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While Trichodesmium erythraeum is prevalent in the semi-closed SW lagoon of New Caledonia, T. thiebautii is dominant in the more open SE lagoon. This led to the comparison of the two species from the results of two high-frequency surveys focused on T. erythraeum (Rodier and Le Borgne, 2008) and T. thiebautii (the present paper). (1) Environmental conditions and triggers of the blooms are the same: calm weather and/or temperature >26 degrees C, and temporary nutrient inputs are required for both; (2) growth rates under favorable conditions are similar (0.14-0.27 d(-1)) but (3) T. thiebautii has lower net ascent rates along the water column during blooming events, due to distinct buoyancy capacities and the resulting vertical distributions; (4) carbon and dinitrogen fixation rates are not significantly different and contribution of Trichodesmium spp. represent less than 35% and 5% of the total phytoplankton carbon and nitrogen requirements, respectively. It is concluded the two species can be mixed in ecological studies, except for their vertical distributions during the bloom periods.

The horizontal and vertical movements of large bigeye tuna (#Thunnus obesus$ Lowe, 1839 ; 25 to 50 kg) captured in the south Pacific Ocean (French Polynesia) were determined using pressure-sensitive ultrasonic transmitters. Bigeye tuna swam within the first 100 m below the surface during the night-time and at depths between 400 and 500 m during the daytime. The fish exhibited clear relationships with the sound scattering layer (SSL). They followed its vertical movements at dawn and dusk, and were probably foraging on the organisms of the SSL. Bigeye tuna did, however, make regular rapid upward vertical excursions into the warm surface layer, most probably in order to regulate body temperature and, perhaps, to compensate for an accumulated oxygen debt (i.e. to metabolize lactate). The characteristics of these dives differ from those reported from previous studies on smaller bigeye tuna (approximately 12 kg) near the main Hawaiian Islands. During the daytime, the large fish in French Polynesia made upward excursions approximately only every 2.5 h, whereas smaller fish in Hawaiian waters made upward excursions approximately every hour. Our data are the first observations on the role of body size in the vertical behavior of bigeye tuna. (Résumé d'auteur)

High resolution hydrographic observations of temperature and salinity are used to analyse the subsurface circulation along the coast of North Brazil, off the Amazon mouth, between 2° S and 6° N. Observations are presented from four cruises carried out in different periods of the year (March–May 1995, May–June 1999, July–August 2001 and October–November 1997). Numerical model outputs complement the results of the shipboard measurements, and are used to complete the descriptions of mesoscale circulation. The Salinity Maximum Waters are here analyzed, principally in order to describe the penetration of waters originating in the Southern Hemisphere toward the Northern Hemisphere through the North Brazil Current (NBC)/North Brazil Undercurrent (NBUC). Our results show that, if the Equatorial Undercurrent (EUC) is fed by Northern Atlantic Waters, this contribution may only occur in the ocean interior, east of the western boundary around 100 m depth. Modeling results indicate a southward penetration of the Western Boundary Undercurrent (WBUC) below the thermocline, along the North Brazilian coast into the EUC or the North Equatorial Undercurrent (NEUC) (around 48° W–3° N). The WBUC in the region does not flow more south than 3° N. The northern waters are diverted eastward either by the NBC retroflection or by the northern edge of the associated clockwise rings. The existence of subsurface mesoscale rings associated to the NBC retroflection is evidenced, without any signature in the surface layer, so confirming earlier numerical model outputs. These subsurface anticyclones, linked to the NBC/NBUC retroflection into the North Equatorial Undercurrent and the EUC, contribute to the transport of South Atlantic high salinity water into the Northern Hemisphere.

Les déplacements verticaux diurnes de trois albacores, observés à partir de marquages acoustiques réalisés dans l'Océan Indien occidental (archipel des Comores), sont mis en parallèle avec la répartion verticale de la température et de la concentration en oxygène dissous. Deux types de comportements, albacores associés à un Dispositif de Concentration de Poisson (DCP) et albacores non associés à un DCP, peuvent être distingués par les relations entre les profondeurs de nage et la structure verticale de la température et de l'oxygène dissous (épaisseur de la couche homogène, de la thermocline et de l'oxycline) observées dans la zone des marquages. Les gradients de température et de la concentration en oxygène dissous se révèlent être les facteurs essentiels qui déterminent les déplacements verticaux des albacores. Un examen des données océanographiques collectées dans l'ensemble de l'Océan Indien tropical occidental (10°N-20°S, 40°E-80°E) révèle que la profondeur d'immersion de l'oxycline correspond très largement à celle où des valeurs de concentration en oxygène dissous de 4,2 à 4,3 ml/l sont observées. (D'après résumé d'auteur)

Modifications of the intertropical thermal structure along the 165°E meridian linked to the 1986-87 ENSO are described and analyzed from results of a series of cruises carried out by ORSTOM from Nouméa, and additional data from the US-PRC western Pacific program cruises. During the evolution of the ENSO event, changes in the vertical distribution of temperature are found to be restricted mostly to the upper 4OO m. Therefore, temperature anomalies and vertical displacemnts of isotherms can be computed in relation to a climatology deduced from XBT data available in the region. Warning and deepening of surface layers are apparent before the onset of the ENSO event, more important in the southern hemisphere warm pool; then, a cooling of thermocline layers appears close to the equator at the beginning of 1987. The corresponding equatorial shaling of isotherms extends progressively poleward in both hemispheres to 10° of latitude, while reaching sub-thermocline layers.

In the deep-sea part of the Gulf of Guinea, zooplankton dry weights (D.W.) are negatively correlated with the top of the nitrate gradient (DNO3) or the shallowest undersaturated oxygen level (Dax). This is the consequence of a two-fold relation: the integrated chlorophyll, Chlai, vs. DNo3 or D, relation in typical tropical structures (i.e. with no nitrate in the mixed layer) and the linear relation between D.W. and Chl ai for offshore oligotrophic areas and the equatorial divergence as well. However this D.W.-Chl ai relation is not valid any more in the Mauritanian coastal upwelling. This point is discussed together with that of the respective vertical distributions of zooplankton and chlorophyll and the limits of the studied relations.

Sea surface chlorophyll concentrations (SSCC) (3369) have been measured from January 1978 to December 1982 in the area 14 to 32°S. 160 to 175°E, on samples taken by voluntary observing ships. A log-transformation is first applied to the data to stabilize the variance; a function of time and latitude is obtained which accounts for 16% of the variance of the logs. It is then subtracted from the logs, resulting in a field with a zero mean and a constant and uncorrelated error, in which the longitude effect is small and will be neglected. The data are distributed into a one-month, two-degree latitude grid; the subgrid noise variance is 0.472 and corresponds to a variation coefficient of 78% after return to the antilog. Objective analysis is applied to obtain optimal estimates of the SSCC in the limits of the study. The most striking feature is an enrichment in austral winter, developing northward to about 20 to 22°S, with SSCC in the range 0.15 to 0.40 mg m−3. A positive correlation between SSCC and integrated (0 to 150 m) chlorophyll (log transformation, r = 0.714, n = 150) is found in the same region using existing data from oceanographic cruises; the results from SSCC data can thus cautiously be applied to the whole euphotic layer. The data from oceanographic cruises support the hypothesis that the winter enrichment results from an enhanced vertical mixing after the cooling of the sea surface. This is confirmed by the direct relationship between the frequency of strong winds and the importance of the winter enrichment. There is some indication that the combination of available light and mixed-layer depth is limiting south of 28°S in June and July, resulting in two maxima in May and August. The wind effect is inverse in the northern part where Trichodesmium spp. patches form during the calms; these patches are only surface features and are not indicative of a biomass increase at deeper levels. The interannual variability is reflected by the ratio of the range of the standard year to that of the monthly anomalies which varies from 0.5 at 15°S to 1.11 at 29°S.