DISTRIBUTION PATTERNS OF LIVE PLANKTIC FORAMINIFERS AS RELATED TO REGIONAL HYDROGRAPHY AND PRODUCTIVE SYSTEMS OF THE MEDITERRANEAN-SEA

Distribution patterns of live planktic foraminifers are examined at tw o times of the year, late summer and late winter, through horizontal p lankton towing along a NW-SE transect across the West and East Mediter ranean Basins and in the Alboran Sea (French oceanographic cruises Vic omed I, II and III, respectively). Foraminiferal production is signifi cantly different in the two sampling periods and across the West and E ast Mediterranean. Foraminiferal density is higher in winter than summ er (e.g. in the western basin: 3700 specimens/1000 m(3) in winter agai nst 200-600 specimens/1000 m(3) in summer). The production, averaged f or the two sampling sets, is higher in the Western Basin than in the E astern one. However, in late summer, peaks of maximal production are o bserved in the Central Mediterranean (Strait of Sicily and Ionian Basi n) (1000 to 2000 specimens/1000 m(3)), where the assemblages are domin ated by Globigerinoides ruber rosea and G. ruber alba. In the Mediterr anean Sea, frontal regions and semipermanent eddies, which are essenti ally controlled by regional hydrography, represent a significant porti on of the regional production. They also elicit a higher diversity of species. Their influence is observed in summer as well as in winter. A t the end of summer, the rather shallow mixed layer hosts spinose symb iont-bearing species, such as G. ruber alba, G. ruber rosea, O. univer sa and G. trilobus. At that season, in the Western Basin, the nutrient content of the mixed layer is generally low, and the surface layer is rapidly nutrient depleted. Summer foraminiferal production remains lo w at the surface, with respect to the winter production. In the Easter n Basin, late summer and autumn correspond to the ''biological spring' '. Summer foraminiferal production in surface waters is higher than in winter. But standing crops are lower than in the Western Basin for th e same species, at the same period. In summer however, frontal structu res and eddies are generally better developed than in winter. In some particular cases (frontal boundaries in the Western Basin; eddies sout h of Cyprus), nutrients from intermediate layers can be upwelled, thro ugh isopycnal or diapycnal mixing, to the photic zone, and allow for s econdary blooms. In the Western Basin these blooms favour the developm ent of grazers at depth (Globorotalia truncatulinoides, G. inflata). S outh of Cyprus, in the Eastern Basin, oligotrophic conditions promote the development of heterotrophic organisms and predatory foraminifers have higher standing stocks (G. ruber alba, G. ruber rosea). In winter , the break down of the thermocline combined with deep turbulent mixin g in the Northwestern Basin, and the deepening of the mixed layer in t he Alboran Sea and along the North African coast, result in increased phytoplankton biomass and favour reproduction and proliferation of spe cies such as G. truncatulinoides or G. inflata. Our observations indic ate that the geographic and depth distribution of live foraminiferal s pecies cannot be entirely explained by the general temperature and sal inity differences among the different Mediterranean Basins but that th ey are strongly related regional hydrographic patterns. Hydrography co ntrols the seasonality and depth of the turbulent mixing (which differ between the Western and the Eastern Basins), the depth of the mixed l ayer and its seasonal stratification, and the strength of the pycnocli ne. Nutrient supply and primary production are mostly regulated by the se dynamics. One may therefore expect that the species assemblages, on the sea floor and in deep-sea cores, will also correlate to the vario us hydrographic parameters of the overlying water masses.