CO2 concentration and pH in the present Southern Ocean.

Phytoplankton ecology in the present Southern Ocean. Southern Ocean phytoplankton are major drivers of global carbon cycling accounting for ~2 Pg carbon of annual primary production. Most of this production relies on large phytoplankton blooms of a few key diatom species and the prymnesiophyte Phaeocystis along the Antarctic continent while pelagic phytoplankton contributes to a smaller degree. The productivity and community structure of oceanic and coastal phytoplankton is primarily controlled by the availability of trace metals, in particular iron (Fe). While coastal and shelf environments are naturally enriched in Fe, the land remote high nutrient low chlorophyll (HNLC) regions are typically Fe-limited. Due to this, primary production and carbon export of oceanic phytoplankton is low while inshore phytoplankton is highly productive and has a high carbon export. Next to iron, light limitation is considered the second important factor which strongly governs Southern Ocean phytoplankton productivity.

CO2 concentration and pH in the future Southern Ocean. Figure: S. Trimborn

Phytoplankton ecology in the future Southern Ocean. Climate models indicate that the Southern Ocean is particularly prone to climate change, with important implications for Southern Ocean phytoplankton ecology. By the year 2100, the ongoing anthropogenic emissions of carbon dioxide (CO2) will likely have increased atmospheric CO2 concentrations from ~390 to >750 μatm, which in turn will reduce the seawater pH of 8.1 by 0.3 units (‘ocean acidification’). Next to the ongoing OA, CO2 as a green house gas has already caused global average temperatures to increase by 0.6 °C over the past 100 years. The strong warming and freshening of Antarctic surface waters will enhance vertical stratification and reduce mixed-layer depth, which in turn will elevate mean underwater irradiances. These physico-chemical changes will not occur independently from each other and their interactive effects will inevitably affect Southern Ocean phytoplankton ecology in numerous and unexpected ways. How will these changes shape Southern Ocean phytoplankton community structure and possibly alter their productivity? Until now, only little information is available on how these environmental factors (trace metals, CO2, light) affect Southern Ocean phytoplankton. In particular research on interactive effects of these factors is still missing. The consequences will, however, most likely differ depending on the region, potentially altering the complex balance of biogeochemical cycles and climate feedback mechanisms.

Approach: The major goal of the working group is to understand how global change will affect Southern Ocean phytoplankton ecology and biogeochemistry. To this end, we follow a multidisciplinary approach that integrates marine biology and chemistry and combines laboratory and field work:

The EcoTrace approach. Figure: S. Trimborn

Manipulation experiments with natural Antarctic phytoplankton assemblages simulating future climate change scenarios will identify phytoplankton species in the field, which are particularly sensitive, but also tolerant towards climate change.

Laboratory experiments with species, that were identified as sensitive and tolerant phytoplankton isolates in the field, and selected ecologically relevant species will provide a mechanistic understanding on physiological processes in response to climate change.

In situ sampling of natural phytoplankton communities and their physiological characterization will identify the parameters primarily, but also secondarily driving the oceanic and coastal ecosystem.

Competition experiments will examine how resource limitation (trace metals) and species interactions affect Southern Ocean phytoplankton community structure.

Our results will be exploited in multidisciplinary contexts ranging from molecular through physiological to ecological processes to improve parameterization of cell, ecosystem and biogeochemical models.

The EcoTrace Group is funded by the Helmholtz Association and hosted at the Alfred Wegener Institute within the Marine Biogeosciences section and at the Bremen Marine Ecology Centre for Research and Education within the Marine Botany Department of the University Bremen.

At the moment, the following projects are on-going:

Elucidating the role of light and carbon dioxide in Southern Ocean phytoplankton succession

Vitamins, zinc and cobalt - potential drivers of Antarctic phytoplankton biodiversity?

The role of the iron source on Southern Ocean phytoplankton growth