Spatial variability in growth and reproduction of the Pacific oyster Crassostrea gigas (Thunberg, 1793) along the west European coast

The Pacific oyster Crassostrea gigas was introduced in Europe for commercial purposes in the mid 1960s. It was initially thought that low winter temperatures would restrain this species' reproduction and settlement; however, its present distribution in areas where no introduction has taken place suggests that natural invasion and expansion has occurred. Along the European coast, wild populations of Pacific oysters are already found from northern Germany to southern Portugal. Whether C. gigas will continue to further expand through northern waters will depend on its physiological performance. In this study, the performance of wild oyster populations has been studied in terms of growth and reproduction at three stations: La Rochelle (France; 46°N), Yerseke (Oosterschelde estuary, The Netherlands, 51°N), and Texel (Wadden Sea estuary, The Netherlands, 53°N). The French population had the lowest somatic-shell mass ratio and an increase in maximum shell length, somatic and gonadal mass was observed from France to the Netherlands. In addition, mean oocyte diameter decreased significantly from south to north. The combination of increasing gonadal mass and decreasing oocyte volume suggests an increasing reproductive output in terms of egg numbers from France to The Netherlands. Differences in temperature between locations will at least be partly responsible for the observed patterns; however, other environmental factors (such as food availability, predation pressure, sediment type and/or seston concentration) cannot be excluded. Since smaller eggs (oocytes) are thought to have a longer development time, the environmental conditions along the Dutch coast may result in increased larval dispersal and possibly in further population expansion.     

Structure and Quaternary tectonic history of the Woodlark triple junction region,Solomon Islands

The Woodlark triple junction region, a topographically and structurally complex triangular area of Quaternary age, lies east of Simbo Ridge and southwest of the New Georgia island group, Solomon Islands, at the junction of the Pacific, Australian and Solomon Sea plates. SeaMARC II side-scan imagery and bathymetry in conjunction with seismic reflection profiles, 3.5 kHz records, and petrologic, magnetic and gravity data show that the active Woodlark spreading centre does not extend into this region.South of the triple junction region, the Woodlark spreading centre reoriented at about 2 Ma into a series of short ESE-trending segments. These segments continued to spread until about 0.5 Ma, when the lithosphere on their northern sides was transferred from the Solomon Sea plate to the Australian plate. Simultaneously the Simbo transform propagated northwards along the western side of the transferred lithosphere, forming a trench-trench-transform triple junction located NNW of Simbo island and a new leaky plate boundary segment that built Simbo Ridge.As the Pacific plate approached, the area east of northern Simbo Ridge was tilted northwards, sheared by dominantly right-lateral faults, elevated, and intruded by arc-related magmas to form Ghizo Ridge. Calc-alkalic magmas sourced beneath the Pacific plate built three large strato-volcanic edifices on the subducting Australian plate: Simbo at the northern end of Simbo Ridge, and Kana Keoki and Coleman seamounts on an extensional fracture adjoining the SE end of Ghizo Ridge.A sediment drape, supplied in part from Simbo and Kana Keoki volcanoes, mantles the east-facing slopes of northern Simbo and Ghizo Ridges and passes distally into sediment ponded in the trench adjoining the Pacific plate. As a consequence of plate convergence, parts of the sediment drape and pond are presently being deformed, and faults are dismembering Kana Keoki and Coleman seamounts.The Woodlark system differs from other modern or Tertiary ridge subduction systems, which show wide variation in character and behaviour. Existing models describing the consequences of ridge subduction are likely to be predictive in only a general way, and deduced rules for the behaviour of oceanic lithosphere in ridge subduction systems may not be generally applicable.     

Effects of benthic biology on bottom boundary layer processes, Dry Tortugas Bank, Florida Keys

 An instrumented tetrapod was deployed for three weeks on the Dry Tortugas Bank at a depth of 26 m in February 1995. Bottom roughness was dominated by shrimp burrows and worm mounds with rms roughness amplitudes ranging from 0.47 to 1.75 cm. Logarithmic velocity profiles show apparent total roughness heights ranging from 0.30 to 1.45 cm, values consistent with observed biological roughness. The bed sediments were weakly bound by an algal crust at the sediment–water interface. When this bound layer was scraped away by a mooring that was accidentally dragged, sharp-crested wave-induced ripples appeared within the resulting swath. We conclude that physically induced roughness is biologically suppressed, but if dominant, would be significantly higher than the prevailing biological roughness.     

Drag force on a circular cylinder in unseparated laminar high Reynolds number nonlinear random oscillatory flow

The stochastic properties of the drag force maxima on a circular cylinder subjected to nonlinear random waves are investigated. Unseparated laminar high Reynolds number flow is considered. A simplified approach based on second order Stokes waves is presented, including the sum-frequency effect only. It is demonstrated how a drag force formula valid for regular linear waves can be used to find the cumulative distribution function of individual drag force maxima for nonlinear irregular waves. Here the [Wang, 1968] drag force coefficient is used.     

Influence of structural flexibility and wave interference on dynamic behavior of idealized offshore platform

An approximate method is presented to estimate the hydrodynamic loading and structural response of an idealized offshore platform subjected to a regular train of linear surface waves. The platform is taken to consist of four bottom-mounted, flexible, circular cylinders supporting a rigid deck and is assumed to be aligned parallel to the incident wave direction. The response of each column is assumed to be one-dimensional and to be governed by linear beam theory. The solution technique for the fluid velocity potential involves replacing scattered waves by equivalent plane waves together with non-planar, first-correction terms, and can be shown to be a large spacing approximation.Numerical results are presented which show the effect of hydrodynamic interference and structural flexibility on the platform response.     

Microearthquakes along the back-arc spreading system in the eastern Bismarck Sea

OBS’s were deployed for 26 to 29 days in the eastern Bismarck Sea to investigate the back-arc spreading. Hypocenters of 186 shallow earthquakes were determined using P- and S-waves from at least five stations. In the western survey area, a transform fault zone is marked by a linear micro-earthquake activity striking N65°W and less than 5 km wide. The predominant type of their focal mechanisms is strike-slip. In the eastern area, several intermittent zones of micro-earthquakes and their strike-slip type focal mechanisms suggest the location of short-length transform faults separating en-echelon spreading ridges.