High-frequency near-bottom acoustic reflection signatures of hydrocarbon seeps on the Northern Gulf of Mexico continental slope

 Acoustic reflection signatures of four hydro-carbon seeps were classified using near-bottom 25-kHz echosounder profiles. Echo patterns were compared with ground-truth data obtained by submersible observations and shallow coring. Six echo types were distinguished: strong reflections from (1) exposed or (2) buried hard substrates, such as authigenic carbonate or gas hydrate; acoustic scattering in (3) unlayered or (4) layered sediments owing to gas, shells, or disseminated carbonates; (5) attenuation caused by gas; and (6) undisturbed sediments. Echo type distributions suggest that high spatial variability indicates a younger, vigorous seep, whereas extensive hard substrate implies an older, encrusted seep. Received: 29 May 1998 / Revision received: 7 October 1998     

Particle trajectories of nonlinear gravity waves in deep water

The paper presents a numerical method for calculating the particle trajectories of nonlinear gravity waves in deep water. Particle trajectories, mass-transport velocity and Lagrangian wave period can be accurately determined by the proposed method. The high success rate of the proposed method is examined by comparing the present results with those of (a) Longuet-Higgins, M.S., 1986, 1987. Eulerian and Lagrangian aspects of surface waves. Journal of Fluid Mechanics 173, 683-707 and (b) Lagrangian moments and mass transport in Stokes waves. Journal of Fluid Mechanics 179, 547-555. It is shown that the dimensionless mass-transport velocity can exceed 10% for large waves, and the Lagrangian wave period is much larger than the Eulerian wave period for large waves.     

Review of recent findings on the water masses and circulation in the East Sea (Sea of Japan)

Recent findings on water masses, biogeochemical tracers, deep currents and basin-scale circulation in the East/Japan Sea, and numerical modeling of its circulation are reviewed. Warming continues up to 2007 despite an episode of bottom water formation in the winter of 2000–2001. Water masses have definitely changed since the 1970s and further changes are expected due to the continuation of warming. Accumulation of current data in deep waters of the East/Japan Sea reveals that the circulation in the East/Japan Sea is primarily cyclonic with sub-basin scale cyclonic and anticyclonic cells in the Ulleung Basin (Tsushima Basin). Our understanding of the circulation of intermediate water masses has been deepened through high-resolution numerical studies, and the implementation of data assimilation has had initial success. However, the East/Japan Sea is unique in terms of the fine vertical structures of physical and biogeochemical properties of cold water mass measured at the highest precision and their rapid change with the global warming, so that full understanding of the structures and their change requires in-depth process studies with continuous monitoring programs.     

Characteristics of bottom dissolved oxygen in Long Island Sound,New York

The variability of bottom dissolved oxygen (DO) in Long Island Sound, New York, is examined using water quality monitoring data collected by the Connecticut Department of Environmental Protection from 1995 to 2004. Self-organizing map analysis indicates that hypoxia always occurs in the Narrows during summer and less frequently in the Western and the Central Basins. The primary factor controlling the bottom DO, changes spatially and temporally. For non-summer seasons, the levels of bottom DO are strongly associated with water temperature, which means DO availability is primarily driven by solubility. During summer, stratification intensifies under weak wind conditions and bottom DO starts to decrease and deviate from the saturation level except for stations in the Eastern Basin. For the westernmost and shallow (<15 m) stations, bottom DO is correlated with the density stratification (represented by difference between surface and bottom density). In contrast, at deep stations (>20 m), the relationship between oxygen depletion and stratification is not significant. For stations located west of the Central Basin, bottom DO continues to decrease during summer until it reaches its minimum when bottom temperature is around 19–20 °C. In most cases the recovery to saturation levels at the beginning of fall is fast, but not necessarily associated with increased wind mixing. Therefore, we propose that the DO recovery may be a manifestation of either the reduced microbial activity combined with the depletion of organic matter or horizontal exchange. Hypoxic volume is weakly correlated to the summer wind speed, spring total nitrogen, spring chlorophyll a, and maximum river discharge. When all variables are combined in a multiple regression, the coefficient of determination (r²) is 0.92. Surprisingly, the weakest variable is the total nitrogen, because when it is excluded the coefficient r² only drops to 0.84. Spring bloom seems to be an important source of organic carbon pool and biological uptake of oxygen plays a more crucial role in the seasonal evolution of bottom DO than previously thought. Our results indicate that the reassessment phase of the Long Island Sound Total Maximum Daily Load policy on nitrogen loading will most likely fail, because it ignores the contributions of the spring organic carbon pool and river discharge. Also, it is questionable whether the goal of 58.5% anthropogenic nitrogen load reduction is enough.     

Sampling the vertical particle flux in the upper water column using a large diameter free-drifting NetTrap adapted to an Indented Rotating Sphere sediment trap

Further development of the large, surface-tethered sediment trap (NetTrap) employed as part of the MedFlux program is described whereby the large collection capacity of the NetTrap is combined with an Indented Rotating Sphere/Sample Carousel (IRSC) sediment trap (IRSC–NT). This trap is capable of collecting particle flux either in a time series or settling velocity mode; settling velocity mode allows the collection of particles that fall within discrete settling velocity intervals. During short field deployments in the Mediterranean Sea the IRSC–NT configured in the settling velocity mode successfully collected unpoisoned samples for chemical and microbiological experiments. In addition to the development of the IRSC–NT, particle-settling behavior above and below the swimmer-excluding IRS valve was tested during on-deck experiments using a specially constructed water-tight trap. Chemical analyses of settling materials (published elsewhere) suggested that separation of particles by settling velocity was achieved. However, due to the motion of the ship, it was not possible to directly measure particle-settling velocities within the trap. Particle release from the IRS did not bias the apparent settling velocity spectrum. Rotation of the IRS did not engender turbulence at the surface of the sphere or within the skewed funnel below. Tests of different ball designs over the course of the MedFlux program showed that a “ridge and saddle” pattern was optimal for efficiently transferring particles under the IRS seal while still reducing swimmer entrance to the collection funnel. The large size of the IRSC–NT did not prevent it from drifting effectively with the current. Several modifications of the present design are proposed that should improve the accuracy of the settling velocity measurements.     

Implications of mycosporine-like amino acid and antioxidant defenses in UV-B radiation tolerance for the algae species Ptercladiella capillacea and Gelidium amansii

Ultraviolet-B (UV-B) radiation (0.5, 1.0, 1.5, and 3.0 W m^?2) induced higher H₂O₂ production and lipid peroxidation in alga Gelidium amansii inhabiting in lower subtidal regions than upper subtidal alga Ptercladiella capillacea. Compared to G. amansii, mycosporine-like amino acid (MAA) concentration in P. capillacea was higher and can be increased by 0.5–1.0 W m^?2 UV-B, while carotenoid concentration was lower but also increased by 1.5–3.0 W m^?2 UV-B. UV-B increased ascorbate concentration, but to a higher degree in P. capillacea. UV-B decreased glutathione concentration, but to a higher degree in G. amansii. UV-B increased ascorbate peroxidase (APX) and glutathione reductase (GR) activities in P. capillacea but decreased them in G. amansii. UV-B increased superoxide dismutase and catalase activities, but to a higher degree in G. amansii. So, G. amansii suffered greater oxidative stress from UV-B radiation. P. capillacea can effectively reduce UV-B sensitivity by increasing sunscreen ability and antioxidant defense capacity.     

Adaptive Surface Interference Suppression for Matched-Mode Source Localization

Localizing a quiet submerged target in the presence of loud interfering surface ships is an important problem for matched-field processing (MFP) in shallow water. Typically, a data-driven interference suppression scheme is employed which requires neither prior information of the interferer's location nor filter design optimization and iterative estimation. However, the target and the interferers are usually in motion resulting in spreading or mixing of signal energies in their subspaces, thus making it difficult to determine the interference subspace dimension. In this paper, we exploit the difference in modal amplitudes for surface and submerged sources by eigenanalysis of the modal cross-spectral density matrix (CSDM). Simulation and experimental data results show that the interference subspace can be estimated adaptively and the beam output for the target is enhanced.      

Composition of the phytoplankton pigments in the surface waters along a transect from the Northwestern to the southeastern Pacific Ocean

Multidisciplinary oceanic investigation was undertaken in Aug–Sep. 2003 along a transect from Northwestern (Busan, Korea) to Southeastern Pacific (Talcahuano, Chile) to understand the physical, chemical and biological features in the surface water, and to depict their interaction with the atmosphere. Among the twenty parameters measured, we describe the physical, chemical and biological features. Physico-chemical data were analyzed in conjunction with the geographic position and yielded 7 peculiar surface water masses. The first water mass (28.4°N, 130.8°E to 21.5°N, 139.5°E) was warm and low in phosphate and nitrate content, and high in silicate. The concentration of phytoplankton pigment was one of the lowest. The second (20.4°N, 140.7°E to 2.2°S, 162.9°E) was the warmest and the least saline. Nitrate and phosphate concentration were one of the lowest. Chlorophyll a (Chl a) concentration was the lowest among the surface waters. The third (3.4°S, 164.0°E to 14.5°S, 173.3°E) was warm. Nitrate concentration was the lowest. CHL-a, peridinin (Perid), violaxanthin (Viola), zeaxanthin (Zea), chlorophyll-b (Chl b) and β-CAR were abundant. The fourth (18.6°S, 177.5°E to 31.8°S, 123.9°W) was saline and poor in nutrient concentration. The contributions of 19′-butanoyloxyfucoxanthin (But-fuco), 19′-hexanoyloxyfucoxanthin (Hex-fuco), and CHL b to CHL a were non-negligible. The fifth (32.4°S, 122.1°W to 33.8°S, 117.2°W) was relatively cold and well oxygenated. Concentration of Fuco, But-fuco, Hex-fuco and Chl b was high. The sixth (34.2°S, 115.4°W to 37.4°S, 92.1°W) was cold, well oxygenated and enriched with phosphate and nitrate. Concentration of phytoplankton pigment was, however, one of the lowest. The seventh, located off the Chilean coast, from 37.2°S, 87.2°W to 36.1°S, 74.1°W was well oxygenated and highly enriched with nitrate and phosphate. Phytoplankton pigments such as Fuco, Perid, But-fico, and Hex-fuco were rich. The 7 surface water masses are partially attributed to Kuroshio Current, North Equatorial Current and North Equatorial Countercurrent, South Equatorial current, South Pacific Subtropical Gyre, South Pacific Current, Subtropical Front and Chilean coastal water. The differences in physicochemical characteristics and the history of the surface water resulted in difference in quantity and composition of the phytoplankton pigment.     

Interaction of water waves with vertical cylinders using null-field integral equations

The scattering of water waves by bottom-mounted vertical circular cylinders is solved by using the null-field integral equations in conjunction with degenerate kernels and Fourier series to avoid calculating the Cauchy and Hadamard principal values. In the implementation, the null-field point can be exactly located on the real boundary owing to the introduction of degenerate kernels for fundamental solutions. An adaptive observer system of polar coordinates is considered to fully employ the properties of degenerate kernels. For the hypersingular equation, vector decomposition for the radial and tangential gradients is carefully considered. This method can be seen as a semi-analytical approach since errors attribute from the truncation of Fourier series. Neither hypersingularity in the Burton and Miller approach nor the CHIEF concept was required to deal with the problem of irregular frequencies. Five advantages of free of calculating principal value, well-posed algebraic system, convergence rate of exponential order, meshfree and elimination of boundary-layer effect, are achieved by using the present approach. Numerical results are given for the forces and free-surface elevation around the circular boundaries. Also, the near-trapped behavior arisen from the physical resonance is detected. A general-purpose program for water wave impinging several circular cylinders with arbitrary number, radii, and positions was developed. Several examples of water wave structure interaction by vertical circular cylinders were demonstrated to see the validity of the present formulation.