Monthly sampling with a sledge type gear at 24 stations in the shallow coastal area in front of the Dutch Delta (southern bight of the North Sea) shows that high densities of a variety of animals are present in the lower 1 m of the water column: the hyperbenthos. Hyperbenthic community structure is strongly dominated by seasonal fluctuations due to the sequential appearance, high abundance and disappearance of the different species of temporary hyperbenthos. Averaged over the year, spatial patterns are consistent with the hydrodynamic regime in the system. Low current velocities and protection from wave action thus create an environment suitable for settlement of macrobenthic larvae and for sedimentation of the phytoplankton bloom.
For marine environments where aggregates are composed of greater than or equal to 50% organic matter, various parameters relevant to aggregation are quantified and compared with those in other aggregation regimes. Some possible environmental effects of the rheological properties of marine organic aggregates are reviewed. The interaction between shear forces in the water and aggregate rheology is likely to determine: firstly, the sinking speed of aggregates and hence vertical organic flux rates; secondly, the volume of microzones, with implications for equilibria and kinetics of chemical reactions in the pelagic milieu; and thirdly, the degree of turbulence damping between aggregates, due to absorption of energy through viscous dissipation inside viscoelastic aggregates during their deformation. Some preliminary results are given of the rheological properties of some laboratory-produced marine organic aggregates.
Sewage sludge from the Barcelona-Besos wastewater treatment plant has been discharged to the middle continental shelf off Barcelona through a pipeline 4 km in length since 1979. Around the mouth of the pipeline a waste deposit has settled in the transition area between the muddy prodeltaic sediment and the sandy relict sediment of the outer part of the shelf, covering an area of 1 km super(2) to a maximum thickness of about 2.5 m. A fraction of the particles discharged through the pipeline rises to the surface water and can generate a column of suspended waste rising from the mouth of the pipeline to the surface. These particles concentrate at the surface levels of the water column, forming a patch of several hundred square metres of very turbid water. Dispersion of the particles depends on hydrographic and dynamic processes. The concentration of Cr, Cu, Cd and Ni in the suspended waste is higher than that detected in the waste deposit and the Pb concentration in the suspended waste is slightly lower than in the surface of the waste deposit.
Michel MONZIERa, Jacques DANIEV, Patrick MAILLET' ORSTOM, BP AS. Noumea, Nouvelle-Caledonie b QRSTOM, Dcpartmcnt o f Geology. La Trobe Universily, Bundoora. Victoria, Australia 3083 a et b GOR «Genese et Evolution des Domaines Oceaniques», Brest, France. Recu le 15,00/89, revise le 17,{)5!90, accepte le 31,{)S,9(1 La collision entre le saillant fonne a 22°S par la ride des Loyaute, appartenant a la plaque indo-australienne, et la temlinaison meridionale de l'arc des Nouvelle-Hebrides a debute il y a environ 300 000 ans. Toutefois. par son action sur le bombement de la plaque plongeante, la ride influcncc probablement, depuis 1 a 2 Ma, le fonctionnement de la terminaison meridionale arquee de la zone de subduction des Nouvelles-Hebrides. L'enmcinement el la floltabi li te de cette ride massive contrarient la flex ure de la plaque plongeante ct rendent difficile sa subduction. Ainsi, les failles normales qui decoupent le saillant sont peu actives; la zone de contact entre les plaques, defin ie par la sismicite superficielle, s'est rapprochee de la fosse; le mur interne, localement souleve, montre un debut d'accret ion et le front des deformations compressives a migre sur le mur externe. L'approche de la ride des Loyaute, puis la collision avec la terminaison meridionale arquee de la zone de subduction des Nouvelles Hebrides, ont provoque une evolution differente, de part et d 'autre de 22°S, du dispositif de convergence. Vers 22°S, un decrochement estouest senestre, d 'amplitude regionale, decoupe l'arc et affecte le bord de la plaque plongeante. Au nord de cct accident, la coll ision, bien qu'intense, ne suffit pas, pour l'instant, a contrecarrer totalement la convergence, nonnale a la fosse, car l 'effet d 'entrainement lateral induit par la subduction de la lithosphere oceanique du bassin nord-Loyaute est dominant. Au sud de cet accident, la convergence oblique sous la terminaison arquee est arretee, et une microplaque, fortement couplee a la plaque ind()-australienne, est apparue. Au droit de la zone de coll ision, la faib le longueur de la zone de 8 enioff, le developpement limite voire l'absence de la chaine volcanique sommitale, la terminaison des fosses intra-are du Coriolis, ainsi que le decrochement important de l'axe de la dorsale active du bassin nord-fidjien ne sont probablement que des consequences de l'approche de la ride et
The western part of the Pacific plate, where the oceanic crust is of Jurassic age, has undergone exceptional activity during the Cretaceous. It has produced numerous volcanoes (oceanic islands, guyots, seamounts) as well as very thick accumulation of volcanogenic material in the deep basins. The Mesopac cruise of the R/V Jean Charcot in the Nauru Basin obtained multichannel seismic profiles for the first time in these regions. This technique aimed at estimating the thickness of the cretaceous volcanics and locating the top of the underlying Jurassic crust. The results, when compared with those of DSDP drill holes, demonstrate that the abnormally shallow depth of the Nauru Basin results from an accumulation of flows and sills that constitute a volcanic complex at least 2.4 km thick, and not from a thermal uplift of the lithosphere.
Seamewe 1 cruise has been devoted to the survey of the projected track of a telecommunication cable between Djibouti and Suez. The data include seabeam bathymetric maps at various scales, mud penetrator, magnetic and gravimetric records along the track lines. Temperature measurements, Kullenberg cores, grab samples and sea-floor photographs were performed at several places. Sidescan sonar pictures were recorded in Suez Gulf and over Djibouti shelf.
Specimens of Riftia pachyptila and Alvinella pompejana were collected during the Biocyarise expedition to hydrothermal vents on the East Pacific Rise at 13 degree N. Quantitative data describing the concentrations of metals in Riftia pachyptila showed that numerous metals are highly concentrated. Analyses of tissues reveal that the plume and the trophosome are the main sites of metal accumulation. Bioconcentration links with the mineral composition of the vent environment are discussed. Mechanisms for metal detoxication were investigated in both species: the presence of metal-rich granules and metallothionein-like proteins were observed. The role of chemolithotrophic bacteria in these mechanisms is discussed.
Available hydrographic data for the Catalano-Balearic sea indicate the presence of a ridge-shaped elevation of the isopycnals, forming a front parallel to the Catalan coast, approximately mid-way between the Iberian peninsula and the Balearic islands. This structure appears as a divergence between the southwest-flowing coastal current on the Catalan side and a flow to the northeast on the island side, and is associated with consistent regularities in the biological distributions. Repeated studies of the mesoscale distribution of phytoplankton organisms have shown patterns that may be interpreted in relationship with the presence of the front.
Ship data collected in the 1960s and recent data from ship, aircraft and satellites indicate that the flow in the Strait of Gibraltar does not move in the form of continuous currents but as tidal-induced pulses. A descriptive model based on these data is presented. The model indicates that the pulses are a result of increases in the speed of the tidal streams as they encounter the constrictions of the regional bathymetry (especially the Camarinal Sill and between the Camarinal Sill and Tarifa). Periodic increases modify the regional flow so that during each tidal cycle, the eastward-flowing surface Atlantic water and westward-flowing deep Mediterranean water are alternately emitted as large pulses into the Mediterranean Sea (Atlantic water) and Atlantic Ocean (Mediterranean water).
The intensification of the South Equatorial Current at the surface and the corresponding decrease of the Equatorial Undercurrent below are associated on a seasonal time scale with a decrease of the temperature at and below the surface; the abrupt increase and decrease of the surface and subsurface currents in February-March 1983 lead, by geostrophic adjustment, to an uplift of the 20 degree C isotherm, contained within the thermocline, of 14 m. In October-November, the decrease of the equatorial undercurrent at 85 m depth induces by the same way a vertical displacement of the 17 degree C isotherm of 16 m. Vertical mixing takes place during the two cooling periods; maxima and vertical shear are found for both years in June-July between 10 and 35 m depths and in October-November between 35 and 60 m depths.
The supper ocean heat content and seasonal sea level changes in the eastern Tropical Atlantic do not depend only upon the vertical displacements of the shallow tropical thermocline and changes in the temperature and thickness of the tropical surface waters. These seasonal changes also depend heavily upon the changes in thickness of the South Atlantic central waters, advected to the east into the Gulf of Guinea by the three equatorial counter-currents, and polewards from the Equator along the African shelf edge by the coastal counter-currents. The heat content of the tropical surface waters and the South Atlantic central waters are compared: heat gain and loss between these two water masses seem to be in balance.
: The changes of heat storage of the Somali Basin in the western Indian Ocean, as shown from 55 detailed temperature sections along the western sea lane (tanker XBT program 1975-1979), are only partly accounted for by the net heat gain at the sea surface. Vertical movement of the thermocline and horizontal advection appear to be the cause of major changes both between and during the monsoons. During the southwest monsoon the large amount of heat stored in the 0 to 400-m layer during northern spring (up to 31 x 10 to the 8th power J/sq m) is redistributed with a heat loss of approximately 13 x 10 to the 8th power J/sq m in the upper 0 to 100-m layer and the gain of a similar amount in the 100 to 400-m layer, caused to a large extent by the dynamic response of these layers to the monsoon wind stress. In late autumn this heat is then advected from the region. Reprints.
A study of the size structure of chlorophyll a (Chla) covering the major part of the equatorial Atlantic Ocean from 5 degree N to 5 degree S leads to the conclusion that seasonal nutrient enrichment in the open eastern equatorial Atlantic does not drastically affect the size distribution of the primary producers: 90% of the total Chla is everywhere contained in the 20 mu m fraction on the average. In the coastal upwelling near Dakar, this percentage is less than 60%, for the same range of chla concentrations. In the equatorial region the percentage of 1 mu m Chla is the same in the deep chlorophyll maximum located at the top of the nitracline during the warm season and in the subsuperficial maximum of the upwelling.
Using long-term current observations from 10 moorings in the central Pacific (1 degree 30'N)-1 degree 30'S, 163 degree 15'W-167 degree W) in February-March 1980, estimations of vertical velocities are made from numerical integration of the continuity equation over the box model. Vertical motion in the upper 300 m layer is directed upwards, and its velocity ranges 1.0 to 8.0 x 10 super(-3) cm.s super(-1).
Recent evidence from long-term monitoring of the deep sea suggest that conditions in this supposedly stable environment may be subject to seasonal forcing. For example, measurements both of flux of particles from the surface and in the velocity of bottom currents suggest seasonal fluctuations occur at the benthic boundary to which the reproductive activity of benthic populations of certain species of echinoderm and bivalve molluscs may be attuned. The small egg size and high fecundity of species showing seasonal periodicities in reproduction suggests planktotrophic development. Larval development occurs in the early part of the year, suggesting that flux of organic particulates from surface production may be an ultimate selective factor for the determination of a reproductive periodicity within the deep-sea environment.
Seven years (1970-1976) of density data on three harpacticoid copepods were analysed by Maximum Entropy spectral Analysis. The spectra are very different, indicating population regulation is also different. Tachidius discipes is regulated by external seasonal factors, mainly the annual light cycle, and has a very simple spectrum. Canuella perplexa has much longer cycles in its spectrum and can be treated as a single species problem with logistic dynamics Paronychocamptus nanus has also a complex spectrum that can partly be explained by competitive interactions.
The water repartition in the Arcachon Basin is as follows: 1) Exterior neritic water, purely marine (the inlet area); 2) Interior neritic water (bottom of the basin); 3) Middle neritic water found between the two above mentioned types of water (central part of the basin). Analyses carried out since 1972 show an increase of average trace metal contents (Zn = 100%, Cu = 400%, Pb = 400%) and of organic particulate carbon. The Lagoon center, poor in metal content, is considered as a geochemical baseline of the basin. Since 1977, this baseline has been showing an increase of metallic elements. These concentrations, however, may be easily compared to the average metal content, noted for the same type of sediments. Inversely, the decrease of metal contents in the coastal area in 1980, constitutes a new phenomenon, likely to improve the quality of the whole Lagoon. A study of the pollution impact shows that lead is the most easily diffused element in the whole of the ecosystem.
One of the ways man modifies lagoons is by placing artificial structures along their shorelines, along the barriers that separate them from the open ocean, and upstream in the rivers that drain into them. Structures built adjacent to or on barrier islands including groins, seawalls, dikes, and breakwaters change normal washover processes; jetties and tidal gates interfere with currents and sedimentation at tidal inlets; and structures on the mainland alter sediment supply, currents, biota, salinity, and water quality. Projects within the lagoon itself, including reclamation and maintenance of canals and dock and harbor facilities modify lagoon form and consequently sedimentation and circulation.
The super(210)Pb radioactive dating technique on two cores from the northern part of the Lagoon of Venice gives an average sedimentation rate of 0.7 cm/year. This is used to date the vertical profiles of member of DDT, Hg, Cd, Pb, Cr, Cu, Ni, Co, Zn and Fe concentrations. A maximum member of DDT concentration is found at a depth of 10 cm, corresponding to about 1955, in accordance with the time of the largest use of this pesticide in the area drained by the Dese river. Due to antropogenic origin the concentrations of heavy metals in the upper sediment layers are 1.3-12.7 times higher than those in the lower ones. The average concenttrations of these elements in the layers below 80 cm are tentatively proposed as natural background concentrations for the Lagoon of Venice.
The three main sources of organic matter in oceanic basins are marine plankton, terrestrial higher plants and undifferentiated residual organic matter which has been highly degraded in subaerial environments. The time and areal distribution of marine organic matter is mainly controlled by the conditions of preservation. In deep oceans accumulation occurs mainly in confined anoxic basins, and also in open ocean, where local conditions generate a surplus of supply over bacterial degradation. Moderately degraded terrestrial organic matter is derived from land plants and carried away by rivers and currents. It is mainly deposited in sediments of the river fans. Residual organic matter has been highly degraded in subaerial conditions prior to transportation, and may consist of contemporaneous and/or recycled material. It cannot be degraded any more and may be spread over wide areas, regardless of the local environments.
