The near field of internal waves after the moving area of atmospheric pressures

The near wave field generated by the travelling atmospheric pressure zone over a stratified fluid is studied within the framework of the linear theory of long waves allowing for the Coriolis force. The exact solution of the problem as a set of single integrals is deduced for the model distribution of pressures. The characteristics of the near wave field have been computed for the Väisälä-Brunt frequency profile takeu from observations. Their dependences on the geographical latitude and the wave generator travelling rate are analysed.Translated by Mikhail M. Trufanov.     

Interactions of cadmium with the reproductive cycle of Mytilus edulis L.

Sublethal effects of cadmium on reproduction can have detrimental consequences for the long-term survival, even of species with high LC₅₀ values. The sea mussel, Mytilus edulis, is well-known for its use in biomonitoring work due to its relatively low sensitivity towards many toxicants, including cadmium.¹ Because little is known on sublethal effects of this metal in invertebrates,^2–6 we have studied these effects, with emphasis on reproduction, by exposing mussels to 100 ppb cadmium in sea water. This concentration significantly inhibits follicle development in both male and female gonads, but has subsequently a stimulating effect on the spawning frequency. Due to these opposite effects the consequences of cadmium stress for gamete production are expected to be small, as the average number of gametes released by spawning animals was not significantly changed.     

Equilibrium constants of triethanolamine in major seawater salts

Acid–base equilibrium constants of triethanolamine (TEA) have been determined by potentiometric titrations with a glass electrode, at 25 °C. Ionic strength was kept constant with only one electrolyte (using one of these salts: NaCl, KCl, MgCl₂ or CaCl₂), with binary mixtures of MgCl₂ and CaCl₂, and finally, in a solution with a composition approximately similar to that of natural seawater without sulfate. Equilibrium constants have been expressed in function of ionic strength by means of Pitzer equations and interaction parameters proposed in this theory have been obtained. It has been found that acid–base behaviour of TEA depends greatly on the salt used: basicity of TEA is decreased by CaCl₂, while it is increased by the other electrolytes used in this work.     

Experimental investigation of forces and geometry of a net cage in uniform flow

A scale model of a flexible circular net with different weights attached to the bottom was tested in a flume tank. Global forces and net deformation were measured for different steady current velocities. Three different sizes of bottom weights were used in the tests. The results from these tests are presented and discussed with the emphasis on the dependency between the forces and the geometry. Comparison is also made to empirical based formulas for calculation of drag and lift forces on net structures. Findings show that i) the forces on, and deformation of a flexible net structure are mutually highly dependent on each other; ii) estimates of global forces on a flexible net structure calculated using simple drag formulas derived from stiff net panel experiments give large errors when compared to experimental measurements; iii) numerical models taking into account the dependency between force and deformation should be used to obtain accurate estimates of forces on flexible net structures; and iv) the forces on a flexible net structure are dependent on Reynolds number, and their dependency are similar to that of a regular cylinder.     

Side-scan and acoustic subbottom characterization of the sea floor near the Dry Tortugas, Florida

 Between 2 and 6 February, 1995, a 25 km² area at the Dry Tortugas (Florida Keys) was surveyed with a 100 kHz side-scan sonar system and 3.5-kHz subbottom profiler. The side-scan system revealed a pattern of alternating high and low backscatter. The subbottom profiler showed areas with no acoustic penetration between sediment troughs. The combination of both methods allowed delineation of the boundaries in high-backscatter regions, and sediment samples allowed correlations between high backscatter and coarser-grained sediments.     

Benthic macrofauna changes in areas of Venice lagoon populated by seagrasses or seaweeds.

Two areas of the Venice lagoon populated by seagrasses (three stations covered by Cymodocea nodosa (Ucria) Asherson, Zostera marina Linnaeus, Zostera noltii Hornemann) or seaweeds (two stations: one covered by Ulva rigida C. Agardh and another at present without seaweed biomass) were monitored by means of six surveys over a year in order to study macrofaunal composition and seasonal changes. The seagrass stations showed a mean species richness (28-30 S m(-2)), individual abundance (1854-4018 N m(-2)) and biomass (22.3-37.7 g m(-2) ash-free-dry-weight, AFDW) ca. 3-8 times higher than those populated by seaweeds (10-15 S m(-2), 494-1395 N m(-2) and 5.6-13.7 g m(-2) AFDW). Differences among seagrass or seaweed stations were much lower. The Ulva-dominated station showed a macrofauna completely different both from the other stations and the communities recorded ca. 30 years ago, before the prolific growth of Ulva. In this station, frequent biomass decompositions and anoxic crises created critical conditions for life favouring organisms with reduced life cycles, younger individuals and the epifaunal species instead of the infaunal ones. In particular, Ulva grazers and scrapers such as Gammarus aequicauda Stock and Gibbula adriatica Philippi were found to be by far the most abundant species, whereas the taxa characteristic of the associations found in the past, in the presence of seagrasses or seaweeds and typical of low eutrophicated environments, appear strongly reduced. Marked differences in the macrophyte dominance and in the bio-physico-chemical variables which characterise the main environmental conditions of the Venice lagoon support the different distribution and composition of macrofaunal communities. Seaweed stations appear mainly governed by the seasonal cycles of these un-rooted macrophytes which, by alternating periods of production and decomposition, are responsible for the drastic reduction of macrofauna biodiversity and biomass. Conversely, seagrass stations exhibit a better oxidisation of the environment and show conditions more favourable for macrofauna colonisation, especially in the presence of macrophytes which are characterised by very well developed below-ground systems such as Cymodocea nodosa.     

Design and projected performance of a flapping foil AUV

The design and construction of a biomimetic flapping foil autonomous underwater vehicle is detailed. The vehicle was designed as a proof of concept for the use of oscillating foils as the sole source of motive power for a cruising and hovering underwater vehicle. Primary vehicle design requirements included scalability and flexibility in terms of the number and placement of foils, so as to maximize experimental functionality. This goal was met by designing an independent self-contained module to house each foil, requiring only direct current power and a connection to the vehicle's Ethernet local area network for operation. The results of tests on the foil modules in the Massachusetts Institute of Technology (MIT) Marine Hydrodynamics Water Tunnel and the MIT Ship Model Testing Tank are both used to demonstrate fundamental properties of flapping foils and to predict the performance of the specific vehicle design based on the limits of the actuators. The maximum speed of the vehicle is estimated based on the limitations of the specific actuator and is shown to be a strong function of the vehicle drag coefficient. When using four foils, the maximum speed increases from 1 m/s with a vehicle C/sub D/ of 1.4 to 2 m/s when C/sub D/=0.1, where C/sub D/ is based on vehicle frontal area. Finally, issues of vehicle control are considered, including the decoupling of speed and pitch control using pitch-biased maneuvering and the tradeoff between actuator bandwidth and authority during both the cruising and hovering operation.