VARIABILITY OF WATER MASSES AND OF ORGANIC PRODUCTION-REGENERATION SYSTEMS AS RELATED TO EUTROPHIC, MESOTROPHIC AND OLIGOTROPHIC CONDITIONSIN THE NORTHEAST ATLANTIC-OCEAN

In the eastern part of the tropical North Atlantic, the trade winds ge nerate a permanent coastal upwelling system along West Africa. Eutroph ic conditions resulting from the uplift of nutrient-rich deep waters a re progressively dissipated westwards. The Eumeli 2 cruise (January-Fe bruary 1991) realized in the framework of the France-JGOFS program, ga ve the opportunity to study the internal variability of the water colu mn at three typical sites, eutrophic, mesotrophic and oligotrophic, lo cated near 20-degrees-N of latitude. The hydrological survey at each s ite provides a description of the different water layers. The mixed su rface layer of variable thickness at the three sites covers the Centra l Waters; North Atlantic Central Water and South Atlantic Central Wate r are competing along the Cape Verde Frontal Zone which crosses the me sotrophic and eutrophic zones. Underneath, the Antarctic Intermediate Water and the North Atlantic Deep Water correspond respectively to min imum and maximum salinities. The oxygen and carbon stable isotope comp ositions of these identified water masses bring complementary geochemi cal informations, such as the carbon cycling through production-regene ration. In surface and central waters, the partial derivative O-18 of water and salinities vary linearly with a slope of 0.46, typical of tr opical oceanic areas. The major changes of partial derivative C-13 of SIGMACO2 occur in the upper 900 m. From oligotrophic to eutrophic cond itions, the surficial partial derivative C-13 values decrease by about 1 parts per thousand; the lowest partial derivative C-13 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 partial derivative O-18 and partial derivative C-13 va lues at the three sites, implying that organic matter remineralization occurs at too slow rate at depth to introduce detectable amounts of C -13-depleted CO2.