Objectives

The Southern Ocean plays a major role in the redistribution of carbon dioxide between the atmospheric and oceanic reservoirs. This ocean region, which by surface area covers about 20% of the global ocean, is unusual as it exhibits plenty essential nutrients nitrate, phosphate and silicate in surface water, but somehow the photosynthetic carbon dioxide fixation and plant biomass is low. Several independent global carbon cycle models suggest that whatever limits the uptake of the major nutrients in the Southern Ocean has a overriding influence on atmospheric carbon dioxide. On a basin scale it is the competition between physical processes (Antarctic upwelling) and the biological pump that determines the impact of the Southern Ocean on the atmospheric carbon dioxide level. In recent investigations by several CARUSO partners it has become evident that the carbon dioxide fixation of the plankton (specifically diatoms) is iron-limited. On the other hand, light limitation has been and still is seen as a major factor for explaining the low biological productivity in the Southern Ocean. In the laboratory it was observed that under iron-limited conditions the plankton is less capable of adaptation to ambient light conditions. The interaction between the two major variables in the Antarctic, iron availability and the given variable light regime, is obviously highly relevant to the carbon dioxide fixation in the Southern Ocean. Thus the overall objective of CARUSO may be formulated in one hypothesis to be tested: "The carbondioxide uptake by the Southern Ocean is being dominated by synergistics of light and iron regulating the photosynthetic CO₂ fixation of large diatoms and carbon export into deeper Antarctic waters." For unraveling the physical forcings (light, wind, mixing) and chemical and biological processes which together control the carbon dioxide budget in the surface ocean a carefully designed combination of four lines of observational approach and their integration by modeling is followed: Estimation of the biological and physical carbon dioxide pumps in the Southern Ocean, with application of transient tracers chlorofluorocarbons, 3He/4He and tritium. Investigation of co-limitation of light and iron of bloom forming diatoms. Quantification of iron sources to Antarctic surface waters with the use of n atural isotopic tracer techniques (228Ra and 143Nd/144Nd). Field observations and an in situ iron fertilisation experiment. Modeling study of the biological, chemical and physical systems in the Antarctic.
Workpackage 1: Estimation of the biological and physical pumps of CO₂

Workpackage 2: Investigation of co-limitation by light and iron of Antarctic diatoms

Workpackage 3: Quantification of iron sources to Antarctic surface waters

Workpackage 4: Field observations and in situ iron enrichment experiment

Workpackage 5: Modelling study of Antarctic biological and physical system
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