Sensitivity to stress of the estuarine bivalve Macoma balthica from areas between the Netherlands and its southern limits (Gironde)

Variation in the sensitivity to stress of Macoma balthica was measured in several French and Dutch estuaries. For adult and juvenile Macoma balthica exposed to copper under conditions of starvation, differences in mortality rate, condition, glycogen, burrowing rate and copper content were assessed. No significant differences were observed between adults and juveniles; the influence of treatment and origin was always evident. Animals from the most southern estuaries, Loire and Gironde, near to the species's southern limit of distribution, showed, in the field, the strongest deviations for the ecophysiological traits measured, and were in the experiments the most sensitive to stress.     

Effects of predation and oxygen deficiency on different age classes of the amphipod Monoporeia affinis

Predation by the big brackish-water isopod Saduria entomon on two age-classes (juveniles and adults) of the small deposit-feeding amphipod Monoporeia (syn. Pontoporeia) affinis was studied at different oxygen conditions (normoxia>11 mg/l O₂, and moderate hypoxia 4 mg/l O₂). Vulnerability of juvenile and adult M. affinis was studied in single and mixed age-class treatments. The proportions of juveniles and adults were varied at one total numerical density. The predatory effects were measured as numbers of individuals consumed and of total biomass intake (mg ash-free dry weight). The highest predation rate (in numbers) was detected in the single-prey treatment with juvenile M. affinis, both in normoxia and moderate hypoxia. No preference for any of the two size classes was found in the treatment with equal numbers of adults and juveniles. In normoxia, the total number of prey consumed decreased with increasing proportions of adults, while intake increased in terms of biomass. In moderate hypoxia, high proportions of adults reduced the predation rate of S. entomon to such an extent that also the intake in terms of biomass was significantly negatively affected.     

Morphodynamics of a bar-trough surf zone

A field study was made of the distinguishing morphodynamic processes operating in a surf zone which perennially exhibits accentuated bar-trough topography (the “longshore-bar-trough” and “rhytmic-bar-and-beach” states as described by Wright and Short, 1984). Characteristic features of the morphology include a shallow bar with a steep shoreward face, a deep trough, and a steep beach face. This morphology, which is favored by moderate breaker heights and small tidal ranges, strongly controls the coupled suite of hydrodynamic processes. In contrast to fully dissipative surf zones, the bar-trough surf zone is not at all saturated and oscillations at incident wave frequency remain dominant from the break point to the subaerial beach. The degree of incident wave groupiness does not change appreciably across the surf zone. Infragravity standing waves which, in dissipative surf zones, dominate the inshore energy, remain energetically secondary and occur at higher frequencies in the bar trough surf zone. Analyses of the field data combined with numerical simulations of leaky mode and edge wave nodal—antinodal positions over observed surf-zone profiles, indicate that the frequencies which prevail are favored by the resonant condition of antinodes over the bar and nodes in the trough. Standing waves which would have nodes over the bar are suppressed. Sediment resuspension in the surf zone appears to be largely attributable to the incident waves which are the main source of bed shear stress. In addition, the extra near-bottom eddy viscosity provided by the reformed, non-breaking waves traversing the trough significantly affects the vertical velocity profile of the longshore current. Whereas the bar is highly mobile in terms of onshore—offshore migration rates, the beach face and inner regions of the trough are remarkably stable over time.     

Cd, Pb, Cu, Ni and Co distribution in the German Bight

Cd, Pb and Cu concentrations, dissolved and total, have been determined in the German Bight. In 1975, 1977, 1978, 1980 and 1983 water samples were collected at 215 sampling stations. In the 1983 mission samples were also analysed for Ni and Co. The samples were filtered and acidified immediately after collection. Analysis was performed by voltammetry.The analytical data indicate that a net portion of the Cd, the Weser being a distinct source for it, will cross the estuarine zone. Open sea Cd levels in the German Bight are about a factor 10 higher than North Atlantic values. Pb, being transported mainly with the suspended particulate phase (up to 99.5%), is better eliminated by sedimentation in the estuaries. Ni and Cu occur mainly in the dissolved phase. Pollution by Co is distinct in the Elbe, but the output is rapidly diluted to background contents below 10 ng kg⁻¹. An interpretation of the results from the different years yields a rather steady state situation in the German Bight.     

Mixed-function oxygenases as biological monitors around petroleum hydrocarbon development sites: Potential for induction by diesel and other drilling mud base oils containing reduced levels of polycyclic aromatic hydrocarbons

The use of oil-based drilling muds has been discouraged in hydrocarbon exploration and production in the marine environment but these muds are presently being used to a considerable degree in the United Kingdom and Norwegian sectors of the North Sea. Field studies in the North Sea have demonstrated only localized impacts around individual drilling sites,^1,2 even including those sites where ‘toxic’ diesel oils were employed as base fluids in drilling muds. Yet the question of disposal of cuttings contaminated with oil from drilling muds remains somewhat controversial. The induction of mixed-function oxygenases (MFO) has been validated on a number of occasions in the field as a sensitive index of low level hydrocarbon exposure (reviewed by Payne),³ including sites in the North Sea where diesel-based muds were used.⁴ The present study demonstrates that any potential for induction by hydrocarbon contaminated cuttings will probably be reduced by substitution of low-aromatic base oils for diesel in drilling mud formulations.     

Low-frequency damping of a moored semisubmersible obtained from simulated extinction tests and mean wave drift forces

Low-frequency damping of a moored semisubmersible drilling platform was obtained from numerical extinction tests simulated in still water and in regular waves and from mean wave drift forces calculated at zero forward speed. The influence of drag forces was represented by the modified Morison equation. The platform as used for the 18th ITTC Comparative Mooring Study was analyzed in irregular beam waves. The computed time series of sway as well as the corresponding sway response spectrum compared favourably with model test measurements, demonstrating that this procedure to determine low-frequency damping can be effective.     

Upwelling in the Black Sea by the results of numerical experiments and satellite data

The paper analyses results of the numerical simulation of upwelling events in the north-western part of the Black Sea, mostly near the South Crimea. The calculations were performed using a numerical model based on primitive hydrodynamics equations. Emphasis is laid on the case when a salinity front simulating the Black Sea rim current is prescribed in the initial conditions. The interaction of the Black Sea rim current's stream with the coastline and bottom topography leads to the development of an upwelling near the Crimea's coast, even in the absence of wind forcing. The paper discusses the structure of the three-dimensional circulation of waters in the shelf area of the NW Black Sea. Numerical modelling results are matched up with the satellite data obtained by the HRPT receiving station. Translated by Vladimir A. Puchkin.     

Uplift at thermo-mechanically coupled ocean–continent transforms: Modeled at the Senja Fracture Zone, southwestern Barents Sea

 Estimates of uplift at ocean–continent transform margins vary significantly between models incorporating both thermal conduction and viscous coupling across the transform and models taking only one of these processes into account. More heat is predicted to be conducted and advected into the continental lithosphere in combined models than in conductive models. Nevertheless, tectonic uplift predicted by the combined models is only half that predicted conductive models. This is because viscous coupling implies crustal thinning close to the transform. The form and amplitude of uplift in the combined models agree well with erosion estimates along a seismic transect across the Senja shear margin, in the southwestern Barents Sea. Received: 31 January 1995 / Revision received: 20 July 1995     

The effect of currents on the generation of internal waves by a barotropic tide near an underwater ridge

In the frame of the linear theory for long waves, the paper studies long waves generated by a barotropic tide running at an arbitrary angle over a bottom ridge. The ocean is assumed to be two-layered. In the area of a ridge with a rectangular cross section, geostrophic flows are considered, coupled with the inclinations of the free surface and interface. Wave amplitudes are shown to depend on the angle of an onrunning tide and this allows us to reach a conclusion about the influence of a geostrophic current on the generation of internal waves. Translated by Vladimir A. Puchkin.     

Gravity torques for surface waves

The effects of the gravity torques acting on the angular momentum of surface gravity waves are calculated theoretically. For short crested waves the gravity torque is caused by the force of gravity on the orbiting fluid particles acting down the slopes of the crests and troughs and in the direction parallel to the crests and troughs. The gravity torque tries to rotate the angular momentum vectors, and thus the waves themselves, counterclockwise in the horizontal plane, as viewed from above, in both hemispheres. The amount of rotation per unit time is computed to be significant assuming reasonable values for the along-crest and trough slopes for waves in a storm area. The gravity torque has a frequency which is double the frequency of the waves. For long crested waves the gravity torque acts in the vertical plane of the orbit and tries to decelerate the particles when they rise and accelerate them when they fall. By disrupting the horizontal cyclostrophic balance of forces on the fluid particles (centrifugal force versus pressure force) the gravity torque accounts qualitatively for the three characteristics of breaking waves: that they break at the surface, that they break at the crest, and that the crest breaks in the direction of wave propagation.