Thomalla S.J., Waldron H., Lucas M., Read, J.F., Ansorge, I.J., Pakhamov, E.
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 CO2draw-down in an overall high nutrient low chlorophyll Southern Ocean.

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 Cruise tracks overlaid on bathymetry of the Northbound transect, showing XBT station positions as black dots. Together with the reciprocal Southbound transect, showing CTD station positions as white circles. The cruise tracks followed the Southwest Indian and Madagascar Ridge, between the Prince Edward Islands (PEI) and 31_ S. Productivity stations are shown as pink circles and labelled (NP1–NP6). Black arrows mark the position of the Agulhas current (AC), the Agulhas Return Current (ARC) and the Antarctic Circumpolar current (ACC). Grey lines indicate the mean frontal positions of the Sub Tropical Front (STF), the Sub Antarctic Front (SAF) and the Polar Front (PF) according to Orsi et al. (1995).

Cruise tracks overlaid on bathymetry of the Northbound transect, showing XBT station positions as black dots. Together with the reciprocal Southbound transect, showing CTD station positions as white circles. The cruise tracks followed the Southwest Indian and Madagascar Ridge, between the Prince Edward Islands (PEI) and 31_ S. Productivity stations are shown as pink circles and labelled (NP1–NP6). Black arrows mark the position of the Agulhas current (AC), the Agulhas Return Current (ARC) and the Antarctic Circumpolar current (ACC). Grey lines indicate the mean
frontal positions of the Sub Tropical Front (STF), the Sub Antarctic Front (SAF) and the Polar Front (PF) according to Orsi et al. (1995).

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