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Le système assimilation-régénération des sels nutritifs dans les eaux superficielles de l'océan Austral

Le système assimilation-régénération des sels nutritifs dans les eaux superficielles de l'océan... 227 92 92 3 3 V. Simon Centre d'Océanologie de Marseille Faculté des Sciences de Luminy (U.R.A.41) Case 901 F-13288 Marseille cédex 9 France Abstract Low primary-productivity levels in high-nutrient environments of the Southern Ocean are not yet well understood. An accurate knowledge of nutrient assimilation by phytoplankton, at the base of the pelagic food-web, therefore appears to be essential. A study of the mesoscale hydrological structure and chemical observations in the upper layers of the Indian sector of the Antarctic Ocean (Cruise MD-25 FIBEX of M.S. “Marion Dufresne”, January–February 1981) permitted us to develop a method for estimating nutrient consumption by phytoplankton. After strong vertical mixing in winter, the homogeneous Antarctic Surface Water (ASW) is, in summer, divided into two parts by a well-formed pycnocline. In the upper layer, corresponding approximately to the photic zone, we observed nutrient depletion resulting from photosynthetic activity, while the underlaying thermal minimum layer displayed unaltered winter characteristics, including nutrient concentrations typical of winter surface-water conditions. Taking into account the nutrient depletion in summer, we calculated the assimilation ratios for Antarctic phytoplankton as follows: ((NO 3 - lower)-(NO 3 - upper)):((PO 4 ≡ upper))=ΔN:ΔP=11.2±2.4 and ΔN:ΔSi=0.27±0.05. These ratios are lower than the classical Redfield ratios (ΔN:ΔP=16 and ΔN:ΔSi=1), but are in good agreement with the elementary composition of phytoplankton samples collected during the same cruise. Moreover, they agree with previously published data on stoechiometric determinations of particulate matter in surface-water samples. Besides, in the “transition layer” between the lower layer and “Warm Deep Water” (WDW), the calculated mineralization ratios were slightly higher than the assimilation ratios: ΔN:ΔP=14.6 and ΔN:ΔSi=0.37. From these ratios, it would appear that, in these surface layers of the Southern Ocean (down to 250 m), assimilation-regeneration mechanisms operate in such a way that phosphate and silicate contents decrease much more than nitrate content during the northward drift of the surface waters. The determination of assimilation ratios taking into account the nutrient depletion of the upper layer allowed us to evaluate the carbon net-production which integrates time-space variations in the photic layer over the whole early summer period. We estimated a production rate (0.4 g C m -2 d -1 ) of the same order of magnitude as the mean value obtained by the 14 C method (0.2 g C m -2 d -1 ). The theoretic silicium consumption by phytoplankton along a meridian transect, calculated using an estimated assimilation ratio of ΔC:ΔSi=1,85, indicates that the marked south-north decrease in silicate concentration in the Antarctic Surface Water would mainly result from biological activity. The silicate concentrations thus calculated were in good agreement with concentrations measured along a previous transect across the Southern Ocean. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Marine Biology Springer Journals

Le système assimilation-régénération des sels nutritifs dans les eaux superficielles de l'océan Austral

Marine Biology , Volume 92 (3) – Aug 1, 1986

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References (38)

Publisher
Springer Journals
Copyright
Copyright © 1986 by Springer-Verlag
Subject
Life Sciences; Biomedicine general; Oceanography; Ecology; Microbiology; Zoology
ISSN
0025-3162
eISSN
1432-1793
DOI
10.1007/BF00392684
Publisher site
See Article on Publisher Site

Abstract

227 92 92 3 3 V. Simon Centre d'Océanologie de Marseille Faculté des Sciences de Luminy (U.R.A.41) Case 901 F-13288 Marseille cédex 9 France Abstract Low primary-productivity levels in high-nutrient environments of the Southern Ocean are not yet well understood. An accurate knowledge of nutrient assimilation by phytoplankton, at the base of the pelagic food-web, therefore appears to be essential. A study of the mesoscale hydrological structure and chemical observations in the upper layers of the Indian sector of the Antarctic Ocean (Cruise MD-25 FIBEX of M.S. “Marion Dufresne”, January–February 1981) permitted us to develop a method for estimating nutrient consumption by phytoplankton. After strong vertical mixing in winter, the homogeneous Antarctic Surface Water (ASW) is, in summer, divided into two parts by a well-formed pycnocline. In the upper layer, corresponding approximately to the photic zone, we observed nutrient depletion resulting from photosynthetic activity, while the underlaying thermal minimum layer displayed unaltered winter characteristics, including nutrient concentrations typical of winter surface-water conditions. Taking into account the nutrient depletion in summer, we calculated the assimilation ratios for Antarctic phytoplankton as follows: ((NO 3 - lower)-(NO 3 - upper)):((PO 4 ≡ upper))=ΔN:ΔP=11.2±2.4 and ΔN:ΔSi=0.27±0.05. These ratios are lower than the classical Redfield ratios (ΔN:ΔP=16 and ΔN:ΔSi=1), but are in good agreement with the elementary composition of phytoplankton samples collected during the same cruise. Moreover, they agree with previously published data on stoechiometric determinations of particulate matter in surface-water samples. Besides, in the “transition layer” between the lower layer and “Warm Deep Water” (WDW), the calculated mineralization ratios were slightly higher than the assimilation ratios: ΔN:ΔP=14.6 and ΔN:ΔSi=0.37. From these ratios, it would appear that, in these surface layers of the Southern Ocean (down to 250 m), assimilation-regeneration mechanisms operate in such a way that phosphate and silicate contents decrease much more than nitrate content during the northward drift of the surface waters. The determination of assimilation ratios taking into account the nutrient depletion of the upper layer allowed us to evaluate the carbon net-production which integrates time-space variations in the photic layer over the whole early summer period. We estimated a production rate (0.4 g C m -2 d -1 ) of the same order of magnitude as the mean value obtained by the 14 C method (0.2 g C m -2 d -1 ). The theoretic silicium consumption by phytoplankton along a meridian transect, calculated using an estimated assimilation ratio of ΔC:ΔSi=1,85, indicates that the marked south-north decrease in silicate concentration in the Antarctic Surface Water would mainly result from biological activity. The silicate concentrations thus calculated were in good agreement with concentrations measured along a previous transect across the Southern Ocean.

Journal

Marine BiologySpringer Journals

Published: Aug 1, 1986

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