Jon L. Watkins

Climatically driven fluctuations in Southern Ocean ecosystems

Determining how climate fluctuations affect ocean ecosystems requires an understanding of how biological and physical processes interact across a wide range of scales. Here we examine the role of physical and biological processes in generating fluctuations in the ecosystem around South Georgia in the South Atlantic sector of the Southern Ocean. Anomalies in sea surface temperature (SST) in the South Pacific sector of the Southern Ocean have previously been shown to be generated through atmospheric teleconnections with El Niño Southern Oscillation (ENSO)-related processes. These SST anomalies are propagated via the Antarctic Circumpolar Current into the South Atlantic (on time scales of more than 1 year), where ENSO and Southern Annular Mode-related atmospheric processes have a direct influence on short (less than six months) time scales. We find that across the South Atlantic sector, these changes in SST, and related fluctuations in winter sea ice extent, affect the recruitment and dispersal of Antarctic krill. This oceanographically driven variation in krill population dynamics and abundance in turn affects the breeding success of seabird and marine mammal predators that depend on krill as food. Such propagating anomalies, mediated through physical and trophic interactions, are likely to be an important component of variation in ocean ecosystems and affect responses to longer term change. Population models derived on the basis of these oceanic fluctuations indicate that plausible rates of regional warming of 1oC over the next 100 years could lead to more than a 95% reduction in the biomass and abundance of krill across the Scotia Sea by the end of the century.

An anticyclonic circulation above the Northwest Georgia Rise, Southern Ocean

Data from a variety of sources reveal a warm-core anticyclonic circulation above the Northwest Georgia Rise (NWGR), an similar to2000-m high bathymetric feature north of South Georgia. The sense of the circulation is opposite to the general cyclonic flow in the Georgia Basin. The circulation shows the characteristics of a stratified Taylor column: dimensional analysis shows that the local bathymetry and hydrography are conducive to the formation of such. ERS2 altimeter data show that the column, whilst not fully permanent, is nonetheless a recurring feature. High concentrations of chlorophyll-a are observed at the centre of the circulation, indicating that the modulation of the physical environment has significant consequences for the local biogeochemical system via enhanced primary production. Enhanced chlorophyll-a extends in a long plume from the NWGR along pathways indicated by drifters; this passive redistribution may have consequences for the larger (basin-) scale ecosystem.

Southern Antarctic Circumpolar Current Front to the northeast of South Georgia: Horizontal advection of krill and its role in the ecosystem

During December 2000 and January 2001 we conducted a high-resolution hydrographic and bioacoustic transect (RRS James Clark Ross cruise 57) that extended across the South Georgia shelf from close to Cumberland Bay, across the shelf break and slope and into the deep waters of the Georgia Basin beyond. We observed a high biomass of zooplankton between 53.8degrees and 53.4degreesS associated with the inshore, northwestward flow of the Southern Antarctic Circumpolar Current Front (SACCF) that occurred in around 2500 m of water close to the base of the slope. There was very little zooplankton biomass present in the more offshore, eastward flowing waters where a second manifestation of the SACCF was also present on the section. The region of enhanced zooplankton biomass was over 50 km in horizontal extent with the highest densities (>10 g m(-3)) in the area of strongest flow (>35 cm s(-1)). The majority of the zooplankton present on the section was Antarctic krill and most of it occurred in the upper 100 m. The rate of physically mediated transport of Antarctic krill across the off-shelf sections (similar to10 km) of the transect showed marked variation, with highest rates (>10(6) g s(-1)) associated with the northwestward flow of the SACCF. Farther offshore, where the krill biomass and flow rates were much reduced, the flux of krill was very low. The integrated horizontal flux of krill across the offshore sections was large (192 x 10(3) t d(-1)) and to the northwest. A second occupation of the transect showed that the krill flux is highly variable, and we discuss the various physical and biological factors that will generate such variability. We show that horizontal flux of krill in ocean currents is a major factor in determining the abundance of krill around South Georgia.