Anne Lebourges-Dhaussy

A whirling ecosystem in the equatorial Atlantic

[1] The equatorial Pacific and Atlantic oceans exhibit remarkable meridional undulations in temperature and chlorophyll fronts visible from space over thousands of kilometers and often referred to as tropical instability waves. Here, we present new observations of an ecosystem ranging through three trophic levels: phytoplankton, zooplankton and small pelagic fish whirling within a tropical vortex of the Atlantic ocean and associated with such undulations. Cold, nutrient and biologically rich equatorial waters are advected northward and downward to form sharp fronts visible in all tracers and trophic levels. The equatorward recirculation experiences upwelling at depth, with the pycnocline and ecosystem progressively moving toward the surface to reconnect with the equatorial water mass. The observations thus indicate that it is a fully three-dimensional circulation that dominates the distribution of physical and biological tracers in the presence of tropical instabilities and maintains the cusp-like shapes of temperature and chlorophyll observed from space.

Vinciguerria nimbaria (micronekton), environment and tuna: their relationships in the Eastern Tropical Atlantic

Abstract Micronekton is a major component of oceanic tuna diet. Within micronekton species, Vinciguerria nimbaria, was found to constitute the main forage fish for tuna in the [10–20° W, 0–5° N] area where a large seasonal tuna fishery occurs. The relationships linking the Vinciguerria abundance, its spatial distribution and behaviour to its dynamical or biological environment, were investigated as part of the Picolo program, devoted to the study of the mechanisms leading to the high seasonal tuna concentration in that area. During the Picolo 1 cruise, in January–February 1997, the 1° S–4° N transect was sailed nine times back and forth along 15° W. Micronekton and Vinciguerria were acoustically surveyed, hydrological conditions sampled, phytoplankton and zooplankton biomasses measured. South of 0° 30’ N, a marked divergence (upwelling) was found, with high abundance of zooplankton and micronekton. From there to 4° N a stable situation occurred with a well mixed surface layer, a strong Deep Chlorophyll Maximum (DCM) but less zooplankton and micronekton. While most micronekton performed large diel vertical migrations, schools of Vinciguerria remained at the surface by day in the stable zone, therefore becoming vulnerable to tuna in contrast to the upwelling area. It is concluded that Vinciguerria fits its behaviour according to zooplankton abundance, having to spend more time in the surface layer for feeding in poor areas. More precisely, they remain during the day near the strong DCM where they find aggregated zooplankton, and they become available for tuna. This could explain why a rather poor area may hold and sustain a high biomass of tuna.

Spatial and temporal variability of zooplankton off New Caledonia (Southwestern Pacific) from acoustics and net measurements

Spatial and temporal distribution of zooplankton off New Caledonia in the eastern Coral Sea was studied during two multidisciplinary cruises in 2011, during the cool and the hot seasons. Acoustic measurements of zooplankton were made using a shipborne acoustic Doppler current profiler (S-ADCP), a scientific echosounder and a Tracor acoustic profiling system (TAPS). Relative backscatter from ADCP was converted to biomass estimates using zooplankton weights from net-samples collected during the cruises. Zooplankton biomass was estimated using four methods: weighing, digital imaging (ZooScan), ADCP and TAPS. Significant correlations were found between the different biomass estimators and between the back-scatters of the ADCP and the echosounder. There was a consistent diel pattern in ADCP derived biomass and echosounder backscatter resulting from the diel vertical migration (DVM) of zooplankton. Higher DVM amplitudes were associated with higher abundance of small zooplankton and cold waters to the south of the study area, while lower DVM amplitudes in the north were associated with warmer waters and higher abundance of large organisms. Zooplankton was largely dominated by copepods (71–73%) among which calanoids prevailed (40–42%), with Paracalanus spp. as the dominant species (16–17%). Overall, zooplank-ton exhibited low abundance and biomass (mean night dry biomass of 4.7 6 2.2 mg m 3 during the cool season and 2.4 6 0.4 mg m 3 during the hot season) but high richness and diversity (Shannon index ∼4). Substantially enhanced biomass and abundance appeared to be episodically associated with mesoscale features contributing to shape a rather patchy zooplankton distribution.

A new multifrequency acoustic method for the discrimination of biotic components in pelagic ecosystems: Application in a high diversity tropical ecosystem off Northeast Brazil

Underwater acoustics have an unrealized potential for multicomponent ecosystem characterization. A variety of methods (e.g., backscattering differences, clustering, Gaussian model) are currently used for multifrequency classification. In this study we propose and implement a new method based on the distribution of scatters on multifrequency spatial planes. Our approach is based on both the sum and the difference of backscattered energy of up to five bi-frequency pairs. This method was developed using four frequencies (38, 70, 120 and 200 kHz) data collected in the frame of the ABRAÇOS (Acoustics along the BRAzilian COaSt) project, around Archipelago of Fernando de Noronha and Atoll das Rocas; an area characterized by low productivity but high biodiversity. By applying the method we could discriminated six groups: Fish like, two types of High Resonant at 38 kHz associated to gelatinous, Fluid Like associated to crustacean macrozooplankton, High Resonant at 70 kHz associated to pelagic algae, and a group of Unclassified echoes. The algorithm was validated using in situ sampling performed by mesopelagic trawl and zooplankton nets. Results are coherent in terms of distribution pattern of each group allows for a 3D representation of organisms distribution around the oceanic islands. Among other, we reveal the importance of gelatinous are the dominant group in the vicinity of the islands where they form a dense layer above the thermocline. These results open new perspectives to improve knowledge on the patterns of distribution and the interaction of a variety of functional groups in tropical and other systems.