DISCOVERY-FOOD-WEBS: Scotia Sea FOOD-WEBS

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.