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.     

Measurement of the redox speciation of iron in seawater by catalytic cathodic stripping voltammetry

Catalytic cathodic stripping voltammetry (CSV) preceded by adsorptive collection of complexes of 1-nitroso-2-napthol (NN) can be used to determine iron in seawater. It is shown here that iron(II) is effectively masked in the presence of 2,2-dipyridyl (Dp) so that iron(III) is measured selectively. The concentration of iron(II) is then calculated as the difference between the concentrations of reactive iron (Fe_R) in the absence and presence of 2 μM Dp, Fe_R being defined as that which was complexed by 20 μM NN at pH 6.9 in the presence of 1.8 mM H₂O₂ and 5 ppm sodium dodecyl sulphate. A 30 min reaction time was allowed for Dp to react with iron(II) in seawater prior to the determination of reactive iron(III) using the same conditions as used for Fe_R. Detection limits of 0.08 nM, 0.077 nM and 0.12 nM were obtained for Fe_R, iron(III) and iron(II), respectively, using a 60 s deposition time.The method was utilised to determine the redox speciation of iron in the northern North Sea. Concentrations of Fe_R ranged between 0.8 and 3.5 nM with nutrient-like depth profiles. Iron(II) was found to be present at concentrations up to 1.2 nM, the highest concentrations occurring in the upper 20 m of the water column.     

Extension of the Narmada — Son lineament on the continental margin off Saurashtra,Western India as obtained from magnetic measurements

Magnetic total intensity values and bathymetric data collected on the continental margin off Saurashtra were, used to prepare magnetic anomalies and bathymetric contour maps. The magnetic anomalies are considered to have been caused by the Deccan Trap flood basalts which underlie the Tertiary sediments. Interpretation of the magnetic data using two-dimensional modelling method suggests that the magnetic basement is block faulted and deepens in steps from less than 1.0 km in the north to about 8.0 km towards the southern portion of the study area. The WNW-ESE trending faults identified in the present study extend across the Saurashtra continental margin between Porbandar and Veraval and appear to represent a major linear tectonic feature. The relationship of these fault lineaments with the regional tectonic framework have been discussed to indicate that they conform better as the northern boundary faults of the Narmada rift graben on the continental margin off Saurashtra.     

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.