Community shifts and productivity changes in the Southern Ocean

The Southern Ocean exerts an exceptionally large influence on the marine carbon cycle and likely played a key role in glacial-interglacial atmospheric CO2 transitions. CO2-related changes in environmental conditions, both directly through ocean seawater acidification and indirectly via increasing thermal stratification, are expected to be particularly pronounced in high latitudes and hence provide the possibility for strong feedbacks on atmospheric CO2. Despite these implications, little information is currently available on the potential CO2 sensitivity of Southern Ocean phytoplankton.

Being one of the three major high-nutrient-low-chlorophyll (HNLC) areas, macronutrient concentrations are generally high while much of the Southern Ocean is strongly Fe-limited. Moreover, large seasonal changes in solar irradiance, sea ice cover and vertical mixing affect growth conditions for phytoplankton. These specific characteristics of the Southern Ocean have to be considered when assessing the sensitivity of Southern Ocean phytoplankton to global change.
Key species in this region are diatoms belonging to the genera Fragilariopsis, Chaetoceros or Pseudo-nitzschia and the prymnesiophyte Phaeocystis antarctica. The latter species is known to be a prodigious producers of DMS(P), a sulphur compound ultimately leading to the release of volatile aerosols which induce atmospheric cloud formation. Large seasonal blooms of these species occur in coastal and open ocean regions and drive most of the vertical carbon fluxes. Both groups have very specific effects on elemental cycles, and changes in their distribution have pronounced impacts on higher trophic levels.
Laboratory and field experiments assess the sensitivity of key diatom species and Phaeocystis to ocean acidification under different iron availability and light regimes. Their responses in these acclimations will be described based on standard measurements (e.g. growth rates, elemental ratios, pigment composition). Since energy availability, imposed by both iron limitation and light regime, plays a central role for phytoplankton in the Southern Ocean, different techniques are combined to measure cellular carbon and energy fluxes under the respective future scenarios. The laboratory research is complemented by field studies during Southern Ocean cruises. The fieldwork includes on-deck perturbation experiments using natural phytoplankton assemblages. Next to classical 14C-based assays and fluorescence measurements a seagoing MIMS with a novel cuvette/inlet system, currently being developed at the AWI, will be used.

AWI web site.

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