Gisèle Champalbert

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.

Can tropical freshwater zooplankton graze efficiently on cyanobacteria

Zooplankton may at times graze cyanobacteria. However, their top-down effects are considered to be low, particularly in tropical regions dominated by small-size grazers that may be unable to consume efficiently filamentous or colonial species. Recently, cyanobacteria blooms were reported in the Senegal River hydrosystem. We conducted feeding experiments to assess the ability of copepods (Pseudodiaptomus hessei and Mesocyclops ogunnus), cladocerans (Moina micrura and Ceriodaphnia cornuta), and rotifers (Brachionus angularis, B. falcatus, and Keratella sp.) to control different cyanobacteria (Cylindrospermopsis raciborskii, Anabaena solitaria, A. flos-aquae, and Microcystis aeruginosa). None of the zooplankton species ingested M. aeruginosa. Mesocyclops ogunnus did not consume any of the cyanobacteria. Both cladocerans consumed the smallest filaments of cyanobacteria, whereas all the rotifers and P. hessei consumed a broader food-size spectrum. The functional feeding responses suggest that the concentration and size of the filaments are not the sole criteria for food consumption. The high zooplankton community grazing rates, estimated by applying the clearance rates measured in the laboratory to the in situ zooplankton abundance, indicate that grazing by zooplankton potentially constitutes an important controlling factor for the filamentous cyanobacteria in the tropics.

Ontogenic variations in the phototaxis of larval and juvenile sole Solea solea L.

Abstract Ontogenic variations in phototaxis and photokinesis of larval and juvenile Solea solea L. were studied in horizontal tanks. Short-duration experiments were carried out under artificial white light on dark- and light-adapted animals. The results were compared with those obtained in 24-h experiments under continuous natural illumination. After 4–5-day period of neutrality, young larvae became photopositive in the range of 10 −3 to 10 μW·cm −2 . Associated with the increased in ocular sensitivity, phototaxis was strongly negative at μ W·cm −2 , but only in short-duration experiments; in 24-h experiments, phototaxis always remained positive at diurnal, crepuscular and nocturnal irradiance. At metamorphosis, phototaxis became positive, whatever the intensity, in both short and 24-h experiments; this photopositivity persisted until juveniles reached a length of 30 mm. In 24-h experiments older juveniles were photonegative at both diurnal and nocturnal irradiance, although a slight photopositivity appeared at dusk and dawn. No phototactic circadian rhythm was demonstrated in larvae and juveniles. Kinesis was weak in very young larvae increased in 8–15-day-old larvae; it was maximum at diurnal irradiance. In metamorphosing larvae, a photoinhibition appeared at strong diurnal irradiance and kinesis became maximum at nocturnal levels of illumination. In light gradients, movements of the different stages under vertical illumination reflected the variations in phototaxis. The ecological consequences of phototactic behaviour of the young sole are discussed.

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.

La relation alimentation-fixation benthique chez les jeunes soles Solea solea L. métamorphosées. Evidences expérimentales

Abstract The effect of quality and quantity of the food supply on the settlementof newly metamorphosed soles was studied by automatically recording pelagic swimming activity in vertical tanks. Without food and under normal daylight conditions both fasting and well-fed young soles showed a constant nocturnal pelagic swimming activity and no pelagic swimming during the day. The presence of live benthic food (small polychaetes) or a large amount of living Artemia nauplii induce a rapid reduction and a complete cessation of swimming activity for several days. Offering small amounts of Artemia during the day stimulated immediate pelagic swimming of the young fish. Offering dead prey, fresh or frozen, did not consistently alter swimming behaviour. Young soles reared from eggs in a hatchery showed the same behaviour as young soles collected from the sea. The observed effect of food supply on pelagic swimming behaviour may have important consequences for the ecology of settling newly metamorphosed young soles: when a proper food supply is available on the bottom the fish will remain benthic and settle more or less permanently. When the benthic food supply is insufficient or inadequate, the nocturnal pelagic swimming behaviour of the fish will lead to transportation of the population during the night, followed by testing of the benthic food supply during the day. Thus, the newly metamorphosed fish will accumulate in areas with a proper benthic food supply.

Systme planctonique et pollution urbaine: un aspect des populations zooplanctoniques

Zooplankton populations of a marine area exposed to pollution from the sewage outfall of Marseille-Cortiou have been studied in relation to their horizontal distribution related to distance from the outfall. Samples were collected by various means, and confirmed preliminary results which had indicated that some species tend to cluster in a facies characteristic of a polluted environment. The zone in immediate proximity to the outfall is the most turbid zone; it is extremely poor in fauna but not azoic. Outside this zone and up to 500 m distance from the sewage outlet into the sea, the most euryoecious species (e.g. Acartia clausi, Oithona nana, Euterpina acutifrons, and Oikopleuridae) are frequently present. An intermediate zone then occurs, in which, together with the species mentioned above, other species quite tolerant to pollution (e.g. Clausocalanus spp., Centropages typicus, Paracalanus spp., Candacia armata and some Corycaeidae and Oncaeidae) are well represented. Outside this zone, the zooplanktonic populations show a better defined and more stable structure. Paracalanus spp. are still very abundant, together with Fritillaridae, and fish eggs and larvae. Beneath the turbid surface zone, the impact of pollution seems less marked, and an homogenous zooplankton assemblage is found whose composition is identical to that of species inhabiting unpolluted areas.

Mesozooplankton biomass and composition in the equatorial Pacific along 180

[1] Latitudinal variations in mesozooplankton biomass and composition were investigated along an equatorial transect (8� S–8� N, 180� ) in October–November 1996. This study also included intensive sampling (3-hour intervals for 48 hours) for diel variations in mesozooplankton vertical distributions at 3� S and the equator. Most of the study took place in the high-nutrient, low-chlorophyll (HNLC) area that stretched between 7� S and 5� N. Mesozooplankton latitudinal distributions were influenced by the passage of a tropical instability wave during the equatorial time series station, which brought lower mesozooplankton biomass from the northeast, contrasting with the lack of a similar effect on concentrations of phytoplankton and particulates. South of the equator, the distributions of the mesozooplankton showed variable patterns with respect to chlorophyll and surface nitrate concentrations that could be ascribed to different states of the HNLC ecosystem. Very low diel variations of mesozooplankton biomass in the 0–50 and 0–100 m depth strata, a shallow vertical distribution, and the dominance of the larger size fraction (500–2000 mm) appear to be typical of the equatorial Pacific HNLC mesozooplankton and contrast with tropical oligotrophic ecosystems. Effects of such characteristics are a low active carbon export and a continuous predatory pressure. INDEX TERMS: 4231 Oceanography: General: Equatorial oceanography; 4855 Oceanography: Biological and Chemical: Plankton; 4880 Oceanography: Biological and Chemical: Trophodynamics; 4227 Oceanography: General: Diurnal, seasonal, and annual cycles; KEYWORDS: equatorial Pacific, zooplankton, diel cycles, composition, biomass

Influence of light and feeding conditions on swimming activity rhythms of larval and juvenile turbot Scophthalmus maximus L. : An experimental study

Abstract Turbot larvae are transported towards coastal nursery areas and live in very shallow waters. Food availability is assumed to be an important factor that retains them in such areas. To study the effects of a biotic factor (food) and an abiotic factor (light) that strongly influence behavioural mechanisms, experiments were carried out on laboratory-reared animals: larvae (1 cm), post-larvae (1.2 to 2.5 cm) and early juveniles (6 to 7 cm). Three kinds of apparatus and methods were used to record variations in swimming activity: (1) a phototaxis device to study orientation reactions in horizontal tanks; (2) actographs with infrared photoelectric barriers fitted around vertical cylindrical tanks; and (3) video cameras and cylindrical tanks. Observations were performed in total darkness and under dark–light regimes. Different types and quantities of food were provided to the fish. Larvae and juveniles of turbot exhibited a positive phototaxis from 1 to 1000 μW cm−2. At intensities lower than or equal to 0.1 μW cm−2, they did not exhibit clear reactions toward or away from the light. Turbot larvae and juveniles kept in total darkness did not show a clear rhythm of activity. Under natural illumination as well as in artificial LD conditions of similar periodicity, larvae swam by day and night. Live food (Artemia nauplii or juvenile mysids) induced an immediate increase in activity or the maintenance of a high level of activity, which decreased over the following days. Recently metamorphosed turbot kept under LD conditions exhibited a clear rhythm with a nocturnal maximum. Food given at night did not induce swimming changes as long as food density remained low. At higher prey concentrations, increased activity during feeding was followed by reduced activity for more than 24 hours. A similar response pattern was noted when active food was given in large quantities during the day: juveniles displayed an immediate increase in activity, which subsequently decreased. Regular food supply during the day induced an activity rhythm with a diurnal maximum. Despite slight differences, 6–7 cm juveniles behaved in a similar way. Our results clearly indicate the importance of trophic conditions on the swimming activity rhythms of larval and juvenile turbots and the lesser role of light conditions. They are compared with results in the literature describing the behaviour of the same species, and with juvenile sole, whose behaviour appears to be different.

Metazooplankton communities in the Ahe atoll lagoon (Tuamotu Archipelago, French Polynesia): Spatiotemporal variations and trophic relationships

Metazooplankton abundance, biomass (<80 μm, 200-500 μm and >500 μm) and community structure in the Ahe atoll were studied together with their relationships with environmental factors (temperature, salinity, wind) and trophic factors (phytoplankton, bacteria, heterotrophic nanoflagellates (HNF) and ciliates) during three periods in 2008-2009. Meroplankton, mainly bivalve and gastropod larvae, was dominant. Holoplankton was dominated by copepods, the main species being Oithona spp., Paracalanus parvus, Clausocalanus spp., Corycaeus spp., Acartia fossae and Undinula vulgaris. The results suggest a clear wind influence on the structure and horizontal distribution of the zooplankton communities. The metazooplankton appeared to be controlled mainly by food resources, suggesting a bottom-up control. The low nanophytoplankton biomass in contrast to the high abundance of picophytoplankton, HNF and nano-particle grazers (mainly Oithona spp., Paracalanus and bivalve larvae) highlighted the importance of the microbial loop in the food web.

Rheotaxis in larvae and juvenile sole (Solea solea L.): Influence of light conditions and sediment

Abstract An oval channel with an experimental section containing a sediment bed of thin sand was used to examine the effects of current on the orientation and the patterns of behaviour of larvae and juvenile sole. Behaviour of wild and laboratory-reared fish was compared. Experiments were carried out at constant temperature (12 °C) and salinity (35%.) under different light conditions and water velocities. Compared with the random distribution observed in still water, most larvae and juvenile headed into the current from low water velocities; the number of rheopositive animals increased with increasing speeds from a threshold depending on their age and whether they were in the water column or on the bottom. Larvae and small juveniles began to respond at currents of about 1 cm·s −1 ; larvae swimming in the water column began to be swept back at currents above 4 cm·s −1 , although most remained heading into the current. Near the bottom, most juveniles oriented upstream from a speed of ~ 1 cm·s −1 . Often, they swam off and just over the bottom and some buried. Juvenile sole rheotaxis persisted in the dark. Observations, confirmed by statistical analysis, showed that light conditions modify the relation between rheopositive responses and current speed. The percentage of rheopositive responses was higher in dim light or total darkness than under bright light. The implications of these observations for fish behaviour in the field are considered. The experiments indicate that the effects of current, combined with those of light and endogenous rhythmicity may control larvae and juvenile sole migrations from spawning areas to coastal areas. Nevertheless, swimming activity and orientation with respect to currents differ between day and night and are probably related to vertical, and mainly nocturnal, migrations.