Phytoplankton distribution and nitrogen dynamics in the southwest indian subtropical gyre and Southern Ocean waters

Abstract. 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 chlorophyll concentrations (max = 0.27.3 mg m⁻³ dominated by pico-phytoplankton cells (> 81%) and very low f-ratios (< 0.1), indicative of productivity based almost entirely on recycled ammonium and urea. Micro-phytoplankton growth was limited by the availability of NO₃ (< 0.5 mmol m⁻³ and Si(OH)₄ (< 1.5 mmol m⁻³ through strong vertical stratification preventing the upward flux of nutrients into the euphotic zone. Biomass accumulation of small cells was likely controlled by micro-zooplankton grazing. In subantarctic waters, total chlorophyll concentrations increased (max = 0.74 mg m⁻³ relative to the subtropical waters and larger cells became more prevalent, however smaller phytoplankton cells and low f-ratios (< 0.14) still dominated, despite sufficient NO₃ availability. The results from this study favour Si(OH)₄ limitation, light-limited deep mixing and likely Fe deficiency as the dominant mechanisms controlling significant new production by micro-phytoplankton. The percentage of micro-phytoplankton cells and rates of new production did however increase 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. Open ocean regions such as these provide important areas for local but significant particulate organic carbon export and biological CO₂ draw-down in an overall high nutrient low chlorophyll Southern Ocean.