Based on pronounced differences between these two grazers in feeding, growth, reproductive cycles and internal structure / composition with regards to carbon-, lipid- and protein content, we also expect different impacts on the plankton community. On this cruise we combine field studies with experimental manipulations on board in order to investigate a whole suite of factors controlling plankton dynamics and export in patches with high krill or salp abundances. We take water samples from 4 depths at different onshore and offshore locations to characterize plankton biomass and community structure (bacteria, phyto- and zooplankton of different size classes), as well as the biochemistry of the water (dissolved nutrients and organic carbon). In addition, we take Bongo net tows to collect and characterize mesozooplankton (0.2 - 2mm, Photo 3).

All of these data will be related to krill and salp abundances in the water column determined by net tows as well as to the respective hydrographic conditions.

Furthermore, we conduct on-board experiments with krill and salps. Whereas krill is quite robust to be used in experimental manipulations, salps are very sensitive and fragile and have to be handled with a lot of care. So far, very few experiments have been conducted with Salpa thompsoni in the Southern Ocean and we are very happy that we were able to collect salps alive and in good physiological condition at Deception and Elephant Islands. We conducted two large experiments with them so far, one of which is still running. In both, we incubated krill and salps alone and in combination (salps and krill) with a natural plankton community in cylindrical aquaria that we brought with us (70 liters, Photo 2). During the first experimental period, we investigated direct grazing effects on plankton biomass and community structure, as well as effects on nutrient recycling and carbon export. The treatment combining krill and salps will enable us to study interactive effects of these two major grazers, which co-occurred in the natural habitats sampled at Deception and Elephant Islands. After 4-5 days, we filtered the water preconditioned by krill and salps to remove all of the organisms and incubated a fresh phytoplankton assemblage. Salps and krill presumably recycle nutrients in different amounts and ratios, which may indirectly affect phytoplankton biomass and community structure. In comparison with large-scale observations in the field, our experiments allow us to disentangle and quantify direct and indirect consequences of potential grazer shifts from krill to salps for the plankton community structure and process rates within the food web.

As expected, we did not find Salpa thompsoni further south, but we found lots of krill. The Krill group (AWI) led by Bettina Meyer, investigated the relationship between the bottom topography west off the Antarctic Sound and the respective hydrography and krill abundance and distribution. High krill abundances were observed above the numerous seamounts characterizing this region. Moreover, they conducted krill growth experiments, which are a good indicator of food availability and the fitness of krill.

The Biological Carbon Pump group, led by Morten Iversen (AWI/Marum, Bremen), conducted a 24-hour camera survey of the water column during which they deployed their camera system every six hours to obtain a more detailed insight into the vertical night-day migration of krill and their fecal pellet production. Additionally, they were able to conduct another experiment to assess the fecal pellet production of krill over time, as well as the carbon content of these pellets.

Wednesday morning, we had wonderful weather conditions and a lot of humpback whales came very close to the ship, including a mother-calf pair. Our whale survey group led by Helena Herr (University of Hamburg) had covered the Antarctic Sound the same morning by helicopter and recorded many sightings of humpback whales. In the afternoon, we left this southernmost region of our research transect through an, unfortunately, very foggy Antarctic Sound, to reach our last East transect, northbound, on the southern side of Elephant Island. We are looking forward to find out what conditions will prevail there, compared with the northern side, in terms of krill and salp biomass and distribution, and also regarding fin whale abundance. We hope to fill some last gaps in our experiments and process studies and cannot yet imagine that we will have to pack right afterwards to start our voyage back home.

With best wishes from Polarstern on behalf of the Chief Scientist, Bettina Meyer and all participants,

Stefanie Moorthi