Spatial and temporal operation of the Scotia Sea ecosystem: a review of large-scale links in a krill centred food web.

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.

Physiological Progenesis in Cephalopod Molluscs.

The coleoid cephalopods (cuttlefish, squid and octopus) arose from their shelled ancestors during the late Devonian; they diversified in the Jurassic but did not radiate substantially until the Tertiary. Since then they have coevolved with the fish (1). Squid are less efficient energetically than fish (2) but have survived alongside them by evolving highly opportunistic reproductive and feeding strategies (3, 4) as well as rapid jetting and inking for escape and defense. Little is known about the life history strategies of the fossil forms, but the only surviving shelled cephalopods, the nautiluses, have relatively long life spans and are iteroparous; that is, in common with most members of other molluscan classes, they breed more than once during their lives. In contrast, all other living cephalopods are generally short lived (usually 1 year) and have monocyclic reproduction and a semelparous life history. The short-lived semelparous coleoids are typified by the mid-latitude ommastrephid squid which provide the basic model considered here. This family is relatively primitive and biologically well known. Its members are essentially monocyclic, but some species may spawn their eggs in batches (5, 6) although there is no evidence of this in laboratory spawnings (7). Most loliginid squid, at least in temperate seas, have a life cycle similar to that of the ommastrephids, despite having different spawning habits. A comparison of the lifetime energetics and growth pattern of benthic, iteroparous molluscs with those of the pelagic, semelparous ommastrephids shows that, although some squid may attain a length of 1 m or more, the allocation of their energy resource among growth components is essentially characteristic of the early life, especially the first year, of iteroparous forms. The life-time energy budget of these squid thus seems to have evolved by physiological progenesis, a process in which maturation is accelerated while other aspects of the physiology are more typical of the juvenile.

Cephalopods Occupy the Ecological Niche of Epipelagic Fish in the Antarctic Polar Frontal Zone.

Recent data from research cruises and explorator fishing in the Antarctic Polar Frontal Zone (APFZ) of the Scotia Sea, together with data from dietary studies of Antarctic vertebrate predators, have revealed a large, previously overlooked trophic system in the Southern Ocean (Fig. 1). The upper trophic levels of this open-ocean epipelagic community are exceptional in that they contain no fish species. Fishes are replaced by cephalopods, including the ommastrephid squid, Martialia hyadesi. This squid preys on mesopelagic m.yctophids (lanternfish), which feed largely on copepods. We identify here a geographically distinct, Antarctic, open-ocean food chain which is of importance to air breathing predator species but where Antarctic krill, Euphausia superba, is absent. This system is probably prevalent in areas of higher primary productivity, especially the Scotia Sea and near the peri-Antarctic islands. Squid stocks in the APFZ may have potential for commercial exploitation, but they, and the predators they support, are likely to be sensitive to overfishing. Squid have a short, semelparous lifecycle, so overfishing in a single year can cause a stock to collapse.