Isotopic variation of fishes in freshwater and estuarine zones of a large subtropical coastal lagoon

We used stable C and N isotope ratios of tissues from 29 fish species from a large subtropical lagoon in southern Brazil to examine spatial variability in isotopic composition and vertical trophic structure across freshwater and estuarine habitats. Nitrogen isotope ratios indicated a smooth gradation in trophic positions among species, with most fishes occupying the secondary and tertiary consumer level. Fish assemblages showed a significant shift in their carbon isotopic signatures between freshwater and estuarine sites. Depleted carbon signatures (from −24.7‰ to −17.8‰) were found in freshwater, whereas more enriched signatures (from −19.1‰ to −12.3‰) were obtained within the estuarine zone downstream. Based on our survey of the C₃ and C₄ plants and isotopic values for phytoplankton and benthic microalgae reported for ecosystems elsewhere, we hypothesized that the observed δ¹³C differences in the fish assemblage between freshwater and estuarine sites is due to a shift from assimilating organic matter ultimately derived from C₃ freshwater marsh vegetation and phytoplankton at the freshwater site (δ¹³C ranging from −25‰ to −19‰), to C₄ salt-marsh (e.g. Spartina) and widgeon grass (Ruppia maritima), benthic microalgae and marine phytoplankton at the estuarine sites (from −18‰ to −12‰). Our results suggested that fish assemblages are generally supported by autochthonous primary production. Freshwater fishes that likely were displaced downstream into the estuary during periods of high freshwater discharge had depleted δ¹³C values that were characteristic of the upper lagoon. These results suggest that spatial foodweb subsidies can occur within the lagoon.     

Reduction of hydroelastic responses of a very-long floating structure by a floating oscillating-water-column breakwater system

The hydroelastic responses of a very-long floating structure (VLFS) placed behind a reverse T-shape freely floating breakwater with a built-in oscillating water column (OWC) chamber are analyzed in two dimensions. The Bernoulli–Euler beam equation is coupled with the equations of rigid and elastic motions of the breakwater and the VLFS. The interaction of waves between the floating rigid breakwater and the elastic VLFS is formulated in a consistent manner. It has been shown numerically that the structural deflections of the VLFS can be reduced significantly by a suitably designed reverse T-shape floating breakwater.     

Numerical study of the motions and drift force of a floating OWC device

The motion and the drift force of a floating OWC (oscillating water column) wave energy device in regular waves are studied taking account of the oscillating surface-pressure due to the pressure drop across the duct of the air chamber. The potential problem inside the chamber is formulated by making use of the Green integral equation associated with the Rankine-type Green function while the outer problem with the Kelvin-type Green function. The added mass, wave damping and excitation coefficients as well as the motion and drift force of the OWC device are calculated for various values of parameter related to the pressure drop.     

A sonic well log of the basement complex of the Walvis Ridge

A sonic well log was obtained within the basement complex of the Walvis Ridge during Deep Sea Drilling Project Leg 74. The top of the basement complex is characterized by smooth acoustic reflectors. The rocks recovered within the basement complex consist of basalts with intercalated sediments. According to the log ～-50% of the upper 75 m of basement are igneous rocks and the other 50% sedimentary. Sonobuoy results indicate that the ratio of sediments to basalt increases with depth for an additional 225 m until a typical oceanic velocity structure is observed. Paleontological results suggest that the processes forming this upper 300 m of the basement complex was accomplished within a short time interval.     

Response of the suspended sediment transport system to continental shelf dynamics

Surface currents influenced by a wind-driven upwelling event in San Pedro Bay moved total suspended matter (TSM) confined to the inner shelf on 19 April 1978 seaward, so that by 27 April surface TSM had increased over the outer shelf. Near-bottom concentrations of TSM also increased across the shelf during this time. This is explained by sediment resuspended by large surface waves being advected from the inner shelf seaward by the mean flow after this flow had turned from southeasterly to southerly when upwelling ceased on 26 April. These complex shelf dynamics contribute to the off-shelf transport of mud to the slope and deep basins.     

A Lagrangian model for wind- and wave-induced near-surface currents

A theory is outlined for time-dependent currents induced near the sea surface in deep water, away from coastal boundaries, by a variable wind stress and deep-water wave field. It is based on the theory of Weber (1983) which uses a second-order perturbation expansion of the Navier-Stokes equations in Lagrangian coordinates and includes the Coriolis effect. It uses an eddy viscosity formulation for both wave dissipation and momentum transfer within the current field: the eddy viscosity ν may be allowed to vary with depth. The wind stress may be time-varying and the wave field may vary in both space and time.For the case of a constant ν, the results agree with those of Ursell (1950), Hasselmann (1970) and Pollard (1970) in the limit ν→0, and the steady-state results agree with those of Weber. For a particular case of depth-varying ν, results (obtained from numerical simulations) are in better general agreement with observations of wind-induced surface drift than when a constant ν is used.An outline is given of the application of the theory to the case of a random sea state. There are good prospects for using output data from numerical wave prediction models to drive the equations of this near-surface current model.     

Geohazards surveys in the North Carolina upper continental margin

Geophysical surveys on the upper continental slope and shelf-break area off North Carolina confirm previous suggestions that this highly dissected region is characterized by deposition of sediment swept from the adjacent shelf. Seismic reflection profiling, echo sounding and side-scan sonar studies failed to reveal conclusive evidence for slumping or other gravity processes which have been described for similar areas to the north. Outcrops appear to be lacking and the upper slope is a region of sediment accumulation and/or periodic bypassing.     

Free vibration of semisubmersibles: A parametric study

Like any other engineering structure, the dynamic aspects of semisubmersible offshore platforms require serious consideration. The free vibrations of the semisubmersible structures have been investigated in the present work, in which the effect of the variation of the length, draft and hull spacings on the natural frequencies and mode shapes has been studied and the nature of the variations and their reasons have been discussed.     

Numerical modelling of the seasonal variability of hydrophysical fields in the Black Sea

The results of research into the seasonal variability of hydrothermodynamic characteristics of the Black Sea are presented. These have been obtained using field data on the fields of temperature, salinity, and wind, and by mathematical modelling. The seasonal variability of the current and heat content fields and of the cold intermediate layer and other hydrophysical characteristics is discussed.Translated by Mikhail M. Trufanov.