Phytoplankton distribution and nitrogen dynamics in the Southwest Indian subtropical gyre and Southern Ocean Waters.

During the 1999 Marion Island Oceanographic Survey (MIOS 4) in late austral summer, a northbound and reciprocal southbound transect were taken along the Southwest Indian and Madagascar Ridge, between the Prince Edward Islands and 31 °S. The sections crossed a number of major fronts and smaller mesoscale features and covered a wide productivity spectrum from subtropical to subantarctic waters. Associated with the physical survey were measurements of size fractionated chlorophyll, nutrients and nitrogen (NO₃, NH₄ and urea) uptake rates. Subtropical waters were characterised by low concentrations (<0.27 mg m^-3) of pico-phytoplankton cells (>81%) and very low f -ratios (<0.1), indicative of productivity based almost entirely on recycled ammonium and urea. Diatom growth was limited by the availability of NO₃ (<1 mmol m^-3) and SiO₄ (<1.5 mmol m^-3) through vertical stratification that prevents the upward flux of nutrients into the euphotic zone. Biomass accumulation of small cells was likely controlled by microzooplankton grazing. In subantarctic waters, total chlorophyll concentrations increased (<0.74 mg m^-3) and larger cells became more prevalent, however smaller phytoplankton cells and low f -ratios (>0.15) still dominated, despite sufficient NO₃ availability. The results from this study favour Si limitation, light-limited deep mixing and likely Fe deficiency as the dominant mechanisms controlling significant new production by micro-phytoplankton. Increased concentrations of micro-phytoplankton cells and and rates of new production did however occur at oceanic frontal regions (58.6% and 11.22%, respectively), and in the region of the Prince Edward archipelago (61.4% and 14.16%, respectively). Here water column stabilization and local Fe-enrichment are thought to stimulate phytoplankton growth rates, especially of diatoms. Open ocean regions such as these provide important areas for local but significant POC export and biological CO₂ draw-down in an overall HNLC Southern Ocean. [ABSTRACT FROM AUTHOR]