Diurnal changes in the bio-optical properties of the phytoplankton in the Alborán Sea (Mediterranean Sea)

The changes in the phytoplankton absorption properties during a diurnal cycle were investigated at one station located in the north-western area of the Alborán Sea. The experiment was performed in spring when the water column was strongly stratified. This hydrological situation permitted the establishment of a deep chlorophyll a (chl a) fluorescence maximum (DFM) which was located on average close to the lower limit of the mixed layer and the nutricline. The relative abundance of pico-phytoplankton (estimated as its contribution to the total chl a) was higher in the surface, however, micro-phytoplankton dominated the community at the DFM level. Chl a specific absorption coefficient (a*(λ)) also varied with optical depth, with a* (the spectrally average specific absorption coefficient) decreasing by 30% at the DFM depth with respect to the surface. A significant negative correlation between the contribution of the micro-phytoplankton to the total chl a and a* was obtained indicating that a* reduction was due to changes in the packaging effect. Below the euphotic layer, a* increased three-fold with respect to the DFM, which agrees with the expected accumulation of accessory pigments relative to chl a as an acclimation response to the low available irradiance. The most conspicuous change during the diurnal cycle was produced in the euphotic layer where the chl a concentration decreased significantly in the afternoon (from a mean concentration of 1.1 μg L⁻¹ to 0.7 μg L⁻¹) and increased at dusk when it averaged 1.4 μg L⁻¹. In addition, a* and the blue-to-red absorption band ratio increased in the afternoon. These results suggest that a*(λ) diurnal variability was due to increase in photo-protective and accessory pigments relative to chl a. The variation ranges of a*(λ) at 675 and 440 nm (the absorption peaks in the red and blue spectral bands, respectively) in the euphotic layer were 0.01–0.04 and 0.02–0.10 m² mg⁻¹ chl a, respectively. Approximately 30% out of this variability can be attributed to the diurnal cycle. This factor should therefore be taken into account in refining primary production models based on phytoplankton light absorption.     

Unusual large-scale phytoplankton blooms in the equatorial Pacific

Unusual large-scale accumulations of phytoplankton occurred across 10,000 km of the equatorial Pacific during the 1998 transition from El Niño to La Niña. The forcing and dynamics of these phytoplankton blooms were studied using satellite-based observations of sea surface height, temperature and chlorophyll, and mooring-based observations of winds, hydrography and ocean currents. During the bloom period, the thermocline (nutricline) was anomalously shallow across the equatorial Pacific. The relative importance of processes that enhanced nutrient flux into the euphotic zone differed between the western and eastern regions of the blooms. In the western bloom region, the important vertical processes were turbulent vertical mixing and wind-driven upwelling. In contrast, the important processes in the eastern bloom region were wave-forced shoaling of nutrient source waters directly into the euphotic zone, along-isopycnal upwelling, and wind-driven upwelling. Advection by the Equatorial Undercurrent spread the largest bloom 4500 km east of where it began, and advection by meridional currents of tropical instability waves transported the bloom hundreds of kilometers north and south of the equator. Many processes influenced the intricate development of these massive biological events. Diverse observations and novel analysis methods of this work advance the conceptual framework for understanding the complex dynamics and ecology of the equatorial Pacific.     

Numerical Solution of Lock-Release Gravity Current with Viscous Self-Similar Regime

Lock-release gravity currents with a viscous self-similar regime are simulated by use of the renormalization group(RNG) k - ε model for Reynolds-stress closure. Besides the turbulent regime with initially a slumping phase of a conslant current front speed and later an inviseid self-similar phase of front speed decreasing as t^-1/3(where t is the time measured from release), the viseous self-similar regime is satisfactorily reproduced with front speed decreasing as t^-4/5,consistent with well known experimental observations.     

Modal Wave Number Tomography for South China Sea Front

1 .IntroductionTheSouthChinaSealiestothesoutheastpartoftheChinesecontinent.TheKuroshioflowsbytheeastsideoftheLuzonStraitfromsouthtonorthinwinter ,spring ,andautumn .TheKuroshioaf fectsnearcurrentfieldsandtemperaturefields,changingtheoceanenvironmentparametersandsoundpropagation .Thus ,theoceanacoustictomographytechniqueisofpotentialtolong term ,largescalemonitoringoftheocean .Themodalwavenumbertomographymethod (Rajanetal.,1 987;Frisketal.,1 989)isusedtoobtainthesoundspeedprofileinwatercol…     

Spatial, temporal and diel variations of fish assemblages at two sandy beaches in the Sepetiba Bay, Rio de Janeiro, Brazil

Fish assemblages from two beaches, one in the inner and the other in the outer Sepetiba Bay (latitude: 22°54′–23°04′S; longitude: 43°34′–44°10′W), Southeast Brazil, were sampled by beach seine net, simultaneously, on both seasonal and diel scales, between August 1998 and June 1999. Sites were selected to encompass different environmental conditions which reflect the two bay zones, thus providing a comprehensive assessment of the factors influencing surf zone fish assemblages, and their spatial, seasonal and diel variations. A total of 55 fish species was recorded, mostly young-of-the-year. Anchoa tricolor, Micropogonias furnieri, Gerres aprion, Diapterus rhombeus, Harengula clupeola, Atherinella brasiliensis and Mugil liza were numerically dominant and contributed to 95.2% of the total fish catches. Strong differences in fish assemblages were observed between the two areas, with higher number of species in the outer bay. Increases in fish numbers occurred in winter, while the highest biomass occurred in winter and summer. Transparency, followed by salinity, was responsible for most of the spatial variability and played an important role in structuring fish assemblages. Overall, diel patterns did not reveal any significant trends; however, if we consider each season separately, an increase in fish numbers during the day with peak at sunset was observed in winter, and a higher biomass occurred at night in winter and summer. Species preferences for various combinations of environmental variables are responsible for shifts in the structure and overall abundance of assemblages and dictated some patterns. The sciaenid M. furnieri, the second most abundant species, occurred only in the inner zone, being more abundant in winter. The species of Engraulidae were more abundant in the outer zone in winter/spring during the day. The gerreids G. aprion and D. rhombeus occurred mainly in summer. Overall, temporal fluctuations act more at a specific level than at a structural one, and may be linked to some particular stages of the fish life cycle, but do not significantly influence the spatial organization.     

First results from the North Iceland experiment

Between June 2004 and September 2004 a temporary seismic network was installed on the northern insular shelf of Iceland and onshore in north Iceland. The seismic setup aimed at resolving the subsurface structure and, thus, the geodynamical transition from Icelandic crust to typical oceanic crust along Kolbeinsey Ridge. The experiment recorded about 1,000 earthquakes. The region encloses the Tjörnes Fracture Zone containing the Husavik–Flatey strike-slip fault and the extensional seismic Grimsey Lineament. Most of the seismicity occurs in swarms offshore. Preliminary results reveal typical mid-ocean crust north of Grimsey and a heterogeneous structure with major velocity anomalies along the seismic lineaments and north–south trending subsurface features. Complementary bathymetric mapping highlight numerous extrusion features along the Grimsey Lineament and Kolbeinsey Ridge. The seismic dataset promises to deliver new insights into the tectonic framework for earthquakes in an extensional transform zone along the global mid-ocean ridge system.     

A New Approach to High-Order Boussinesq-Type Equations with Ambient Currents

A new approach to high-order Boussinesq-type equations with ambient currents is presented. The current velocity is assumed to be uniform over depth and of the same magnitude as the shallow water wave celerity. The wave velocity field is expressed in terms of the horizontal and vertical wave velocity components at an arbitrary water depth level. Linear operators are introduced to improve the accuracy of the kinematic condition at the sea bottom. The dynamic and kinematic conditions at the free surface are expressed in terms of wave velocity variables defined directly on the free surface. The new equations provide high accuracy of linear properties as well as nonlinear properties from shallow to deep water, and extend the applicable range of relative water depth in the case of opposing currents.     

Waveform Shaping of Sonar Transducers for Improving the Vertical Resolution in Sub-bottom Sediments Profiling

Vertical resolution is of fundamental importance in sonar exploration and is directly related to the duration of the acoustic pulse generated by the transducer. The shorter the radiated pulse, the higher the vertical resolution. Many sub-bottom profiling sonar systems use piezoelectric transducers because they are reversible and well understood. Piezoelectric projectors are normally resonant transducers, which are intrinsically narrowband. A piezoelectric transducer is usually driven by a tone-burst. However, it is possible to use Fourier techniques to find a pre-compensated electrical driving function so that the transducer radiates a prescribed wider band acoustic waveform. This technique can be applied to synthesize zero-phase cosine-magnitude, Gaussian, and bionic pulses, with a conventional sandwich transducer. Zero-phase cosine-magnitude waveforms provide minimum length pulses (and therefore maximum resolution) within a prescribed frequency band.The aim of this paper is to illustrate the synthesis of wideband acoustic pulses using an underwater piezoelectric projector. The conventional acoustic waveform radiated when a Tonpiltz transducer is transiently excited using a “click” and allows its frequency response function to be measured. This function is used to design the electrical signal which then drives the transducer so that it radiates the shortest pulse compatible with its mechanical response. The significant resolution enhancement of the waveform shaping process is illustrated by its application to a sediment wedge model.     

Measuring and modelling temporal trends of 226Ra in waters of a Spanish estuary affected by the phosphate industry

The presence and temporal evolution (1990-2001) of (226)Ra in a tidal estuary affected by the phosphate industry has been investigated. Water samples collected in the course of four separate sampling campaigns were analysed for (226)Ra content using a gas flow proportional counter following Ba coprecipitation. Two (226)Ra sources have been identified: direct discharges from the industrial complex and run-off from a phosphogypsum pile. Although activity levels are similar, or even higher, than those found in other environments affected by the phosphate industry, there has been a general decrease in contamination since direct discharges ceased in 1998 due to new regulations from the EU. However, sediments are now acting as a source of Ra to the water column due to redissolution processes. A numerical model of the estuary has been developed to describe quantitatively the experimental results. The model solves the hydrodynamics and the dispersion equation of (226)Ra including interactions with sediments. Model results are, in general, in good agreement with observations.     

Fuzzy Fatigue Reliability Analysis of Offshore Platforms in Ice-Infested Waters

1 .IntroductionTheBohaiGulfisinfestedbyiceinthewholewinter,andoffshorestructuresthereexperiencenotonlywind ,waves ,currentsandearthquake ,butalsoiceloadswhicharecomplicatedbecauseofthedynamicinteractionbetweenicesheetsandstructures.Duanetal.( 2 0 0 0 )hadacomprehensivere viewofthelatestadvancesonice inducedvibration ,fatigueandfractureofoffshorestructures ,payingmuchattentiontothedifferencesinicebreakingmodeanddynamicsofstructuresindifferentice in festedwaters .Theypresentedcriticallythelates…