Ocean food webs modify the efficiency with which CO2 is absorbed from the atmosphere into the sea, fixed by phytoplankton in the surface layers and then transported to the ocean interior. This so-called &biological pump& buffers about one-third of the CO2 released from all human activities. Our priority is to understand the Southern Ocean ecosystem and its interconnections sufficiently well to be ... able to make realistic predictions about the likely response to future climate change. In addition, after two centuries of overexploitation, a similar level of detail is essential to allow effective ecosystem-based management into the future. Our challenge is to understand the structure and functioning of the range of food webs present in the southwest Atlantic Sector of the Southern Ocean, from those in relatively unproductive deep ocean systems to the highly productive, krill-dominated systems elsewhere. We hypothesise that these extremes support fundamentally different types of food web, with the latter effecting an efficient transfer of CO2 from the atmosphere into the deep ocean.
Much data have already been collected by various nations on the distribution and diets of the major species within the food web. We will firstly synthesise these data, for example with construction of databases, and target field sampling at some key unresolved issues. The research cruises aboard James Clark Ross will involve large scale transecting across the range of habitats in the Southwest Atlantic sector of the Southern Ocean. We plan three such cruises, sampling in spring, summer and autumn to incorporate elements of seasonal change. These cruises are linked to an ongoing research programme from the shore bases on Signy and South Georgia on the land-breeding predators.
Key issues include: 1) Investigating the distribution, diet and abundance of poorly known taxa such as baleen whales, king penguins, petrels and myctophids (midwater fish), and gap-filling for species where data are lacking for nonbreeders, for particular age, sex or size classes, or at certain times of year (usually winter). Where appropriate this will include indirect approaches such as fatty acid, trace metal and stable isotope analysis in order to construct comprehensive models of food web operation and energy flow. 2) Understanding the role of iron, which is a critical nutrient but is in short supply over much of the Southern Ocean. Where it fertilises the upper ocean (for example in wind-blown dust) it can generate outbursts or &blooms& of phytoplankton which support the food web. 3) Determining the rate of vertical export of carbon to the ocean interior. We will deploy "sediment traps", akin to rain gauges, which collect the material falling out of the productive surface layer.
Objectives include: 1. To describe the spatial and temporal distributions of the major species within each food web. 2. To quantify the major energy flows. 3. To understand the key factors controlling energy flow in the food web including a) responses to variable food supply and, b) implications for carbon export
Bearhop S, Phillips RA, McGill R, Cherel Y, Dawson DA & Croxall JP. (in press) Stable isotopes indicate sex-specific and long-term individual foraging specialisation in diving seabirds. Marine Ecology Progress Series. Cherel Y, Phillips RA and McGill R. (in press). Stable isotope evidence of diverse species-specific and individual wintering strategies in seabirds. Biology Letters. Collins MA & ... Rodhouse PGK, in press. Southern Ocean cephalopods. Advances in Marine Biology. Poncet S, Robertson G, Phillips RA, Lawton K, Phalan B, Trathan PN & Croxall J.P. 2006. Status and distribution of wandering, black-browed and grey-headed albatrosses breeding at South Georgia. Polar Biology. Reid K, Diniz TCD, Hill S & Collins MA. 2005. Mackerel icefish Champsocephalus gunnari in the diet of upper trophic level predators at South Georgia: Implications for fisheries management. Marine Ecology Progress Series 305, 153-161 Trathan PN, Green C, Tanton J, Peat H, Poncet J & Morton A. 2006. Foraging dynamics of macaroni penguins Eudyptes chrysolophus at South Georgia during brood-guard. Marine Ecology-Progress Series. Warren NL, Trathan PN, Forcada J, Fleming A & Jessopp MJ. 2006. Distribution of post-weaning Antarctic fur seal Arctocephalus gazellae pups at South Georgia. Polar Biology 29, 179-188. Phillips RA, Silk JRD & Croxall JP. 2005. Foraging and provisioning strategies of the light-mantled sooty albatross at South Georgia: competition and co-existence with sympatric pelagic predators. Marine Ecology Progress Series 285, 259-270. Phillips RA, Silk JRD, Croxall JP, Afanasyev V & Bennett VJ. 2005. Summer distribution and migration of nonbreeding albatrosses: individual consistencies and implications for conservation. Ecology 81, 2386-2396. Phillips RA, Silk JRD, Croxall JP & Afanasyev V. 2006. Year-round distribution of white-chinned petrels from South Georgia: rela tionships with oceanography and fisheries. Biological Conservation 12 9, 336-347.