The Scotia Sea ecosystem is a major component of the circumpolar Southern Ocean system, where productivity and predator demand for prey are high. The eastward-flowing Antarctic Circumpolar Current (ACC) and waters from the Weddell-Scotia Confluence dominate the physics of the Scotia Sea, leading to a strong advective flow, intense eddy activity and mixing. There is also strong seasonality, manifest by the changing irradiance and sea ice cover, which leads to shorter summers in the south. Summer phytoplankton blooms, which at times can cover an area of more than 0.5 million km2, probably result from the mixing of micronutrients into surface waters through the flow of the ACC over the Scotia Arc. This production is consumed by a range of species including Antarctic krill, which are the major prey item of large seabird and marine mammal populations. The flow of the ACC is steered north by the Scotia Arc, pushing polar water to lower latitudes, carrying with it krill during spring and summer, which subsidize food webs around South Georgia and the northern Scotia Arc. There is also marked interannual variability in winter sea ice distribution and sea surface temperatures that is linked to southern hemisphere-scale climate processes such as the El Niño-Southern Oscillation. This variation affects regional primary and secondary production and influences biogeochemical cycles. It also affects krill population dynamics and dispersal, which in turn impacts higher trophic level predator foraging, breeding performance and population dynamics. The ecosystem has also been highly perturbed as a result of harvesting over the last two centuries and significant ecological changes have also occurred in response to rapid regional warming during the second half of the twentieth century. This combination of historical perturbation and rapid regional change highlights that the Scotia Sea ecosystem is likely to show significant change over the next two to three decades, which may result in major ecological shifts.

No abstract

time it grows to 1 cm in shell diameter 21 . Analysis was carried out on both freshly 90 caught material preserved directly upon collection and on specimens that were 91 incubated under manipulated CO 2 levels (375 to 750 parts per million at 4°C) in order 92 to establish a response index. All freshly-caught and incubated specimens were 93 preserved in 70% ethanol. Subsequently, they were treated to dehydrate shell-layers 94 and to remove the periostracum (Fig. 3) so that the state of the underlying shell matrix 95 could be examined using SEM. 96 97 Different degrees of dissolution were identified in incubated shells of live pteropods 98 (see supplementary information). We categorised them into three main levels 99 according to the degree of encroachment upon the upper prismatic layer and into the 100 upper shell layer (Fig. 4). Specimens were scored blind and then correlated back to 101 5 the experimental conditions. Incubations in which even only a slight degree of 102 undersaturation was experienced for 8 d (Ω A 0.94-1.12, pCO 2 of 675 µatm) was 103 sufficient to cause substantial dissolution of the shell matrix relative to the 104 supersaturated control (Ω A 1.62-1.78, Fig. 5). We then examined freshly caught 105 material, preserved directly upon collection, for signs of such shell dissolution. 106 107 L. helicina antarctica juveniles were found at all sampling stations, with northern 108 stations (<57ºS) containing higher abundances (7.2 x 10 4 to 3.4 x 10 4 ind. m -2 ) than 109 those to the south (>57ºS; 2.9 x 10 2 to 1.9 x 10 3 ind. m -2 ). At station Su9, we found L.