Modeling the krill transport pathways in the Scotia Sea: spatial and environmental connections generating the seasonal distribution of krill
A coupled physical-biological model analysis was undertaken to examine the seasonal development of the distribution of antarctic krill (Euphausia superba Dana) in the Scotia Sea. The origin and fate of krill observed during the CCAMLR 2000 survey were studied using output from the OCCAM model. Lagrangian particle tracking for the period prior to the survey showed the expected dominance of the west to east flow of material associated with the main direction of the current flow, but there was no simple association of particle transport with any of the fronts of the Antarctic Circumpolar Current. Most of the krill were associated with areas to the south of the Antarctic Circumpolar Current in the Weddell-Scotia Confluence (WSC) and farther east in Weddell Sea-influenced waters. Examining the pathways of krill transport in relation to satellite-derived sea-ice distributions suggests that particles present in the high krill biomass regions in January would have come from areas that were covered by sea-ice during late winter/early spring (September-October). The results of Eulerian grid-based simulations of the development of the biomass distribution after the survey period showed transport of particles around South Georgia, probably in association with the Southern Antarctic Circumpolar Current Front. However, many of the krill encountered in the eastern Scotia Sea would have exited toward the east, passing north of the South Sandwich Islands, probably in association with the Southern Boundary of the Antarctic Circumpolar Current and Weddell Sea waters that penetrate to the north in this area. These krill may return to more southern regions where further spawning is possible in later years. Simulations of particle tracks that included diurnal vertical migration showed that krill behavior could modify the pathways of transport, although the current flows probably dominate the movement of krill in open ocean regions. This study suggests that the summer distribution of krill in the Scotia Sea is connected to the winter sea-ice distribution and probably to the pattern and rate of the spring sea-ice retreat. Many of the krill in the survey region in the summer of 1999/2000 came from under the sea-ice in the eastern Scotia Sea, the southern Scotia Arc, and the northern Weddell Sea. This highlights that the spatial association of the sea-ice with the Weddell-Scotia Confluence and frontal regions of the Antarctic Circumpolar Current during winter and spring will be crucial in determining the summer krill distribution. Variation in the extent and timing of sea-ice retreat, and fluctuations in Weddell-Scotia Confluence and Scotia Sea flows, will change the pathways of transport resulting in large changes in the distribution of the krill during summer.
Authors: Murphy, E.J., Thorpe, S.E., Watkins, J.L., Hewitt, R.