temperature at South Georgia shows the presence of high levels of autocorrelation, with periodicity evident in temperature anomalies at lag periods of approximately 3 to 4 years. Crosscorrelation analyses with temperature anomaly data for the El Niño 4 region in the Pacific show that variability at South Georgia also reflects temperature fluctuations in the Pacific, with the Pacific leading South Georgia by approximately 3 years.
Biological variability is also apparent at South Georgia, being evident in data from a suite of top predators as well as in data from fish and Antarctic krill. Increased krill biomass appears to coincide with cold anomaly periods. In contrast, periods of reduced predator breeding performance are strongly correlated with warm anomaly periods, but lagged by a number of months. For some predators the most critical periods appear to be prior to the breeding season during the summer and early autumn of the preceding year. Thus, gentoo penguins (Pygoscelis papua) show a strong negative relationship between the number of chicks fledged and sea surface temperatures in the preceding February some 12 months earlier. Antarctic fur seals (Arctocephalus gazella) show a similar negative relationship between the number of pups surviving at birth and the temperature 14 months earlier in the preceding November. Such relationships with the physical environment most likely reflect prey (krill) availability. However, the full explanation of the observed relationships requires a more detailed understanding of the temporal and spatial interactions between predator demography and foraging dynamics, prey biomass and recruitment, and environmental variability.