Biogeochemical and ecological impacts of boundary currents in the Indian Ocean

Monsoon forcing and the unique geomorphology of the Indian Ocean basin result in complex boundary currents, which are unique in many respects. In the northern Indian Ocean, several boundary current systems reverse seasonally. For example, upwelling coincident with northward-flowing currents along the coast of Oman during the Southwest Monsoon gives rise to high productivity which also alters nutrient stoichiometry and therefore, the species composition of the resulting phytoplankton blooms. During the Northeast Monsoon most of the northern Indian Ocean boundary currents reverse and favor downwelling. Higher trophic level species have evolved behavioral responses to these seasonally changing conditions. Examples from the western Arabian Sea include vertical feeding migrations of a copepod (Calanoides carinatus) and the reproductive cycle of a large pelagic fish (Scomberomorus commerson). The impacts of these seasonal current reversals and changes in upwelling and downwelling circulations are also manifested in West Indian coastal waters, where they influence dissolved oxygen concentrations and have been implicated in massive fish kills. The winds and boundary currents reverse seasonally in the Bay of Bengal, though the associated changes in upwelling and productivity are less pronounced. Nonetheless, their effects are observed on the East Indian shelf as, for example, seasonal changes in copepod abundance and zooplankton community structure. In contrast, south of Sri Lanka seasonal reversals in the boundary currents are associated with dramatic changes in the intensity of coastal upwelling, chlorophyll concentration, and catch per unit effort of fishes. Off the coast of Java, monsoon-driven changes in the currents and upwelling strongly impact chlorophyll concentrations, seasonal vertical migrations of zooplankton, and sardine catch in Bali Strait. In the southern hemisphere the Leeuwin is a downwelling-favorable current that flows southward along western Australia, though local wind forcing can lead to transient near shore current reversals and localized coastal upwelling. The poleward direction of this eastern boundary current is unique. Due to its high kinetic energy the Leeuwin Current sheds anomalous, relatively high chlorophyll, warm-core, downwelling eddies that transport coastal diatom communities westward into open ocean waters. Variations in the Leeuwin transport and eddy generation impact many higher trophic level species including the recruitment and fate of rock lobster (Panulirus cygnus) larvae. In contrast, the transport of the Agulhas Current is very large, with sources derived from the Mozambique Channel, the East Madagascar Current and the southwest Indian Ocean sub-gyre. Dynamically, the Agulhas Current is upwelling favorable; however, the spatial distribution of prominent surface manifestations of upwelling is controlled by local wind and topographic forcing. Meanders and eddies in the Agulhas Current propagate alongshore and interact with seasonal changes in the winds and topographic features. These give rise to seasonally variable localized upwelling and downwelling circulations with commensurate changes in primary production and higher trophic level responses. Due to the strong influence of the Agulhas Current, many neritic fish species in southeast Africa coastal waters have evolved highly selective behaviors and reproductive patterns for successful retention of planktonic eggs and larvae. For example, part of the Southern African sardine (Sardinops sagax) stock undergoes a remarkable northward migration enhanced by transient cyclonic eddies in the shoreward boundary of the Agulhas Current. There is evidence from the paleoceanographic record that these currents and their biogeochemical and ecological impacts have changed significantly over glacial to interglacial timescales. These changes are explored as a means of providing insight into the potential impacts of climate change in the Indian Ocean

Maurice Godelier, L’imaginé, l’imaginaire et le symbolique

Comment l’être humain produit-il du réel ? De quelles manières l’imaginaire accède-t-il au statut de réel ? Comment l’imagination des individus est-elle en partie produite par la société ? Telles sont les questions que nous suggèrent L’imaginé, l’imaginaire et le symbolique de M. Godelier. L’enjeu porte d’abord sur les croyances individuelles et collectives : il s’agit de rendre compte de leur production tant au niveau des individus qu’au niveau d’une société. Il s’agit ensuite de reprendre e..

Structuration acoustique du micronecton de l'écosystème océanique du sud-ouest de l'océan Indien

To understand the functioning of an ecosystem, it is essential to understand the organization of its associated food web. While the distribution of primary production and top predators in the Southwest Indian Ocean's oceanic waters (10-60 ° S, 35-90 ° E) are well known, a lack of knowledge remains regarding the distribution of mid-trophic level organisms, despite their ecological (biological pump and key trophic role) and potentially economic (commercial fisheries in the near future) importance. To fill this gap, this work focused on the horizontal and vertical distribution and composition of micronektonic communities in this region. Micronekton comprises relatively small organisms (1 to 20 cm long) able to swim freely without being unduly affected by currents. Micronekton distributions have been studied at meso- and large-scale using acoustic recordings (38 kHz). A vertical structure in three layers has been identified ; the intensity and thickness of these layers vary depending on the considered time period (diel migration) and ocean conditions (meso-scale oceanic eddies and large-scale oceanic fronts). Horizontaly, the Southwestern Indian Ocean harbours the richest waters (10-800 m) in subtropical latitudes in summer and winter times. Since every organism has its own ecological characteristics, it was important to separatly describe the distribution of biological groups composing micronekton. A bi-frequency acoustic approach (38 and 120 kHz) allowed the distinction of micronektonic crustaceans from small mesopelagic gas-filled swimbladder fish in waters east of Kerguelen where micronektonic crustaceans where dominant in terms of overall acoustic density. Two distinct types of vertical structure were highlighted : an aggregative and a layer structure. Micronektonic crustaceans were characterized by layering, day and night, while other organisms (gas-filled swimbladder fish in particular) exhibited structure mixing aggregates and layers during the day, but only composed of layers during the night. In summary, the acoustic approach used in this work helped to describe the distribution and composition of micronektonic organisms and provides an essential knowledge base on the micronekton of the southwest Indian Ocean.Pour comprendre le fonctionnement d'un écosystème, il est essentiel de comprendre l'organisation du réseau trophique associé. Dans les eaux océaniques du sud-ouest de l'océan Indien (10-60°S, 35-90°E), si la répartition de la production primaire et la distribution des prédateurs supérieurs sont bien connues, un manque de connaissance demeure en ce qui concerne la distribution des différents organismes composant l'échelon trophique intermédiaire et ce, malgré leur importance écologique (pompe biologique et rôle trophique clé) et potentiellement économique (ambitions de pêche dans un futur proche). Pour combler ce manque, ce travail de thèse s'est concentré sur les distributions horizontale et verticale et la composition des organismes micronectoniques de cette région. La distribution du micronecton a donc été étudiée à méso- et large-échelle à partir d'enregistrements acoustiques (38 kHz). En termes de distribution verticale, une structuration en trois couches principales a été mise en évidence ; l'intensité et l'épaisseur de ces couches variant selon la période du jour (migrations nycthémérales) et les conditions océaniques (tourbillons océaniques à méso-échelle et fronts océaniques à large-échelle). En ce qui concerne la distribution horizontale, le sud-ouest de l'océan Indien a été caractérisé par des eaux (10 – 800 m) plus riches aux latitudes subtropicales, en été comme en hiver. Mais chaque type d'organisme ayant ses propres caractéristiques écologiques, il était important de pouvoir décrire les distributions propres à chaque groupe biologique composant ce micronecton. Une approche acoustique bifréquence (38 et 120 kHz) a permis de distinguer les crustacés micronectoniques des petits poissons mésopélagiques contenant une vessie natatoire gazeuse dans les eaux est de Kerguelen. En termes de distribution horizontale et de densité globale estimée, les crustacés micronectoniques dominaient les eaux océaniques à l'est de Kerguelen. Sur la verticale, deux types de structuration ont été mis en évidence : une structure agrégative et une structure en couches. Les crustacés micronectoniques ont été caractérisés par une structuration en couches, de jour comme de nuit, alors que les autres organismes sondés (les poissons ayant une vessie natatoire gazeuse notamment) présentaient une structuration mixte composée d'agrégats et de couches le jour et composée uniquement de couches la nuit. En résumé, l'approche acoustique utilisée dans ce travail a permis de décrire la distribution et la composition des organismes micronectoniques et fournit une base descriptive essentielle sur le micronecton du sud-ouest de l'océan Indien

Pierre-Louis Patoine, Corps/texte. Pour une théorie de la lecture empathique (Cooper, Danielewski, Frey, Palahniuk)

Au quotidien, la lecture est susceptible de provoquer des sensations et des émotions. Comment l’analyse littéraire peut-elle rendre compte d’un tel phénomène ? Pour répondre à cette question, Pierre-Louis Patoine nous invite à nous défaire de quelques habitudes de pensée : la valeur littéraire d’un texte ne provient pas uniquement de ses innovations formelles ou conceptuelles ; une œuvre appartenant au genre pornographique ou à celui de l’horreur n’est pas plus indigne d’être analysé qu’un au..

Mesoscale features and micronekton in the Mozambique Channel: an acoustic approach

International audienceThe oceanic circulation in the Mozambique Channel (MZC) is dominated by mesoscale cyclonic and anticyclonic eddies that are known to play a key role in biological processes of less-productive deep-sea ecosystems by converting physical energy into trophic energy and by restructuring and concentrating biomass across the eddy field. In this study, hydroacoustics was used to investigate the spatial distribution of micronekton according to four classes of mesoscale features and assess whether cyclonic eddies, anticyclonic eddies or eddy edges (divergence and frontal regions) impact the density of micronekton. Acoustic data were collected continuously with a Simrad EK60 split-beam echosounder during three surveys carried out in the MZC within the framework of the MESOBIO programme. First, micronekton ascents and descents during the crepuscular periods (dusk and dawn, respectively) were similar to the well-known process of diel vertical migration, with the largest changes in the shallow layer, much smaller in the deep layer, and almost non-existent in the intermediate layer. Additionally, the acoustic densities for the total water column were greater at night than during the day, suggesting that organisms migrate from layers deeper than the water column that was sampled (740 m). Second, there was evidence of differences in the acoustic responses of micronekton to mesoscale features During two of the three surveys, cyclonic eddies exhibited greater micronekton density than anticyclonic eddies for day and night. In contrast, during the last survey, the greatest micronekton density was observed in anticyclonic eddies. To explain this discrepancy, several hypotheses are proposed, including the eddy generation site and trajectory throughout the life of the eddy, eddy-eddy interactions, seasonality and difference in monsoon wind regime, the depth of influence of eddies and a low dependence of movements of larger micronektonic organisms on the mesoscale gradients. Furthermore, this study demonstrated that mesoscale features could be predicted using acoustic responses at several acoustic frequencies

Acoustic distribution of discriminated micronektonic organisms from a bi-frequency processing: the case study of eastern Kerguelen oceanic waters

International audienceDespite its ecological importance, micronekton remains one of the least investigated components of the open-ocean ecosystems. Our main goal was to characterize micronektonic organisms using bi-frequency acoustic data (38 and 120 kHz) by calibrating an algorithm tool that discriminates groups of scatterers in the top 300 m of the productive oceanic zone east of Kerguelen Islands (Indian sector of the Southern Ocean). The bi-frequency algorithm was calibrated from acoustic properties of mono-specific biological samples collected with trawls, thus allowing to discriminate three acoustic groups of micronekton: (i) “gas-bearing” (ΔSv,120-38 v,120-38 > 2 dB), and (iii) “undetermined” scatterers (-1 v,120-38 < 2 dB). The three groups likely correspond biologically to gas-filled swimbladder fish (myctophids), crustaceans (euphausiids and hyperiid amphipods), and other marine organisms potentially present in these waters and containing either lipid-filled or no inclusion (e.g. other myctophids), respectively. The Nautical Area Scattering Coefficient (NASC) was used (echo-integration cells of 10m long and 1m deep) between 30 and 300m depth as a proxy of relative biomass of acoustic targets. The distribution of NASC values showed a complex pattern according to: (i) the three acoustically-defined groups, (ii) the type of structures (patch vs. layers) and (iii) the timing of the day (day/night cycle). NASC values were higher at night than during the day. A large proportion of scatterers occurred in layers while patches, that mainly encompass gas-bearing organisms, are especially observed during daytime. This method provided an essential descriptive baseline of the spatial distribution of micronekton and a relevant approach to (i) link micronektonic group to physical parameters to define their habitats, (ii) investigate trophic interactions by combining active acoustic and top predator satellite tracking, and (iii) study the functioning of the pelagic ecosystems at various spatio-temporal scales

Acoustic micronektonic distribution is structured by macroscale oceanographic processes across 20–50°S latitudes in the South-Western Indian Ocean

International audienceMicronekton constitutes the largest unexploited marine biomass worldwide. It is one of the most conspicuous and ecologically important components of the still poorly known mesopelagic ecosystem. Acoustic data were collected from both fishing and research vessels along 18 transects for a total of 47 682 linear kilometers to investigate large-scale distribution of micronekton over a long latitudinal gradient (20–50°S) and two contrasted seasons (summer and winter) in the South-Western Indian Ocean. Acoustic backscatter at 38 kHz was used as a proxy of mid-water organisms' abundance (0–800 m depth). Two consistent features were diel vertical migration of backscatters and vertical distribution of micronekton in three distinct layers, namely the surface (SL), intermediate (IL) and deep (DL) layers. Satellite remote sensing data was used to position oceanic fronts, and hence define water masses, from the tropical to low Antarctic zones. A key finding of this study was the significant correlation observed between abundance and distribution of acoustic backscatter and position relative to these front and water masses. Total backscatter peaked in the subtropical zone, with low abundances in the colder Polar Frontal Zone. The high overall abundances in subtropical waters resulted mainly from high backscatters in the IL and DL that contrasted with low SL values, especially during the day (2–11%). The warmer the waters, the higher SL backscatter was, with the highest absolute and relative (38–51% of the total abundance) values observed at night in the Tropical Zone and the lowest abundance in the Antarctic Zone. No significant seasonal pattern was found, but SL backscatters were very low in winter compared to summer in the Polar Frontal Zone. Moreover, the Northern winter shift of the fronts induced a Northern latitudinal shift of the peak in abundance from summer to winter. The present study highlights the value of building large acoustic databases collected from both research and fishing vessels. The method provides unique opportunities to gather basic information on micronekton and is an essential step to describe oceanic zones of relevant biological interest in terms of trophic ecology

Acoustic Survey of an Estuarine Marine Protected Area and of its Close Vicinity : Analysis and Monitoring Prospective

International audienceFacing the loss of fish biodiversity and the decrease of fish stocks since several decades, a proposed solution is the multipli-cation of Marine Protected Areas (MPA). In Senegal, one MPA has been implemented in 2003 in one of the channels composing the Sine Saloum estuary. A monitoring program was carried out based on hydro-acoustic survey and the two main metrics extracted from hydro-acoustic data: Fish biomass proxy and fish size distribution proxy. The surveys were performed using vertical beaming at differ-ent seasons from 2003 to 2010. In the MPA, a light increase of the fish biomass proxy and an increase of the fish size proxy during the dry season were observed. The biomass proxy recorded in a close but not protected tributary were higher than in the MPA, probably due to a strong connection with open marine water, aggregative be-havior of preys in the MPA due to higher predation, and a stronger avoidance phenomenon in the MPA. The high variability of the MPA environment submitted to both estuarine and marine influences and the size of the MPA may explain the lack of clear results. However, the learned lessons enable us to make recommendations to monitor fish population in MPAs using hydroacoustic methods. Evaluation procedures using these methods are proposed highlighting the im-portance of a rigorous survey protocol to insure the exploitation of the entire acquired time series

Acoustic Survey of an Estuarine Marine Protected Area and of its Close Vicinity : Analysis and Monitoring Prospective

International audienceFacing the loss of fish biodiversity and the decrease of fish stocks since several decades, a proposed solution is the multipli-cation of Marine Protected Areas (MPA). In Senegal, one MPA has been implemented in 2003 in one of the channels composing the Sine Saloum estuary. A monitoring program was carried out based on hydro-acoustic survey and the two main metrics extracted from hydro-acoustic data: Fish biomass proxy and fish size distribution proxy. The surveys were performed using vertical beaming at differ-ent seasons from 2003 to 2010. In the MPA, a light increase of the fish biomass proxy and an increase of the fish size proxy during the dry season were observed. The biomass proxy recorded in a close but not protected tributary were higher than in the MPA, probably due to a strong connection with open marine water, aggregative be-havior of preys in the MPA due to higher predation, and a stronger avoidance phenomenon in the MPA. The high variability of the MPA environment submitted to both estuarine and marine influences and the size of the MPA may explain the lack of clear results. However, the learned lessons enable us to make recommendations to monitor fish population in MPAs using hydroacoustic methods. Evaluation procedures using these methods are proposed highlighting the im-portance of a rigorous survey protocol to insure the exploitation of the entire acquired time series