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See detailVariations at different spatial scales of Posidonia oceanica (L.) Delile beds; effects on the physico-chemical parameters of the sediment
Gobert, Sylvie ULg; Kyramarios, Michael; Lepoint, Gilles ULg et al

in Oceanologica Acta (2003), 26(2), 199-207

The biometric parameters (shoot density, length and width of leaves, leaf and epiphyte biomasses, leaf surface) of the P. oceanica meadow present a significant spatial variability at the mesoscale level ... [more ▼]

The biometric parameters (shoot density, length and width of leaves, leaf and epiphyte biomasses, leaf surface) of the P. oceanica meadow present a significant spatial variability at the mesoscale level (100 m2). All the parameters studied, excepting the number of leaves, decrease according to the depth. The horizontal spatial variations (centrifuge and centripetal distributions) reach a maximum in the shallower areas. The analysis of the microstructures (m2) of the meadow has not allowed a relationship to be established between the spatial variations of the biometric parameters and the environmental factors in the sediment (pH, water and organic matter content, red-ox potential, nutrient content). The heterogeneity of the meadow in the Revellata Bay quite probably results from the internal shoot and rhyzome growth dynamics. <br />On the other hand, it appears that the presence of the prairie modifies the physical-chemical characteristics of the sediment. In fact, the sandy patches present different physical-chemical characteristics: lower organic matter, different granulometric and much higher interstitial water nutrient levels, as well as a higher pH and oxydo-reduction potential. [less ▲]

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See detailContrôle du phytoplancton du bassin ligure par le front liguro-provençal (secteur corse)
Goffart, Anne ULg; Hecq, Jean-Henri ULg; Prieur, Louis

in Oceanologica Acta (1995), 18

The evolution of the phytoplankton distribution was followed along the Calvi - Nice axis, from Calvi to 30 miles offshore, in relation with the hydrological features of the Liguro-Provençal front ... [more ▼]

The evolution of the phytoplankton distribution was followed along the Calvi - Nice axis, from Calvi to 30 miles offshore, in relation with the hydrological features of the Liguro-Provençal front (Corsican sector) and the nutrients disponibility. Oceanographic cruises repeated between 1982 and 1991 have emphasized the permanence of the front and the interannual stability of its hydrological characteristics. Moreover, the front and the associated vertical circulation influenced highly, and in a reproducible way, the algal distribution within distances of few kilometres. In March, within the Ligurian current (on the coastal side), the front and the central zone (on the open sea side), stabilization of upper layers of water column, induced by a beginning of vertical stratification, is the main factor controlling the starting of the bloom. As soon as phytoplankton development is initiated, highest values of chlorophyll a and primary production are observed in the frontal area. During spring, a significant amount of the phytoplankton produced in the surface layer was carried down along the isopycnals in relation with the frontal convergence. As a consequence, the chlorophyll a distribution was typically like a plume, extending from the surface to at least 100 m deep. In all zones, during the stratified period, the position of the chlorophyll a maximum in the water column is determined by the opposing gradients of light and nitrate. In May, the maximum of phytoplankton was permanently observed in the frontal area where it was strictly correlated with the nitracline. A sharp core of high chlorophyll a concentrations was also observed in the central zone while the Ligurian current was characterized by very low pigment levels. During that period, the ecological role of the front seems to be particularly important because it delimits the unique area of enhanced phytoplanktonic biomass in an oligotrophic or very oligotrophic environment. In June, a well-developed deep chlorophyll a maximum was observed everywhere, from the central zone to the coast. The contribution of the front to the enhancement of the biomass appears to be less important than during spring. [less ▲]

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See detailProgress from 1989 to 1992 in understanding the circulation of the Western Mediterranean Sea
Lehucher, P. M.; Beautier, L.; Chartier, M. et al

in Oceanologica Acta (1995), 18(2), 255-271

The present paper describes the major results obtained from 1989 to 1992 by the EUROMODEL group in studying the circulation in the Western Mediterranean Sea. Particular emphasis has been given to the ... [more ▼]

The present paper describes the major results obtained from 1989 to 1992 by the EUROMODEL group in studying the circulation in the Western Mediterranean Sea. Particular emphasis has been given to the physical processes responsible for seasonal and mesoscale variabilities. Observations (in situ and satellite), together with theoretical, physical and numerical models, have been widely used in the course of these studies. Attention has been focused on the dynamics of the northern basin (deep water formation, dynamics of the Northern Mediterranean Current, circulation and shelf/slope interaction in the Balearic Sea) and of the southern basin (Alboran Sea circulation, instabilities of the Algerian Current). The straits dynamics have been studied with particular reference to the Corsica channel. Preliminary runs of a basin-scale circulation model of the Western Mediterranean Sea forced by the straits are also presented. They are shown to be capable of generating a cyclonic circulation in the western basin. [less ▲]

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See detailDistribution of POC, PON and particulate Al, Cd, Cr, Cu, Pb, Ti, Zn and deltaC13 in the English Channel and adjacent areas
Dauby, Patrick ULg; Frankignoulle, Michel; Gobert, Sylvie ULg et al

in Oceanologica Acta (1994), 17(6), 643-657

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See detailUSE OF SEDIMENTARY CLAY-MINERALS FOR THE RECONSTITUTION OF PERIODIC PALEOCLIMATIC VARIATIONS IN THE ARABIAN SEA
Fagel, Nathalie ULg; Debrabant, Pierre; DE Menocal, Peter et al

in Oceanologica Acta (1992), 15(2), 125-136

During the Late Neogene, the sediments of the Owen ridge (Arabian Sea) were able to record paleoclimatic variations because of tectonic stability and weak bio-turbation. These changes had a powerful ... [more ▼]

During the Late Neogene, the sediments of the Owen ridge (Arabian Sea) were able to record paleoclimatic variations because of tectonic stability and weak bio-turbation. These changes had a powerful influence on the western, northern and northeastern detrital supplies related to monsoon conditions and low-latitude aridity which developed during glaciary stages. In ODP Leg 117 sites, previous studies on the distribution of clay assemblages show a great variety of mineral origins and their fluctuating contribution to the Neogene sedimentation. Concurrent investigations of the sedimentological, magnetic, biogenic and geochemical properties of sediments have emphasized typical periodic sedimentary responses related to external forced functions of the earth's orbital parameters. This research investigates whether the clay fraction is able to record short-term periodicities. For this purpose we sampled site 721 with high-resolution over a short Plio-Pleistocene interval well known from magnetic susceptibility measurements. We chose the palygorskite/illite peak ratio, directly measured on the XR diffractograms, as a typical clay parameter. The fluctuations of this parameter, between 2.7 and 1.2 My, defined a time-series which has been treated by spectral analysis: autocorrelation, discrete Fourier Transform (DFI) and DFT of the autocorrelation function (DFTA). The results on the global signal (1.5 My) and on four fractions of 375 Ky, are analysed and discussed. On the spectrograms some peaks are close to the classical Milankovitch cycles of the earth's orbital parameters, but additional periods occur with high amplitude. They represent non-linear responses of the eccentricity and of the tilting cyclicities. The periodic influences change with time from a signal dominated by the precession to a signal dominated by the tilting. This shift appears at 2.4 My and reflects the transition from a dominant regional effect (monsoon) to a global and more complex effect (extension of the ice sheets). These results, which show the clay fraction to be an accurate marker of short-term paleoclimate variations, confirm some previous knowledge obtained with magnetic susceptibility data and indicate in greater detail the mode of detrital supply of the Northwest Indian Ocean. Prior to 2.4 My, the palygorskite-illite ratio data vary predominantly at the precessionnal periodicities and record the modulation of the southwest monsoon intensity and associated northwest winds. After 2.4 My the acidification of North-East Africa and Arabia (monsoonal dust areas), with expanded high-latitude ice cover, favours fibrous clay formation in well-developed pericontinental evaporitic basins. Like the illite flux, the palygorskite flux is modulated by a non-linear continental response that varied in response to the orbital obliquity periodicity. [less ▲]

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