The western Atlantic sector of the Southern Ocean and the northern part of the Western Antarctic Peninsula (WAP) in particular is one of the fastest warming areas on the planet, making this part of the Southern Ocean an invaluable observatory of ecosystem responses to climate change. This warming in the WAP region has resulted in a much shorter winter sea ice duration, with direct impact on the marine biota, particularly those relying on the microbial sea ice community as a food source during winter. The past 30 years indicate a significant sea ice decline in the WAP region. This has had profound consequences for the marine biota and resulted in krill stock decline due to reduce larval krill development and survival during winter, increase of salp population, and a decrease in food quality and quantity. Compared to krill, salps prefer warmer ice-free waters and thrive at lower phytoplankton concentrations due to their filtration mode of feeding. Krill and salps are the most important macrozooplankton grazers on primary production in the Atlantic sector of the Southern Ocean, but they differ strikingly in their life history, feeding biology, population dynamics and trophic position in the marine ecosystem of the Southern Ocean. Thus, shifts in dominance between these two groups might trigger a cascade of short- and long-term changes in ecosystem structure and function, affecting both biodiversity and biogeochemical cycles in the pelagic system of the western Atlantic sector of the Southern Ocean. Thus, this system provides a unique natural model system to study the consequences of anthropogenic warming on food web dynamics and concurrent direct and indirect effects on biogeochemical cycles, which are poorly understood. The overall goals of the project are a) to investigate the role of krill and salps in controlling productivity, as well as their role in (re)cycling of macronutrients (N, P, Si) and trace elements (Fe, Zn, Co, Cu, Cd, Pb, with an emphasis on Fe), export (carbon flux), and microbial food web composition in the Atlantic sector of the Southern Ocean.