Le comblement de la ria de Gernika (golfe de Gascogne) à l'Holocène terminal

A study of the recent sedimentation of the ria de Gernika (Bay of Biscay) is undertaken to characterise different steps of its Holocene filling. Sedimentological and micropalaeontological analysis of six cores sampled in the ria, completed with absolute dating (¹⁴C, and based upon ²¹⁰Pb exc.), allow us to identify three phases in the filling of the ria that are limited by two transgressive shifts (3500 and 2500 yr. B.P.) already described in other areas of the Bay of Biscay. In the first phase, lithologies as well as benthic foraminifera and ostracode assemblages are typical of coastal marine palaeo-environment. During the second phase the euryhaline waters progessively occupy the central area of the estuary. Here the presence of allochthonous microfauna, particularly of planktonic foraminifera, indicates an increase of exchanges with marine waters that can reach the inner areas of the estuarine system. The last phase (2500 yr. B.P. to the present), which corresponds to the recent and active sedimentation, is marked by an impoverishment of the microfaunas, the filling of some channels and the increase of salt marshes. The estimated sedimentation rate for the upper decimetres of the cores (0.73-1.29 cm·yr⁻¹) shows faster sedimentary filling of the ria with anthropic action evidenced by the presence of metallic contaminants and hypohaline microfauna.     

Influence d'un élevage ostréicole sur les flux de nutriments et d'oxygène dans un écosystème lagunaire

The impact of suspended oyster culture (Crassostrea gigas, Thunberg) on oxygen and nutrient fluxes has been studied in situ, in a coastal lagoon (Thau, France), during a seasonal cycle. On the first plan of the multiple factorial correspondences analysis (MCA), seasons were well discriminated. The fluxes were maximum in summer and minimum in winter. However, this seasonal pattern was not only linked to the water temperature, as autumn and spring (similar temperatures of about 12 °C) were distinct in the second factorial plan (2.3). Oxygen uptake by the oyster cultures varied between 0 μmol m⁻² h⁻¹ (January) and 11 823 ±377 μmol m⁻² h⁻¹ (July). Ammonia and nitrate-nitrites were released into the water column respectively at a rate of 2905 ± 327 μmol m⁻²h⁻¹ and 891 ± 88 μmol m⁻² h⁻¹ in the summer and 0 μmol m⁻² h⁻¹ and 177 ± 97 μmol m⁻² h⁻¹ in the cold season. During the summer, the nitrate-nitrites flux was about 20 % of the total dissolved inorganic nitrogen production. Phosphate release was low except for two periods during which an important release was measured; in May (1686 ± 44 μmol m⁻² h⁻¹) and in November (2691 ± 800 μmol m⁻² h⁻¹). No linear relation between water temperature and phosphate flux was found. In Thau Lagoon, oyster cultures (oysters and epibiota) by producing 2 × 10⁷ mol-N y⁻¹ play a central role in nitrogen renewal in the water column.     

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.     

Répartition des foraminifères benthiques actuels sur le plateau continental sénégalais au sud de Dakar

Two hundred and twenty-four benthic foraminiferal species were recognized in the total assemblages of fifty samples collected from the Senegalese continental shelf south of Dakar (down to a depth of 200 m). A factor analysis based on nineteen dominant species shows three main factors. The first two seem to be associated with temperature and its seasonal variations; they are depth-related and determine two main faunal changes. The third factor is substrate-related. The main depth-related species are: i) Cribrononion gerthi, Quinqueloculina akneriana, Quinqueloculina lamarckiana, Cribrolinoides curtus, Elphidium fichtellianum and Textularia sagittula in the coastal zone; ii) Discorbinella bertheloti, Cancris aff. C. sagrum, Textularia truncata, Nonion fabum, Bulimina elegans and Hanzawaia solei in the inner shelf; iii) Trifarina bradyana, Trifarina fornasinii, Uvigerina peregrina and Cassidulina laevigata in the outer shelf. The species that are substrate-related are: i) N. fabum, B. elegans, C. gerthi and H. solei in the fine-grained sands and ii) Lobatula lobatula, C. curtus, T. sagittula and D. bertheloti in the coarse-grained sands. The reduction in planktonic percentage and diversity in the outer shelf are related to the upwelling seasonal influence.     

Dense aggregations of Pygospio elegans (Claparède): effect on macrofaunal community structure and sediments

Epibenthic biogenic structures such as polychaete tubes are conspicuous features of many marine soft-bottom habitats. This paper compares the benthic macrofauna in patches with high and low densities of the tube-dweller Pygospio elegans on intertidal sandflats in eastern Scotland (UK). The main aim of this study was to determine potential differences in the macrofaunal community structure, the size distribution of individual species and sediment properties.Multivariate data analyses revealed that the macrofaunal community composition (excluding P. elegans) within patches was always significantly different from outside patches, mainly due to variability in the abundances of Cerastoderma edule and Corophium volutator. In addition to P. elegans, 5 taxa were sufficiently abundant for univariate analyses, 4 of these (Capitella capitata, C. edule, Macoma balthica and C. volutator) being significantly more abundant within P. elegans patches than in surrounding, non-patch sediments. The size distribution of P. elegans was significantly different between patches (bimodal distribution) and non-patches (skewed distribution). Similarly, there was a greater proportion of larger C. capitata individuals within patches compared to non-patch sediments. Sediment organic content and silt/clay fraction were always significantly higher in patch sediments while redox profiles showed no differences except at the end of the study period when the top 2 cm within patches were more positive and more negative at 4 cm. These results imply that even relatively small (1–1.5 m²) P. elegans patches can have large effects on the spatial variability of macrofaunal community structure on intertidal sandflats.Towards the end of the study there were marked visual changes in the P. elegans patches, such as wave-ripple marks on the surface, which signified their demise. This coincided with dramatic changes in the invertebrate community structure within patches. Along with the decline in P. elegans numbers, dramatic increases in the densities of the 2 bivalve species C. edule and M. balthica occurred, while remaining comparatively stable outside patches. This suggested that the conditions within P. elegans patches were particularly favourable to successful bivalve spat settlement. We postulate that P. elegans patches have limited longevity and propose that enhanced bivalve competition within them leads to rapid decreases in P. elegans numbers.     

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.     

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.     

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.     

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.     

Absorption et régénération de l'azote dans le système brassé de la Manche: productions nouvelle et régénérée

Nitrate and ammonium uptake rates were measured in spring and summer in deep and shallow wellmixed waters of the English Channel during different cruises between 1986 and 1994. In the deep waters, nitrate uptake was relatively low during phytoplankton development and ammonium uptake represented more than 70 % of the total uptake irrespective of the season. In the shallow waters, nitrate uptake during spring phytoplankton growth was high and represented about 75 % of the total uptake. Ammonium uptake became substantial towards the end of spring and summer. The high contribution of ammonium to the nitrogenous nutrition of phytoplankton over the whole of the well-mixed waters is related to a high rate of recycling of nitrogen in the water column. Ammonium regeneration by microheterotrophs can satisfy between 62 % and the totality of the phytoplankton nitrogen requirements. The high primary production in deep well-mixed waters is not supported by allochthonous nitrogen supply but by an intense in situ regeneration of nitrogen. The situation is similar in shallow well-mixed waters, except during the spring bloom.