Modeling phytoplankton blooms and carbon export production in the Southern Ocean: dominant controls by light and iron in the Atlantic sector in Austral spring 1992

The high nutrient low chlorophyll (HNLC) conditions of the Southern Ocean were explored with an ecological model (SWAMCO) describing the cycling of C, N, P, Si and Fe through different, aggregated, chemical and biological compartments of the plankton ecosystem. The structure of the model was chosen to take explicitly into account biological processes of importance in the formation and mineralization of carbon biomass in surface waters and in carbon export production. State variables include major inorganic nutrients (NO3, NH4, PO4, SI(OH)(4)), dissolved Fe, two groups of phytoplankton (diatoms and nanoflagellates), bacteria, heterotrophic nanoflagellates, microzooplankton, labile DOC and two classes of dissolved and particulate organic polymers with specific biodegradability. The model is closed by export production of particulate organic matter out of the surface layer and, when relevant, by metazooplanton, the grazing pressure of which is described as a forcing function. Parameterization was derived from the current knowledge on the kinetics of biological processes in the Southern Ocean and in other 'HNLC' areas. For its application in the Atlantic sector in spring 1992, the SWAMCO model was coupled 'off-line' to a 1D physical model forced by in situ meteorological and sea-ice conditions. The predictions of the model were successfully compared with chemical and biological observations recorded in the Antarctic circumpolar current (ACC) during the 1992 cruise ANTX/6 of RV Polarstern. In particular, the model simulates quite well the diatom bloom and carbon export event observed in the iron-enriched Polar Frontal region and the lack of ice-edge phytoplankton blooms in the marginal zone (MIZ) of the ACC area. Model analysis shows that sufficient light and iron concentrations above 0.5 mu mol m(-3) are the necessary conditions for enhancing diatom blooms and particulate carbon export production in the Southern Ocean. Low iron availability prevents diatom growth but is still adequate for nanophytoplankton, the biomass of which is, however, kept to Chi a levels less than 1 mg(-3) due to the loss by the ubiquitous micrograzers. Little carbon export is predicted under iron-limitation conditions. Sensitivity tests conducted on the parameters describing iron and silicon uptake by diatoms reveal the complex nature of Fe and Si limitation in regulating the magnitude and extent of diatom blooms and carbon and opal export production in the Southern Ocean. (C) 2000 Elsevier Science Ltd. All rights reserved.