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Phytoplankton Distribution and Nitrogen Dynamics in the Southwest Indian Subtropical Gyre and Southern Ocean Waters : Volume 7, Issue 4 (23/07/2010)

By Thomalla, S. J.

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Book Id: WPLBN0003985271
Format Type: PDF Article :
File Size: Pages 57
Reproduction Date: 2015

Title: Phytoplankton Distribution and Nitrogen Dynamics in the Southwest Indian Subtropical Gyre and Southern Ocean Waters : Volume 7, Issue 4 (23/07/2010)  
Author: Thomalla, S. J.
Volume: Vol. 7, Issue 4
Language: English
Subject: Science, Ocean, Science
Collections: Periodicals: Journal and Magazine Collection (Contemporary), Copernicus GmbH
Publication Date:
Publisher: Copernicus Gmbh, Göttingen, Germany
Member Page: Copernicus Publications


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Thomalla, S. J., Ansorge, I. J., Lucas, M. I., Pakhomov, E., Waldron, H. N., & Read, J. F. (2010). Phytoplankton Distribution and Nitrogen Dynamics in the Southwest Indian Subtropical Gyre and Southern Ocean Waters : Volume 7, Issue 4 (23/07/2010). Retrieved from

Description: Department of Oceanography, University of Cape Town, Private Bag, Rondebosch, Cape Town 7701, South Africa. 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.

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

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