Front Off Research Articles

Diet of large-eye dentex Dentex macrophthalmus (Pisces: Sparidae) off Angola and Namibia

The large-eye dentex Dentex macrophthalmus is a major target species of the demersal fishery off Angola, and is taken as bycatch off northern Namibia. Its prey spectrum off Angola and Namibia is described from the stomach contents of 593 individuals collected between northern Namibia (21°S) and northern Angola (6°S) during a bottom trawl survey carried out in 1997. In all, 20 prey types were recorded in the diet, demonstrating that it is an opportunist feeder. The main food items were fish (80.9%), euphausiids (12.0%), polychaetes (4.8%), crustaceans (2.5%) and cephalopods (0.1%). The variety of prey increases with fish size, but there is no systematic effect of depth on the diet. The dominant prey types varied as a function of fish size as well as spatially, demonstrating the different ecological niches provided by the Angola Current off northern Angola, the cold Benguela Current off northern Namibia, and the Angola-Benguela Front off southern Angola.

Nature and Distribution of the Deformation Front in the Luzon Arc-Chinese Continental Margin Collision Zone at Taiwan

Marine seismic reflection profiles from offshore SW Taiwan combined with onland geological data are used to investigate the distribution and nature of the deformation front west of Taiwan. Locations of the frontal structure west of Taiwan are generally connected in a linear fashion, although the alignment of frontal structures is offset by strike-slip faults. The deformation front begins from the northern Manila Trench near 21°N and continues northward along the course of the Penghu Submarine Canyon in a nearly N–S direction north of 21°N until it reaches the upper reaches of Penghu Canyon at about 22°15′N. The deformation front then changes direction sharply to the northeast. It connects to the Chungchou thrust fault or the Tainan anticline in the coastal plain and continues northwards along the outer Western Foothills to the northern coast of Taiwan near 25°N. Characteristics of structural style, strain regime, sedimentation and tectonics vary along the trend of the deformation front. Ramp anticlines, diapiric intrusion and incipient thrust faults are commonly associated with the deformation front. Variations in structural style along strike can be related to different stages of oblique collision in Taiwan. The deformation front (collision front) west of Taiwan can be considered as a boundary between contraction in the Taiwan orogen and extension west of the collision zone. The deformation front east of the Tainan Basin and its northward extension along the outer limit of the Western Foothills is the surface trace separating the foreland thrust belt from the nearby foredeep, not a boundary between the Chinese and Taiwan margins. The submarine deformation front off SW Taiwan is the surface trace separating the submerged Taiwan orogenic wedge from the Chinese passive continental margin, not a surface trace of the plate boundary between the Eurasian and Philippine Sea plates.

Tidal Diffusivity: A Mechanism for Frontogenesis*

It is hypothesized that tidal mixing may provide a “diffusivity” mechanism for frontogenesis. It stems from the fact that tidal diffusivity varies in the opposite sense from the water depth, so the vertically integrated diffusivity may exhibit a minimum at midshelf, thus giving rise to a maximum in the property gradient—even in the absence of flow convergence. An analytical model assuming a tidal diffusivity dominated by shear dispersion is used to elucidate the mechanism, which shows additionally that the front is located at a water depth that is about twice the tidal frictional depth—a prediction not inconsistent with some observed fronts. The proposed frontogenesis is demonstrated by numerical calculations using the Princeton Ocean Model (POM), which show the emergence of a front from an initial field of uniform gradient after tides are switched on, and the diagnosis of the numerical solution and its parameter dependence has corroborated the analytical model. It is suggested moreover that this diffusivity mechanism may be extended to the wind-induced mixing to explain the shelfbreak front off of the northeastern United States.

On the dynamics of an observed thermal front off central eastern Australia

Results from a short observational program off central eastern Australia (28.4°S) are presented. A series of events involving sharp temperature fluctuations and strong pulses in the alongshore currents are measured by a mooring array over the inner shelf. With use of AVHRR images, ADCP, and CTD surveys, it is reasoned that these events correspond to the periodic advection of a thermal front past the inshore moorings. The front involves a temperature change of 1°–2°C, with a southward jet of greater than 0.5 m s−1 over a width of 1–2 km in 25 m of water. The dynamical balances of the momentum equations are examined. It is demonstrated that at the onset of each event the local tendency of alongshore velocity is balanced by across‐shore advection. Additionally, a quasi‐balance between the surface and bottom stress terms is noted throughout the experiment demonstrating the prominence of mixing over the inner shelf.

Ecology of neonate loggerhead turtles inhabiting lines of downwelling near a Gulf Stream front

Neonate sea turtles disperse from nesting beaches into the open ocean and develop during a multi- year growth period at sea, but data that characterize their behavior, feeding, and habitat during this devel- opmental period have been few. Limited information has suggested that neonate sea turtles associate with lines of floating debris and biota at areas of surface downwelling. Data from the present study come from measurements of habitat, turtle behavior, and apparent foraging preferences in areas where neonate (post- hatchling) loggerhead turtles (Caretta caretta) were ob- served and captured. Turtles were observed (n=293) and captured (n=241 of the 293 observed) in down- welling lines that had formed in the slope water near the Gulf Stream front off east-central Florida, USA. Catch- per-unit-effort averaged 12.4 turtles/h from a vessel moving at 2.5 knots. Turtles were largely inactive and were closely associated with floating material, especially pelagic species of Sargassum. Turtles captured along with samples of floating material and given a gastric- esophageal lavage showed a preference for animal ma- terial (35.5% of volume in habitat, 70.9% in lavage) over plant material (60.3% of volume in habitat, 22.5% in lavage). Ingested anthropogenic debris included tar (20% of turtles) and plastics (15% of turtles). Ingested animals were principally small (most <1 mm) and were typically slow-moving or non-motile species or stages. Ingested plants were most commonly Sargassum frag- ments or seagrasses that bore epiphytic animals. Pre- ferred or commonly ingested animals were hydroids, copepods, and pleuston such as Janthina, Creseis, Porpita ,a ndHalobates. Data support a hypothesis describing post-hatchling loggerheads as facultatively active but principally low-energy float-and-wait foragers both within and outside of downwelling lines. Pelagic dispersal of turtles may be best predicted by a ''smart'' drifter analogy wherein turtle buoyancy, surface advec- tion, and minimal oriented movement determine their distribution at sea. Conservation implications of plastic and tar ingestion are discussed.

Subtropical Shelf Front off eastern South America

Historical hydrographic data from the continental shelf off eastern South America are used to examine the thermohaline properties of the water masses in the region between 20°S and 40°S. The continental shelf water masses are originated by dilution of open ocean waters of the western boundary currents of the South Atlantic Ocean. On the basis of temperature‐salinity relation, two distinct water masses are identified, namely, the Subantarctic Shelf Water and the Subtropical Shelf Water. Subantarctic Shelf Water originates by dilution of Subantarctic Water, primarily in the southeast Pacific, due to excess precipitation and continental runoff and enters the continental shelf near 55°S. The Subtropical Shelf Water is modified South Atlantic Central Water diluted by continental runoff from the coast of Brazil. In addition, substantial dilution of the upper shelf waters takes place at the mouth of Río de la Plata (approximately located at 36°S) and, in a lesser extent, at the Patos‐Mirim Lagoon (at 32°S). The Río de la Plata and the Patos outflows form a low‐salinity tongue that caps the shelf water leading to a salinity decrease to values &lt;30. The low‐salinity tongue extends northward over the shelf penetrating farther north in winter than in summer. The extent of the low‐salinity water has a strong impact on the vertical stratification and acts to limit winter convection to the layer above the halocline. There is little or no indication of mixing between Subantarctic Shelf Water and Subtropical Shelf Water. An intense temperature, salinity, and nutrient front separates these water masses. The front is oriented along the north‐south direction, located on average near the 50 m isobath at 32°S and extends southward toward the shelf break near 36°S. Between 32° and 34°S the Subtropical Shelf Front follows the 100 to 200 m isobaths and separates Subantarctic Shelf Water from the oceanic South Atlantic Central Water. On the basis of the temperature and salinity distributions, beneath the low‐salinity surface layer, the Subtropical Shelf Front appears as an extension of the Brazil‐Malvinas Confluence over the continental shelf of South America. Thus the location of the Subtropical Shelf Front may be linked to the migrations of the separation point of the Brazil‐Malvinas Confluence from the continental slope.

南海トラフ付加体の温度構造と間隙流体による熱輸送

The thermal structure of the Nankai subduction zone has been investigated through conventional surface heat flow measurements and estimations of heat flow from the depths of gas hydrate BSRs (bottom simulating reflectors). In the middle and western parts of the Nankai Trough, heat flow on the trough floor is generally higher than that expected from the age of the subducting Shikoku Basin lithosphere, while it is consistent with the age in the eastern part. This high heat flow anomaly might be attributed to advective heat transfer by pore fluid flows from deeper parts of the accretionary prism along the decollement zone, but the amount of pore fluid expelled from the accreted sediments may not be enough to produce the observed anomaly. The deficit in the amount of pore fluid compared to the thermal anomalies is a problem common to other accretionary prisms. Local heat flow anomalies around the deformation front off Shikoku and around thrust faults off the Tokai district indicate the existence of channelized fluid flows along fault zones.A detailed heat flow survey was conducted on the Nankai prism off Shikoku in 1999. Over 60 new heat flow values were obtained along a 80km long line perpendicular to the trough axis, which greatly improved the knowledge of the surface heat flow distribution on this accetionary prism. The heat flow profile landward of the deformation front is generally smooth and seems to correlate well with the thickness of the prism. Local high heat flow anomalies were observed in the vicinity of the deformation front and may correspond to thrust faults. This excellent set of data, combined with BSR heat flow data, should be able to give better constraints for the thermal structure of the accretionary prism and the seismogenic zone.More direct evidence of heat transfer by fluid flows has been obtained in the Barbados and Cascadia accretionary prisms. In ODP drill holes within 15km of the deformation front of the Barbados prism, anomalies in temperature and pore water chemistry were detected around the decollement and thrust faults, indicating localized fluid flows. Some of the anomalies must have been produced by episodic flows. The heat flow around the deformation front in the Barbados prism considerably varies along the arc, which is similar to the observation in the Nankai Trough. A marked increase in temperature in the vicinity of a thrust fault was detected during a long-term monitoring experiment in a drill hole in the Cascadi accretionary prism, and is believed to represent an effect of an episodic flow event.

Ingestion rates of phytoplankton by copepod size fractions on a bloom associated with an off-shelf front off NW Spain

Herbivory by copepods was studied from the coast towards the ocean, during a bloom in May 1994 off NW Spain.Ingestion rates were estimated by the gut chlorophyll content method in three size fractions. The chlorophyll content displayed significant daily cycles. Three different water bodies were described: coastal, shelf break and oceanic; the latter two zones separated by a thermo haline frontal structure. Marked differences in plankton species composition, vertical distribution and biological rates were found between zones. Theiiighest phytoplankton biomass, dominated by chain- forming diatoms, occurred in the oceanic zone associated with low primary production rates. Copepod feeding had a low effect on oceanic phytoplankton; up to 0.2% of carbon stock and<3% of carbon production was consumed daily. In contrast, medium-sized and large copepods removed 3% of carbon stock and 12% of primary production daily near the coast, where phytoplankton were dominated by small flagellates in active growth. The highest variability in both plankton composition and ingestionrates was found in the shelf-break zone, probably due to displacements of the front. Copepods exerted a moderate predation pressure on phytoplankton in coastal waters. Meanwhile, the impact of copepods on the offshore bloom was negligible and the fate of the accumulated particulate carbon would be mostly determined by sedimentationand water dynamics.

Feeding in a larval fish assemblage: the nutritional significance of an estuarine plume front

Gut fullness of larval fishes was used to determine the nutritional significance of an estuarine plume front off Botany Bay, SE Australia, on three days in March/April 1990. Fishes were captured in three different water masses (estuarine plume, front and shelf water), each separated by ≃ 200 m, using a 260 μm mesh purse-seine net. Overall, the gut-fullness index (GFI) of 260 fish larvae combined from eight families (Gerreidae, Mugilidae, Mullidae, Sparidae, Blenniidae, Kyphosidae, Monodactylidae, Pomacentridae), was significantly greater in the plume and front water on two of the sampling occasions, but no difference was detected on the third occasion. Trends in GFI among families were inconsistent with respect to the front. The mugilid (Liza argentea) fed equally and abundantly in all water masses (gut fullness>90%), while the kyphosid (Kyphosus spp.) had a significantly greater GFI in the plume compared to the shelf water. In general, the response of GFI to the front varied between dates and amongst taxa. Diet analysis showed that mugilids selected for copepod nauplii within the plume and shelf water [alpha selectivity index (ASI) of (0.22 to 0.98)] and for harpacticoid copepods within the front (ASI=0.92), representing characteristic diets identified by canonical discriminant analysis. Kyphosids selected positively for copepod nauplii in all three water masses on both occasions (ASI>0.96). In general, diets of larval fish were taxon-specific, and responded variably to the three habitats. Comparison of diets across water masses was complicated by the large number of families, which rarely occurred in all water masses. The nutritional significance of an estuarine plume front varies between species of larval fish, and there are no obvious trends that can be applied to the larval fish community in general.

The large‐scale summer circulation of the California Current

Satellite data from the Geosat altimeter and the Advanced Very High Resolution Radiometer (AVHRR) are used to show the large‐scale structure of the surface circulation of the California Current System in summer. These data show the connection between an equatorward jet and temperature front off Oregon that lies within 100 km of the coast, similar to that first observed in the 1960's and 1970's, and a jet that meanders along the convoluted offshore edge of a temperature front off California, as repeatedly observed in the 1980's.

Relationship between egg productivity and RNA/DNA ratio in Paracalanus sp. in the frontal waters of the Kurshio

The relationship between RNA/DNA ratio and egg productivity of adult Paracalanus sp. females was investigated in the frontal area of the Kuroshio current off Kinkazan Island and Boso Peninsula, Pacific coast of Japan in 1989 and 1990, respectively. In the Kuroshio front off Kinkazan Island, the RNA/DNA ratio of females in the surface layer ranged from 3.87 to 6.92 along a transect 8 miles across the front. In the Kuroshio front off Boso Peninsula, the RNA/DNA ratio varied between 5.15 and 7.27 for 32 h in the course of tracking a drifter launched in the front. The RNA/DNA ratio of the females in both frontal areas tended to increase together with ambient chlorophyll-a concentration, and the relationship fitted to Ivlev's formula (P<0.05). The number of eggs laid in filtered sea-water during 24 h (egg productivity) was also positively correlated with the ambient chlorophyll a concentration (P<0.05). There was a positive correlation (P<0.001) between RNA/DNA ratio and egg productivity, and RNA/DNA ratios accounted for about 70% of variation of the egg productivity. Judging from RNA/DNA ratios and female egg productivity in both frontal areas, relatively large physiological or reproductive changes could possibly occur in small temporal and spatial scales in association with change in phytoplankton abundance.

Near‐inertial motion on the shelf‐slope front off northeast Spain

Near‐inertial motion on the shelf‐slope front off northeast Spain was monitored using surface drifters and moored current meters. On the shelf, strong inertial currents were generated by a wind burst. The inertial current amplitude was about 70 cm/s at the surface, 30 cm/s at the base of the mixed layer, and 10 cm/s in the interior. The observed near‐inertial frequency on the shelf was about 10% lower than the local inertial frequency, suggesting that the near‐inertial motion was embedded in region of strong anticyclonic shear. Also, the phase of near‐inertial motion increased through the water column, indicating that the energy propagation was downward. By contrast, the surface inertial currents were only about 10 cm/s in the center of the shelf‐slope front. Indirect evidence suggests that the observed small surface inertial currents were the result of rapid downward transfer of near‐inertial energy in the front.

Short-term effects of coastal upwelling and wind reversals on epiplanktonic cnidarians in the southern Benguela ecosystem

Patterns in the distribution of epiplanktonic cnidarians collected along the west coast of South Africa during June 1986 were examined. Upwelling-favourable south-easterly winds prevailed prior to sampling. These winds are believed to have transported coastal cnidarian assemblages offshore, resulting in a blurring of the inshore-offshore zonation of species. A wind reversal, to northerly winds in the north and north-westerly winds in the south, followed upwelling and persisted during sample collection. This resulted in the development of a distinct thermohaline front off Cape Columbine. Another consequence was the advection of oceanic water shorewards in the south, disrupting the original pattern of species zonation and redistributing cnidarian assemblages in a longshore pattern. Three different assemblages are differentiated: one north of Cape Columbine, consisting of coastal species, one south of Cape Columbine with a mixture of neritic and oceanic species, and one single-species community with coastal ...

From transtension to transpression along the northern Caribbean plate boundary off Cuba: implications for the Recent motion of the Caribbean plate

Marine geophysical surveys using Seabeam, single-channel seismic reflection, gravimetric and magnetic measurements have been conducted along a segment of the northern Caribbean transcurrent plate boundary (SEACARIB II cruise). The data allow a better definition of the geometry and the tectonic regime of this major strike-slip area. They support the following results: 1. (1) Along the southern Cuban margin, the Oriente fault displays a discontinuous trace, mainly composed of dextral offset, “en echelon” segments. Some pull-apart basins are located between fault segments (Cabo Cruz basin, Chivirico and Baitiquiri basins). In the Windward Passage area, the plate boundary enters into the Tortue Channel and is not connected with the subduction front off northern Hispaniola. 2. (2) The eastern part of the Oriente Deep and the Santiago Promontory are characterised by active compressional tectonics. They form the Santiago Deformed Belt, described here for the first time. This deformed belt can be divided longitudinally into three main segments, each one characterised by a particular tectonic style. Its development is related to a transpressional mechanism along the left-lateral Oriente strike-slip fault. Our observations suggest that a tectonic and kinematic reorganisation occurred recently in this area, probably in the Late Pliocene, which may be compared with the recent geological events recorded on land in the northern Caribbean domain. The precise knowledge of both geometry and structures along the Oriente strike-slip fault south of Cuba provides new constraints for the recent kinematic evolution along the northern Caribbean transcurrent plate boundary: it leads us to infer the existence of a convergence component associated with the slip component along the Oriente transform fault.

Coincident seismic-wave velocity and reflectivity properties of the lower crust beneath the Appalachian Front, west of Newfoundland

We have determined the seismic velocity structure of the crust in the vicinity of the Appalachian deformation front off western Newfoundland and the adjacent Gulf of St Lawrence. These measurements were made from two perpendicular wide-angle seismic refraction profiles, one of which is collinear with a previously recorded deep seismic reflection profile. The Grenville foreland crust, about 45 km thick, is characterized by velocities of 6.35 km/s in its upper part and 6.7 km/s in its lower part. Close to the coast of Newfoundland, a deep crustal reflective wedge is bounded by a northwest-dipping reflector and by the crust–mantle boundary, which is at only 39 km depth beneath the wedge. In the wedge, velocities of 7.2–7.3 km/s may indicate the presence of mafic and ultramafic rocks. We speculate that several processes could have caused the high velocities and the high reflectivity. The most attractive is perhaps crustal extension with consequent underplating during the formation of the Iapetus Ocean or during later reactivation by Carboniferous strike-slip movements.

Upwelling off the Gulfs of Panama and Papagayo in the tropical Pacific during March 1985

Satellite measurements of sea surface temperatures (SST) were made in the eastern tropical Pacific Ocean during March 1985. Unusually persistent upwelling was observed southwest of the Gulf of Panama and Gulf of Papagayo from March 7 to 22. The SST was depressed by 6° to 10°C relative to the surrounding waters. The cooler waters off Panama extended southwestward for over 1000 km and reached the Galapagos Islands. The northern SST front off Panama was wavelike in appearance, and long waves of 600‐km length moved southwestward at a phase speed of about 30 km/d. These waves resemble the long waves that appear each year along the northern boundaries of the equatorial SST fronts in the Pacific and the Atlantic oceans. The SST fronts off the Gulf of Papagayo remained relatively stationary, with a large area of colder water centered at 88°W and 10°N. During these events, the upwelling off the Gulf of Tehuantepec was relatively weak. These satellite observations provide a view of an unusual oceanic response on the leeward side of mountain pass wind jets.

Kinematics of the upwelling front off southern Africa

Mesoscale circulation features have been shown to play an important role in the cross-frontal mixing of upwelling cells, their frontal morphology and in their interaction with oceanic water masses. With three years of detailed thermal infra-red satellite information on the South-East Atlantic upwelling system available, it proved possible to present a preliminary study of four prevalent frontal features intrinsic to the short-term behaviour of upwelling in this area. Upwelling filaments are shown to extend between 50 and 600 km seawards of the main front and are found, as are upwelling plumes, predominantly off the recognized major upwelling cells. Frontal eddies have a range of diameters and are found distributed over the full area of upwelling and on both sides of the main upwelling front. Warm filaments of Agulhas Current origin are advected preferentially along the western border of the Agulhas Bank and follow closely the front of the southernmost upwelling cells, where they may play a catalytic role ...

Undulations of the Nova Scotia current

Abstract Analysis of infrared images of a surface temperature front off Nova Scotia, taken over a two‐week period, reveals horizontal oscillations with a mean wavelength of 50 km and comparable amplitudes. The oscillations travel downstream with the Nova Scotia Current at a rate of 0.06 m s−1. A comparison is made with a simple model of baroclinic instability, and it is possible that this mechanism could cause the oscillations. However, more definite tests and data are required. These oscillations could contribute significantly to the exchange of water between the inner and outer shelves.

Relationship between physical and biological processes at an upwelling front off Peru, 15°S

Continuous sampling of temperature, fluorescence and nutrients was performed as part of the JOINT II expedition of the Coastal Upwelling Ecosystem Analysis program. A 500 square kilometer region off the coast of Peru at 15°S was surveyed on 8 nights over a 4-week period in May and June 1976. In this region, a surface thermal front is repeatedly observed, temperature increasing as much as 1°C per horizontal kilometer. The surface mixed layer shoals as the front is approached from offshore. A maximum in fluorescence corresponds to maxima in productivity and biomass, and is consistently displaced on the order of a kilometer to the warm water side of the thermal front. The intensity of the displaced fluorescence maximum is positively correlated with the sharpness of the surface temperature gradient at the front. The suggested mechanisms for this feature are a combination of growth facilitated by ammonium as a nitrogen source and of distribution of the phototropic Gymnodinium splendens during night-time sampling throughout the shoaling mixed layer.

Simulation of the formation of Ushant thermal front

The formation of the Ushant thermal front off northwestern France is simulated numerically with a one-dimensional model of the vertical thermal structure of the sea. This simulation is derived from a Niiler and Kraus (1977 , Modelling and prediction of the upper layers of the ocean, pp. 143–172) mixed layer model in which bottom friction is introduced. The model inputs are the meteorological parameters measured at Ushant and the barotropic tidal currents computed with a numerical model ( Mariette et al., 1982 , Oceanologica Acta, 5, 149–159). Both very fine time steps ( Δt = 1h) and spatial steps (horizontal mesh size: Δx = Δy = 2nmi; vertical step: Δz = 1m) are used. This study is intended to be mainly concerned with the processes acting during the formation of the Ushant front. Comparison of the model results with satellite pictures and in situ measurements shows good agreement. We point out that biological blooms could occur according to a spring-neap cycle, during the thermal front formation. The prevailing effect of local processes such as tidally induced bottom friction and energy budget at the sea surface on the thermal evolution of the Mer d'Iroise is then emphasized. During the formation of the front, these effects seem to be more important than the advective processes which are not considered in this work.