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Controls on Southern Ocean phytoplankton production - A systems approach


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Molina Balari, E 2012 , 'Controls on Southern Ocean phytoplankton production - A systems approach', PhD thesis, University of Tasmania.

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Plankton ecosystems play a significant part in the global cycle of carbon by
contributing ~40% of total annual primary productivity. Changes are expected in
response to warming, increased ocean stratification, altered supply of iron as a
limiting micronutrient, and ecosystem structures. To diagnose these effects, numerical
models of plankton ecology have become a key approach. This thesis examines the
Nutrient Phytoplankton Zooplankton Detritus (NPZD) ecosystem model of Oschlies
& Garcon [1999], and especially the modifications required to simulate phytoplankton
biomass in naturally iron-fertilized Southern Ocean waters. This provides
perspectives on the sensitivity of Southern Ocean ecosystems to global change.
The NPZD model was used to simulate the phytoplankton bloom that forms in
natural iron-fertilized waters of the Kerguelen plateau and adjacent High Nutrient
Low Chlorophyll (HNLC) waters. Sensitivity to mixed layer depth and temperature
was found to be negligible in comparison to zooplankton grazing, which also strongly
modulated the ability of increased iron availability to affect biomass via
phytoplankton growth rates. Direct comparison with satellite chlorophyll estimates
showed that the base model could not achieve chlorophyll concentrations greater than
~0.6 mg m-3, in comparison to the ~2.7 mg m-3 of chlorophyll observed in the
Kerguelen bloom. Incorporating an indirect simulation of iron-fertilization provided
only a small improvement. An increase in zooplankton grazing (by a factor of 7) was
required to achieve the high peak bloom biomass. To match the seasonal cycle and
both summer maximum and winter minimum biomass concentrations, it was also
necessary to impose seasonal cycles in key photosynthetic parameters (α - initial
slope of the photosynthesis/irradiance curve, and μmax - maximal growth rate).
The same model was used to examine the response of plankton ecosystems to
temporary iron fertilizations. The model was able to simulate the phase and amplitude
patterns of the SOIREE bloom in Antarctic waters, given increases in α and μmax
similar to those observed in the field (a factor of 2.5 over 13 days). Simulating a
series of SOIREE-like experiments at different times of the seasonal cycle showed
that while blooms induced in spring resulted in high surface chlorophyll levels, only
blooms induced in summer resulted in both high chlorophyll and high potential
carbon export. These findings also emphasized the importance of low overwintering biomass of phytoplankton and zooplankton to the spring biomass peak. Expanding the
simulations to all latitudes of the Southern Ocean revealed distinct variations in the
intensity of responses to iron fertilization, and suggested that the optimal time and
location for iron fertilization in terms of potential carbon export is early summer in
the Subantarctic zone.

Item Type: Thesis - PhD
Authors/Creators:Molina Balari, E
Keywords: Southern Ocean, biogeochemistry, modelling, primary production
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