Colonisation d’un récif frangeant de l’île de La Réunion par les larves de poissons coralliens

The colonization of a fringing reef of Reunion Island by fish larvae takes place essentially from August to February with two peaks, in August-September, when the water temperatures are coldest, and in February when they are at a maximum. The number of fishes, as well as the number of sampled species, present similar values over two consecutive years with, nevertheless, a strong variation in the species involved. In Reunion Island, the number of post-larvae which colonise the reef is one hundred to four hundred times lower than on larger and more structured reefs. This phenomenon could be explained by the geographical characteristics of the island. The stock control of the adult fish in Reunion could largely be the result of the arrival of post-larvae on reefs and would correspond to the model of “recruitment limitation”.     

Variabilité du panache turbide de la Gironde par télédétection. Effets des facteurs climatiques

The Gironde estuary (France) discharges to the ocean an important amount of suspended particulate matter in the form of turbid plumes. The surface plume is more particularly studied from coastal oceanographic surveys and NOAA/AVHRR satellite data collected during a French programme of coastal oceanography (PNOC-Atlantic). The AVHRR reflectances are atmospherically corrected according to an algorithm based on the clear water concept. The comparison with suspended sediment concentrations are realised by direct and indirect calibrations. The correlations obtained show that at a same concentration the reflectance varies, probably because of varying particle (floc) size and composition. The shape and the surface measured from low resolution (4 km × 4 km) and high resolution (1 km × 1 km) AVHRR data are then compared to the forcings introduced by the fluvial output, the tide, neap and spring tide and the wind variations. The latter have an important effect on the orientation of the distal part of the turbid plume.     

Phases d'érosion—comblement de la vallée de la Papenoo et volcanisme subrécent à Tahiti,en relation avec l'évolution des iles de la Société (Pacifique Sud)

A detailed mapping and data from borings support the conclusion that there is a late lava flow inside the Papenoo valley, the main valley of Tahiti island. This last volcanic manifestation which took place 400,000 years ago came after the phase of erosion corresponding to the Illinoian glacial period. It is one of the four phases of erosion followed by filling observed in the cross section of the Papenoo valley.     

Méthodologie pour une simulation des transformations granulométriques de sables de la zone d'avant-côte

The grain size of mature coastal sands changes rapidly in response to the variation of the energy regime which provokes either erosion or deposition of fractions of the textural assemblage. Whatever the dynamic conditions, questions of paramount importance such as the size of grain populations transported simultaneously, the most common size of the grains in movement and the grain-size range of the sediment fraction involved are still poorly understood. The present paper deals with an attempt to describe this fraction without any consideration of the processes responsible for the observed changes. A major problem results from the fact that the distribution of the mobilized fraction cannot be known and described by direct comparison between the initial and the final grain-size curves. The only way to characterize the mobile fraction between two subsequent grain-size states is to simulate various kinds of removal or deposition. The first question is to express the shape of the actual distributions of the sediments mathematically. The simple Gaussian approximation being irrelevant, distributions can be expressed using advanced computer programmes which can provide mathematical expressions for all distributions. The cubic-spline approximation was chosen. The mobile fraction was assumed to have a Gaussian distribution. Various tests were made in order to simulate qualitative phenomena observed in nature (bimodality, grain-size parameter changes…) and to assess the three parameters which define the characteristics of the fraction deposited or removed: modal value, dispersion of the mobile population and relative amplitude of the change. An application was carried out on the Senegalese coast offshore from the Senegal delta and on the Mediteranean coast in the Gulf of Lions. It suggests that the dynamic agents have a great selectivity. Further developments are considered.     

Procédure Optimisée Hors-contaminations pour l'Analyse des eléments nutritifs dissous dans l'eau de mer

This work describes a complete procedure to simplify pre-analytical operations and to reduce problems of contamination during nutrient analysis which prove the importance of the various potential contaminations: fingerprints, atmosphere, particulate matter. For instance, on a fingerprint there is enough urea to bring about 80 μmol litre⁻ to an autoanalyzer cup, i.e 150 times the mean value in seawater. Atmospheric contact increases the ammonia concentration by about 1·5 μmollitre⁻ per hour. The overestimation due to the interference of particulate matter reaches 15–80% of dissolved phosphate in summer in coastal seawater.In order to minimize tese contaminations, the sample is never poured out of its sampling flask. Thus, when the level of suspended matter is too high, centrifugation of the sample is performed directly in the vial, just before Analysis, instead of the usual filtration. Moreover, a Technicon Sampler IVhas been modified to receive vials up to 125 ml. In this way any filling of cups is avoided and atmospheric contamination is greatly reduced because of the larger volume of sample.The results show the improvement induced by our modifications: with reduced handling results are expected to be more reliable and representative.     

Biodiversité et dynamique des peuplements ichtyiques de la mangrove de Guaratuba,Brésil

The Bay of Guaratuba, whose area is approximately 45 km², is a basin, situated to the south of Brazil (25° 52′ S; 48° 39′ W) and connected to the Atlantic ocean. In its northern part, a region of mangrove is found, under the influence of seasonal fluvial contributions, dominated by Laguncularia racemosa. The depth exceeds 6 m in some places. According to the period of the year, salinity varies between 3 and 37, and temperature between 15 and 28 °C. Several of the present fish species are exploited by commercial and recreational fisheries. This work describes characteristics of the fish populations occupying this mangrove, and temporal and spatial evolution of populations of the main species from available data on their biology. Between September 1993 and August 1996, fish samplings have been undertaken monthly by trawl, according to the same fishing effort (20 min), in channels of this ecosystem. Size, numerical abundance and weight of each individual of species were measured, as well as surface and bottom temperatures and bottom salinity. Species richness, Shannon-Weaver's diversity index were calculated and abundances were processed by the multivaried analysis. The global species richness is 61 species, distributed in 28 families; the better represented species are Sciaenidae (10), Gerreidae (5), Carangidae (5), Ariidae (4), Haemulidae (3), Serranidae (3), Bothidae (3). Each year, the number of species varies between 41 and 44, and a group of eight species represents between 80 to 90 % of the density and 75 % of the biomass. The constancy of this dominance is the main characteristic of this fish assemblage. The most represented species in abundance is Pomadasys corvinaeformis is the first two years, then Stellifer rastrifer. The group of eight main species is shared by 15 species in total, of which 13 for the density and 14 for the biomass. The most important monthly catches take place in winter season, during which one observes a decrease of water temperature and an increase of salinity. These species, however, being likely to be replaced by others according to the year or the season; one never finds the same composition in the assemblage which preserves always the same structures. Seasonal differences between seasons under marine (winter) and continental (summer) influences play a major role on the dynamics of fish in the Guaratuba mangrove. This ecosystem plays a functional role in the achievement of the life cycle of many permanent or temporary species. Those having succeeded to reach high levels of abundance are not represented in a uniform way in the course of the year, even when they reside permanently in the ecosystem. Periodic displacement strategies for self regulation of stocks abundance, are necessary mechanisms for these populations, which thus success to coexist in occupying the mangrove.     

Les observations de poissons tropicaux et le réchauffement des eaux dans l'Atlantique européen

A recapitulation of the records of tropical fishes from European Atlantic waters shows that 67.6% were fishes caught from the upper slope, between approximately 200 and 600 m; 19.8% were fishes caught from the continental shelf; and 13.5% were specimens caught from the middle slope, between 700 and 1 300 m. Since 1963, the upper slope species have made regular northward range extensions off of south Portugal to north-western Ireland (about 55° 30′N), more and less rapidly, about 30 years for Cyttopsis roseus and only 6 years for Sphoeroides pachygaster. The continental shelf species, observed from 1969 but mostly from 1981, have a northward range to south-eastern Ireland (about 52° N), but 65.2% of them have been caught off the south of the Bay of Biscay. The middle slope species, recorded only from 1991 according to the development of the deep fishery, were caught between 48° N and 60° N. The northward range extension of upper slope species and the higher frequency of records of continental shelf species from the southern part of the Bay of Biscay coincide with the investigations on the warming of the south-north current in the upper slope of northern Spain and of the south French Atlantic continental shelf.     

Prélèvements systématiques et caractérisation des réflecteurs acoustiques: nouvelle étape dans la compréhension de la géologie de la Méditerranée occidentale

A sampling survey in the Ligurian Sea and north of the Balearic Islands (Mediterranean Sea) brought back samples containing the following elements: (1) lithified Quaternary crust and deep-lying corals; (2) Early Pliocene marls corresponding to reflector level M in some cases; (3) red sediments tentatively assimilated with the Messinian continental equivalent; (4) rocks coming from the basement; (5) volcanic rocks.For the first time, basalts with olivine were discovered in the Algero-Provencal Abyssal Plain.A volcano 500 m high was sampled by dredging north of Minorca.     

Bioaccumulation des métaux chez le mytilidae hydrothermal Bathymodiolus sp. de la ride médio-atlantique

The hydrothermal environment is characterised, among other things, by a high metal concentration, related to the convective sea-water circulation inside the oceanic crust and its interaction with basaltic rocks. The biological communities associated with the hydrothermal system can survive in this toxic environment owing to their ability to regulate their intracellular metal levels by excretion or accumulation of metal ions in non-toxic forms. Various detoxification processes have been previously studied within an hydrothermal organism: immobilisation and precipitation of the metal in lysosomal systems, or binding to specific and soluble ligands such as metallothioneins. The later are heat stable and characterised by a high content of cysteine. The quantification of Fe, Zn, Cu, Mn, Cd, Ag, Ba and Sr in the tissues of the vent mussel Bathymodiolus sp. indicates a high accumulation of these metals in two target organs, the gill and the digestive gland. This accumulation is in relation with high concentrations of metallothioneins, but the subcellular distribution of metals indicates a higher contribution of the insoluble compartment for detoxification processes. These results are compared with the literature data about metal bioaccumlation in Bathymodiolus thermophilus collected at the Galapagos Rift.     

Distribution et écologie des Méduses (Cnidaria) du golfe de Tunis (Méditerranée sud occidentale)

The Medusae of Tunis Gulf (25 species) are qualitatively the third zooplanktonic group after Tintinnids and Copepods. Anthomedusae (eight species), Trachymedusae (five species) and Leptomedusae (four species) represent the major part among the 20 species of Hydromedusae identified during our study (December 1993–November 1995). However, the Limnomedusae (two species) and the Narcomedusae (one species) are poorly represented. In the Bay of Tunis, Obelia spp., the most common and dominant species, command the quantitative fluctuations of Hydromedusae and reach exceptionally the density of 25 800 ind. m^–3. Within the Scyphomedusae (five species), Pelagia noctiluca is the most common species which outbreaks in winter and autumn. Tunis Gulf seems to be composed of two jellyfish communities; the first one is composed of endogenous species showing a constant period of blooming like Olindias, Cladonema, Aurelia, Cotylorhiza and Rhizostoma and sometimes permanent like Obelia spp. and Clytia spp.; the second is represented by exogenous species considered as “visitors” like Velella, Sminthea or Pelagia whose presence on the coast is dependent on local and particular winds and currents.     

Etude de la variabilité des teneurs en Hg,Cu, Zn et Pb de trois espèces de mollusques de l'étang de Berre (France)

The bioaccumulation of Hg, Cu, Zn and Pb ions was studied in three mollusc species at three sites in the Berre lagoon (Marseille, France), which have been selected according to their hydrodynamic conditions. The degree of the contamination of these mollusc populations was calculated in order to determine the levels of the metal ions present. The uptake of metal ions was regularly monitored for two successive years; the causes of the high variability was observed throughout the two years for all the species studied are discussed.     

Étude isotopique (SrNd) de l'origine des dépôts fins holocènes du littoral atlantique (S-O France)

The fine-grained sediments of the West Gironde mud patch derive mainly from materials supplied via the Gironde estuary. Between 1500 and 1200/1300 years BP and 300/400 years BP and the present day, the Sr isotopic and chemical compositions of the estuarine silty clay inputs are the same. In contrast, these compositions were different during the period 1200/1300 to 300/400 years BP. These variations probably reflect anthropogenic activities including cultivation and agriculture, in the Garonne and Dordogne drainage basins during the medieval period. Modem silty clay sedimentation in the mud flats of the Marennes-Oleron basin and Perthuis Breton is characterised by the occurrence of important quantities (between 40 and 90 % of the total) of detritus that issues from the Gironde estuary. The autochthonous silty clays derived from outcrops of Jurassic green marls and Flandrian deposits are mixed with estuarine allochthonous clays delivered by coastal surface currents. In the Anse de l'Aiguillon, the muds are mainly composed of these autochthonous silty clays. The silty clay infilling of these mud flats has been highly influenced by the development of the oyster and shellfish aquaculture.     

Influence de la circulation à long terme sur la répartition des organismes zooplanctoniques dans la Baie de Campeche,Mexique

A space-time approach is adopted in an attempt to understand the long-term influence of currents on the spatial pattern distribution of zooplankton, ichthyoplankton, and fish larvae communities in the Bay of Campeche, Mexico.The analysis is performed using results of a hydrodynamic numerical model, and zooplankton and ichthyoplankton abundances.Three fish larvae communities were identified in the Bay, on the basis of its physiography, i.e.: “neritic fluvio lagoon influence”, “neritic” and “oceanic” communities.A cyclonic eddy is formed in the Bay and drifts westward, facilitating transport and distribution of planktonic organisms. The eddy provides larvae transport from oceanic areas to the coastal areas and vice-versa. The seasonal distribution of fish larvae is highly influenced by the position of the eddy.The biomass pattern and fish larvae density observed in the coastal area off the Grijalva-Usumacinta delta are governed by the coastal front.     

La dispersion des apports de la Gironde sur le plateau continental. Données in situ,satellitales et numériques

Coastal waters are generally a product of mixing between continental and oceanic originated water masses. Near river mouth areas, as in the Gironde estuary, oceanic waters are modified by the influence of freshwater discharge that introduces suspended and dissolved substances, including pollutants. Within PNOC (Programme National d'Oceanographie Côtière) our objective is to determine the extent of the influence of the Gironde estuary on to the Aquitanian continental shelf. Several cruises for collecting hydrological data have been carried out in this area. The data of every cruise were coupled with a NOAA-11 satellite passage. The recorded images have been analysed and were compared with the results given by a 3D mathematical model. The interaction among different dynamical factors (density, wind-driven and tidal circulation) controls the mixing and the transport of low salinity waters.     

Distribution des sédiments sur la marge du golfe de Tehuantepec (Pacifique Oriental). Exemple d'interaction tectonique-eustatisme

During the PROGEMA I cruise which took place on the south-western Pacific coast of Mexico (between 14° and 16° N, 92° and 96° W), 3.5 kHz echosounding profiles, surface samples and cores as well as hydrological and oxygen minimum measurements down to 2250 m water depth were made in the Gulf of Tehuantepec. Chemical, sedimentological, mineralogical, X-ray and BEM analyses of waters, core and surface samples provided new data which permit a better knowledge of the Gulf of Tehuantepec margin, where tectonic activity is due to an active subduction.On the shelf of the gulf, the distribution pattern of quaternary deposits is affected by a double tectonic forcing and by eustatic oscillations of the sea level. The first tectonic control is linked to the existence of north-south tectonic troughs in the Tehuantepec Isthmus, in which the atmospheric circulation between the Gulfs of Mexico and Tehuantepec is accelerated by the Venturi effect. The resulting winds, called “Tehuanos”, play an important role in the forcing of surface circulation in the Gulf of Tehuantepec, inducing a deviation of the general circulation and thus forcing the distribution pattern of terrigenous sediment supply according to two physiographically and mineralogically distinct provinces, inherited from the past tectonic evolution, which are separated by the Tehuantepec axis.

• •-|A southeastern province related to the Caribbean Plate and fed by: (1) the rivers draining the Chiapas Massif; and (2) the Costa-Rica Current driving tropical and subtropical waters from Central America.
• •-|A northwestern province part of the North American Plate and fed by: (1) the Rio Tehuantepec which drains the Sierra de Oaxaca; and (2) the Californian Current.

The second tectonic control is linked to the tectonic structure of the margin influenced by:

• •-|shallow banks where the products of a present-day phosphatogenesis are concentrated because they are swept by upwellings. The latter are reinforced because the banks are situated south of the wind corridor;
• •-|tectonic depressions acting as sediment traps;
• •-|numerous canyons cutting the slope along brittle accidents, driving weak turbidity currents but which play an important role in the forcing of upwelling currents. Antarctic diatoms are found which are conveyed by the deep circulation from the South-Eastern Pacific and provide evidence that Antarctic Cold Water masses (AABW) rise towards the shelf.

The influence of eustatism is attested by the presence on the shelf of the Gulf of Tehuantepec of three paleoshorelines at 40 , 75 and 130 m water depth, linked to the Würmian sea level and to the postglacial sea level rise as on many passive margins.     

Les recifs coralliens Au-Vent de l'ile Maurice (Archipel des Mascareignes,Océan Indien): Geomorphologie et Bionomie de la pente externe

The outer slope of the eastern reefs of Mauritius shows: from 0 to 15 m depth, a spurs-and-grooves zone of coral morphogenesis, sometimes replaced by a flagstone of coral morphogenesis (down to 20 m) or a spurs-and-grooves zone of volcanic morphogenesis; from 20 to 50 m, a volcanic flagstone with a thin coral-built layer and a buttresses-and-valleys zone of volcanic morphogenesis. The living cover of the upper part (0–15 m) is also quite similar to the cover of the leeward side. On the contrary, the volcanic basement in the lower part is poorly covered by reef-building organisms, while soft or slightly calcified algae are of widespread occurrence.