Traditionally, we have collected all particles as bulk material in sediment traps. However, this does not address the mechanisms that control the export and turnover of material as it sinks through the water column. Examining the particle size-distributions in conjunction with measured fluxes and zooplankton distributions and behavior provides a powerful new approach to the understanding of carbon flow. To accomplish this we are combining careful mapping of the abundance and horizontal and vertical distribution of krill and salps with detailed studies of their grazing and fecal pellet production rates. This approach provides us with a clear understanding of how much suspended phytoplankton is packed into large, fast-sinking fecal pellets for each study region. We are using camera profiles (Photo 1) and modified sediment traps (Photo 2) equipped with a viscous gel that preserves the size, shape, and structure of all sinking particles, including the fragile fecal pellets from salps and krill. In this way we can determine the proportion of fecal pellets that will fall all the way to the seafloor, the proportion that will be re-processed by either microbes or zooplankton in the water column. To understand the role of microbes and zooplankton for the turnover of sinking fecal pellets we are measuring size-specific sinking velocities, microbial respiration, and biogeochemical composition of the pellets in the laboratory. Additionally we brought large cylindrical aquariums called ‘Kreisel’ (Photo 3) in which we video record the feeding behaviour of both krill and salps when offered a diet of their own fecal pellets. All recordings are illuminated with infrared light to emulate their nightly foraging conditions in the ocean surface waters. With these studies we hope to understand the fate of krill and salp fecal pellets in Antarctica.

One may ask why it is interesting to study the waste products of krill and salps. The answer is closely linked to recent discoveries of a shift from krill to salps in the Southern Ocean. Currently we do not know the consequences of such a shift for the ecosystem structure and function. Salps are known as non-selective feeders that graze on anything from the size of one bacterium up to the largest particle that can fit through its mouth opening. This means that more phytoplankton might be packed into large, rapidly sinking salp fecal pellets compared to krill fecal pellets, and a shift from krill to salps may enhance the carbon flux and the efficiency of the biological carbon pump. A shift from krill to salps may also channel less primary production to the large pelagic, krill feeding whales. It is therefore important to study the waste products of krill and salps because it will tell us what the future of the large marine mammals in the Southern Ocean might look like. Our studies will also tell us how the carbon sequestration in the Southern Ocean will function after a shift from krill to salps.

We are currently steaming towards the Weddel Sea to look for the krill spawning grounds and to find krill larvae. Our hope is that we will mainly find krill in these southern, cold waters and that we acquire a better understanding of the export and turnover processes in an ecosystem dominated by krill. By comparing the results with those obtained in the salp dominated system around Elephant Island we will learn a lot about the influence of salps on the ecosystem structure and the export of carbon.

With best wishes from Polarstern on behalf of the chief scientist, Bettina Meyer and all participants,

Morten Iversen