Geoscience Reference
In-Depth Information
Biogeochemical properties
The Southern Ocean is known to be a high nutrient
low productivity area.
Normally availability of nutrients controls the productivity of an ocean area.
When phytoplankton consumes nutrients and no supply occurs by upwelling of
nutrient-rich deeper water, the surface layers are soon depleted of nutrients. In
deeper layers there is not enough light for plants to grow. In the Southern Ocean
relatively high nutrient concentrations occur in the near surface layers but still the
production is low, with only some exceptions. Deep mixing, due to strong winds and
weak strati
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cation, could restrict production because it limits the time during which
phytoplankton can remain in the vicinity of the surface to receive enough light for
growth. More recent investigations suggest that the missing micronutrient iron
limits the biological production. Iron is provided to the ocean through dust which
is transported by the wind from land, e.g. from the Sahara or Patagonia into the
ocean. However, in the Southern Ocean land masses which are able to provide dust
are remote and, in consequence, the natural concentration of iron is very low. The
hypothesis that missing iron is the origin of high nutrient
low productivity areas can
be tested by controlled iron fertilisation of particular ocean areas. Recent iron
fertilisation experiments support this hypothesis and suggest to some people that
geo-engineering with added iron could be one way to solve the global increase in
carbon dioxide.
The uptake of atmospheric carbon dioxide is an important component in
the biogeochemical cycles of the Southern Ocean. Increasing carbon dioxide uptake
from the atmosphere increases the acidity of the ocean waters. Increasing acidity
affects biogeochemical processes. In particular, zooplankton organisms which form
calcareous skeletons are affected because the solubility of calcium carbonate
decreases with increasing acidity. When reaching a critical level, skeletons cannot
be formed any more or will be dissolved. The potential lack of such zooplankton
organisms will perturb the food web with important consequences both for the
biogeochemical cycle as well as life in all the higher trophic levels. This trend seems
intimately linked to global carbon dioxide levels and may now be unstoppable.
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Variations and changes
The conditions in the Southern Ocean are subject to intensive
fluctuations,
which cover a wide range of timescales. Surface waves, small-scale turbulence
and internal waves represent the high frequency part of the spectrum. Eddies and
meanders with space scales from metres to hundreds of kilometres cover the
timescales from days to months. Seasonal, annual, multi-annual and decadal
variations form the low period part of the spectrum. Fluctuations of all timescales
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