Variability of water masses and of organic production-regeneration systems as related to eutrophic, mesotropic and oligotrophic conditions in the northeast Atlantic Ocean
Pierre, C.; Vangriesheim, A.; Laube-Lenfant, E. (1994). Variability of water masses and of organic production-regeneration systems as related to eutrophic, mesotropic and oligotrophic conditions in the northeast Atlantic Ocean. J. Mar. Syst. 5(2): 159-170
In: Journal of Marine Systems. Elsevier: Tokyo; Oxford; New York; Amsterdam. ISSN 0924-7963; e-ISSN 1879-1573, more
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| Authors | | Top |
- Pierre, C.
- Vangriesheim, A.
- Laube-Lenfant, E.
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| Abstract |
In the eastern part of the tropical North Atlantic, the trade winds generate a permanent coastal upwelling system along West Africa. Eutrophic conditions resulting from the uplift of nutrient-rich deep waters are progressively dissipated westwards. The Eumeli 2 cruise (January-February 1991) realized in the framework of the France-JGOFS program, gave the opportunity to study the internal variability of the water column at three typical sites, eutrophic, mesotrophic and oligotrophic, located near 20°N of latitude. The hydrological survey at each site provides a description of the different water layers. The mixed surface layer of variable thickness at the three sites covers the Central Waters; North Atlantic Central Water and South Atlantic Central Water are competing along the Cape Verde Frontal Zone which crosses the mesotrophic and eutrophic zones. Underneath, the Antarctic Intermediate Water and the North Atlantic Deep Water correspond respectively to minimum and maximum salinities. The oxygen and carbon stable isotope compositions of these identified water masses bring complementary geochemical informations, such as the carbon cycling through production-regeneration. In surface and central waters, the delta18O of water and salinities vary linearly with a slope of 0.46, typical of tropical oceanic areas. The major changes of delta13C of sigmaCO2 occur in the upper 900 m. From oligotrophic to eutrophic conditions, the surficial delta13C values decrease by about 1‰; the lowest delta13C values which mark the level of maximum regeneration rate are reached very close to the surface at the eutrophic site (at about 50 to 100 m), and deeper at the mesotrophic and oligotrophic sites (respectively at about 200 m and 500 m). The NADW appears as a very homogeneous water layer, with almost constant delta18O and delta13C values at the three sites, implying that organic matter remineralization occurs at too slow rate at depth to introduce detectable amounts of 13C-depleted CO2. |
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