Iron limitation and cycling in contrasting Southern Ocean Provinces under current and future climate

12 berths on board the RV Polarstern in collaboration with Prof. Christel Hassler (University of Geneva) from 16 February 2016 to 8th April 2016 (PS97, ANT-XXXI/3).

The ProIron team onboard Polarstern in February 2016. Photo: S. Trimborn (Photo: Alfred-Wegener-Institut)

The Southern Ocean ecosystem exerts a disproportional control on the global carbon cycle and is considered to be responsible for 40% of uptake of anthropogenic CO2, playing therefore a pivotal role in the global climate system. Southern Ocean phytoplankton are major drivers of global carbon cycling accounting for 20% of the global annual primary productivity. One of the most challenging issues is to understand how trace metal limitation and cycling operates and how global change will impact the Southern Ocean ecosystem. The availability of trace metals, in particular iron, is considered the key factor in controlling Southern Ocean phytoplankton productivity and community structure. However, phytoplankton are not isolated from other microorganisms and biological interactions can have profound effects on the dynamics of the system.

Seawater sampling in the Drake Passage and along the South Shetland Islands. Picture: S. Trimborn (Photo: Alfred-Wegener-Institut)

This project aims to study how microbial processes are driving and interconnecting the biogeochemical cycles of carbon as well as of trace metals such as iron, zinc, cobalt and manganese. Here, we will focus on viral-bacterial-phytoplankton interactions considering biologically excreted organic matter as a control for trace metal chemistry, bioavailability to phytoplankton and productivity. As the increase in atmospheric CO2 has already caused significantly higher aquatic CO2 concentrations and lower pH values (‘ocean acidification’) compared to pre-industrial times potentially affecting plankton community structure as well as iron chemistry this study will also evaluate the sensitivity of phytoplankton of different regions to trace metal input and climate change scenarios in order to predict their response to future changes. These aims will be addressed mainly by ship-board incubation experiments with phytoplankton of contrasting water masses from the Southern Ocean; namely the naturally iron-enriched waters of the Antarctic Peninsula and the South Orkney Islands and two open ocean sites of the iron-limited waters of the Drake Passage and the Scotia Sea. The proposed research will significantly improve our current understanding of how trace metals and carbon biogeochemical cycle inter-connect, what are the key organic material present in situ and establish how microbial interactions control the biogeochemical cycle of trace metals in the Southern Ocean under current and future climatic scenarios. Improved knowledge on the functioning and the sensitivity of the Southern Ocean ecosystem is pivotal to improve our existing predictive tools (e.g. modelling) and increase our understanding on the mechanisms, by which the Southern Ocean affects climatic processes at global scale.