Interannual variability of the South Georgia marine ecosystem: biological and physical sources of variation in the abundance of krill

Abstract

Interannual variability is a characteristic feature of the Southern Ocean ecosystem, yet the relative roles of biological and physical processes in generating these fluctuations are unknown. There is now extensive evidence that there are years when there is a very low abundance of Antarctic krill (Euphausia superba) in the South Georgia area, and that this variation affects much of the ecosystem, with the most obvious impacts on survival and breeding success of some of the major predators on krill. The open nature of the South Georgia ecosystem means this variability has large‐scale relevance, but even though there are unique time series of data available, information on some key processes is limited. Fluctuations in year‐class success in parts, or all, of the krill population across the Scotia Sea can generate large changes in the available biomass. The ocean transport pathways maintain the large‐scale ecosystem structure by moving krill over large distances to areas where they are available to predator colonies. This large‐scale physical system shows strong spatial and temporal coherence in the patterns of the interannual and subdecadal variability. This physical variability affects both the population dynamics of krill and the transport pathways, emphasizing that both the causes and the consequences of events at South Georgia are part of much larger‐scale processes.

Transport of Antarctic krill (Euphausia superba) across the Scotia Sea. Part I: Circulation and particle tracking simulations

“…Therefore, a modeling approach is used in this study to further the understanding of krill transport via ocean currents. Recently, the idea of krill transport to South Georgia from an upstream source was tested with modeling studies using a composite flow field Murphy et al, 1998). These studies show that the currents associated with the ACC can transport particles from the Antarctic Peninsula to South Georgia in 140-160 days.…”

“…Recent reviews suggest that krill may not be considered passively drifting particles, but that their behavior and swimming capabilities as adults may let them move to areas of their choice (Nicol, 2003a,b). However, it has been shown that advection by ocean currents is a major factor in determining the large-scale distribution (Marr, 1962;Murphy et al, 1998). Unfortunately, there have only been few studies of the potential effects of behavior in analyses of large-scale distribution (Murphy et al, 2004a).…”

Variability of the southern Antarctic Circumpolar Current front north of South Georgia

“…The krill population at South Georgia is believed not to be self-sustaining (Everson, 1984) but to be maintained instead by oceanic transport of krill from recognised breeding grounds upstream of South Georgia in the vicinity of the Antarctic Peninsula, the South Orkney Islands and the Weddell Sea (e.g. Everson, 1977;Murphy et al, 1998) [ Fig. 1(a)].…”

Advection in polar and sub-polar environments: Impacts on high latitude marine ecosystems

“…Knowledge of the scales and links between physical and biological processes impacting zooplankton in both polar ecosystems is critical, as the dominant species are relatively long lived (copepods 1-4 years, Arctic euphausiid species 3-4 years, Antarctic euphausiids >5 years, and possibly as many as 9 years) (Ikeda and Thomas, 1987;Murphy et al, 1998;Falk-Petersen et al, 2009). These zooplankton are important grazers on microplankton and ice algae, and form important couplings to the upper trophic levels.…”

“…Investigations of zooplankton transport in the Southern Ocean have focused primarily on Antarctic krill, given their importance as a key link in the Antarctic marine food web and as the focus of a commercial fishery (e.g., Hofmann et al, 1998;Murphy et al, 1998Murphy et al, , 2004aMurphy et al, , 2004bHofmann and Murphy, 2004;Fach and Klinck, 2006;Piñones et al, 2013aPiñones et al, , 2013b. Krill have a heterogeneous circumpolar distribution, with about 70% of the stock occurring between 0 and 90°W, including the open waters of the Antarctic Circumpolar Current in the Atlantic sector (Scotia Sea) and the western side of the Antarctic Peninsula (Atkinson et al, 2008).…”