Apports de nutriments et développement phytoplanctonique en baie de Seine

As part of the Programme National d'Océanographie Côtière, the nutrient dynamics of the Bay of Seine were studied between 1992 and 1994 in order to complement work on ecological modelling. Firstly, the River Seine's nutrient fluxes were established: 80 000–130 000 t a⁻¹ of dissolved inorganic nitrogen, 6 400–8 4001 a⁻¹ of dissolved phosphorus and 20 000–77 000 t a⁻¹ of dissolved silicium. Estuarine processes were taken into account. Consequences of nutrient loading for the bay were then evaluated at the pelagic level (nutrient and chlorophyll enrichments) and the benthic level (distribution of various phosphorus fractions in superficial sediments). The large continental inputs always induce concentration gradients in the water from the mouth of the river to the northwest of the bay. The northward spreading of fine particulate matter controls the distribution of adsorbed phosphate and iron-aluminium bound phosphate in sediments. In contrast, calcium bound phosphate, the main fraction in sediments, is not affected by river inputs. Organic phosphorus in sediments is related to phytoplankton blooms, with increasing concentrations during productive periods; afterwards the fast recycling prevents sedimentary accumulation. The nutrient depletions observed beyond the turbid plume during spring 1992 enabled the calculation of nutrient uptake rates, and the comparison of these rates with phytoplankton biomass (chlorophyll a + phaeopigments). Ratios of nutrient consumption to pigment concentration were estimated at 1 μmol μg⁻¹ for nitrogen, 0.05 for phosphorus and 0.5 for silicate. These values, as N/P and Si/N uptake ratios (respectively 17.5 and 0.4), were similar to usual values.     

Relations milieu-ressources: impact de la conchyliculture sur un environnement lagunaire méditerranéen (Thau)

Shellfish farming leaves its mark on the environment in which it has developed, and the men who depend upon it. These changes have altogether balanced the lagoon cycle and have caused disastrous episodic events. Increased water clarity caused by the uptake of particulate material by shellfish farming allows seagrass to grow in deeper areas of the lagoon (down to five metres). Shellfish farming nutrient transformations increase ecosystem productivity, even if the filtration pressure keeps phytoplankton biomass at a low level. Storage of phosphorus and nitrogen in animal tissue limits eutrophication in this ecosystem. Transfer of oysters from growout facilities increases animal and vegetal specific diversity. The presence of large amounts of shellfish allows for the development of a masive benthos, while organic enrichment from biodeposition changes the specific composition of soft-bottom benthos. In the deeper areas, (less than six metres), where summer thermoclines limit oxygen transfer from surface water, the organically enriched substrate induces oxygen depletion and ammonium and nitrogen sulfide accumulation in the water column. This ecosystem dysfunction kills benthic populations, and sometimes reaches pelagic populations and affects the shellfish farming economy.     

Données géomorphologiques sur la région de Fresh Creek,Ile Andros (Bahama)

A geomorphological study of the east coast of Andros (Fresh Creek area) shows the existence of a paleotopography represented by low-altitude hills (few metres). This paleotopography is protected by the presence of a calcitic Quaternary crust which covers Pleistocene calcarenite.In the western part of the area, there are long woody axes, oriented NE-SW, parallel to the channels of the creek. They end at two kilometres from the coast, along which is a second kind of lower hills, orthogonal to the first.The first axes can be interpreted as megaripples as seen at the present time on modern deposits (on the Great Bahama Bank) and fossilized by the upper crust. The second direction is made by accretion ripples along the coast.The surface of the Bahamian calcarenite has been studied. The Bahamian karst presents two topographical forms: “blue holes” like those outside the island, which are 60–80 m in diameter and both sparse and deep; and “washtub” dolines; these are numerous and shallow, and, from low altitude, exhibit a honeycombed aspect on the surface. This karstic topography with dolines and blue holes is also seen through the water of the Creek the hard bottom of which is covered only here and there with a few centimetres of sediments. Hence, there is a submerged karstic topography, made of the same elements as the aerial karst, but submerged by the Holocene transgression. The present karstic relief, in relation with the different eustatic levels of the Quaternary, has begun 120,000 years ago, according to the isotopic ages, and might be composed by different steps, difficult to show now, in the topography.The blue holes in the interior of the island of young and little evolved karst, were formed more by solution than by collapse of the karstic caves, because of the absence of a real river to drain the Andros shelf at the time of low sea levels. Blue holes of the inside of the island, as they are called, with submarine openings, have the same salinity as the water of the creek (17.5 g/l). The dolines with very low salinity (0.7 g/l to 3.8 g/l) are filled with stromatolites and charophytes, slowly forming sediments made up essentially of high-magnesian calcite.It seems that the Andros Island karst can be compared with that of the Yucatan, where there are round and deep open pits, called cenote, of which the Bahamian equivalent would be the blue holes which were drowned by the Holocene transgression.     

Comparaison de méthodes d'évaluation de la production secondaire d'une population de l'annélide polychète Ditrupa arietina (O.F. Muller)

We compared the ability of three direct methods (increment summation, loss summation and instantaneous growth rate coefficient), the size-frequency method, and eight indirect methods (Banse and Mosher, Benke, Brey, Edgar, Morin and Bourassa, Plante and Downing, Robertson and Schwinghamer et al.) to measure or to predict the secondary production of a Mediterranean population of the polychaete Ditrupa arietina. This comparison was carried out both on the 1994 cohort during its two-year lifespan and on the entire population during the two years of the study (1994–1996). Our results showed that all three direct methods are more or less equivalent. Results referring to the year 1994–1995 showed a strong overestimation of the actual production by the size frequency method. This is attributed to the existence of an average individual maximal size below which the actual production equals zero. During the same year, the results obtained using indirect methods were highly variable, partly, but not solely, due to the type of environment for which the regression models were built. The incorporation of temperature as an independent variable within the most recent regression models did not contribute to improving the quality of the predictions. Results referring to the year 1995–1996 confirm the incapacity of both the size frequency and the indirect methods to predict changes in the P/B ratio in relation with age structure and recruitment irregularities. These results are discussed in the context of inferring productivities both at the population and the community level.     

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.     

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.     

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.     

Sur l'existence d'une topographie antérécifale dans la région de Tuléar (côte sud ouest de Madagascar)

The outline of the Pleistocene coastal geomorphology in the Tuléar area (Madagascar) can be redrawn from the analysis of the setting and characteristics of present-day coral reefs. Initial outer reef tracts, which grew in Tatsimian times (according to Battistini's terminology), followed in their development the river pattern, since they settled upon the divides between valleys instead of growing uniformly on the outer shelf. These primary reef chains coalesced subsequently. Inner reefs and coral banks also followed, although not so closely, the river pattern.     

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.