Modeling phytoplankton blooms and carbon export production in the SouthernOcean: 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 w ere explored with an ecological model (SWAMCO) describing the cycling of C, N, P, Si and Fe through different, aggregated, chemical and biological com partments of the plankton ecosystem. The structure of the model was chosen to take explicitly into account biological processes of importance in the f ormation and mineralization of carbon biomass in surface waters and in carb on export production. State variables include major inorganic nutrients (NO 3, NH4, PO4, SI(OH)(4)), dissolved Fe, two groups of phytoplankton (diatoms and nanoflagellates), bacteria, heterotrophic nanoflagellates, microzoopla nkton, labile DOC and two classes of dissolved and particulate organic poly mers with specific biodegradability. The model is closed by export producti on of particulate organic matter out of the surface layer and, when relevan t, by metazooplanton, the grazing pressure of which is described as a forci ng 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 SWAM CO model was coupled 'off-line' to a 1D physical model forced by in situ me teorological and sea-ice conditions. The predictions of the model were succ essfully compared with chemical and biological observations recorded in the Antarctic circumpolar current (ACC) during the 1992 cruise ANTX/6 of RV Po larstern. In particular, the model simulates quite well the diatom bloom an d carbon export event observed in the iron-enriched Polar Frontal region an d the lack of ice-edge phytoplankton blooms in the marginal zone (MIZ) of t he ACC area. Model analysis shows that sufficient light and iron concentrat ions above 0.5 mu mol m(-3) are the necessary conditions for enhancing diat om blooms and particulate carbon export production in the Southern Ocean. L ow iron availability prevents diatom growth but is still adequate for nanop hytoplankton, the biomass of which is, however, kept to Chi a levels less t han 1 mg(-3) due to the loss by the ubiquitous micrograzers. Little carbon export is predicted under iron-limitation conditions. Sensitivity tests con ducted on the parameters describing iron and silicon uptake by diatoms reve al the complex nature of Fe and Si limitation in regulating the magnitude a nd extent of diatom blooms and carbon and opal export production in the Sou thern Ocean. (C) 2000 Elsevier Science Ltd. All rights reserved.