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.