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Special issueThe wrapping up of the IDEADOS project: International Workshop on Environment, Ecosystems and Demersal Resources, and Fisheries.-- 11 pages, 5 figures, 5 tables, The pelagic decapod crustacean fauna of two different zones (Sóller and Cabrera) with different hydrographic dynamics and oligotrophy levels was studied around Mallorca (western Mediterranean), the latter with a higher degree of oligotrophy than the former. Samples were taken with a Pelagic Trawl and an IKMT in the upper 600. m of the water column, targeting larger and middle-sized nektonic species, respectively. Fourteen species were collected: five dendrobranchiate shrimps, eight caridean shrimps and one scyllarid lobster. Some species were restricted to the shelf-break: Chlorotocus crassicornis and Plesionika heterocarpus. Others were exclusive of the middle slope: Pasiphaea multidentata, and Sergia robusta. Pasiphaea sivado and Gennadas elegans occurred in all pelagic strata. Multivariate analyses showed several distinct assemblages related to bathymetry and sampling depth. No significant differences were found concerning zone or sampled seasons. Bathymetrically, Deep Scattering Layers showed the highest diversity. No decapod crustaceans occurred in epipelagic daytime samples. The pelagic decapod community sampled was structured by both the geomorphology (and associated hydrographic characteristics over the shelf-break) and the influence of light in the water column. Size analysis showed species-specific patterns concerning size/age movements into the water column throughout the day-night cycle. © 2013 Elsevier B.V., Funding was provided by research projects CTM2008-04489-C03 and CTM2010-22218-C02. DSS acknowledges the pre-doctoral fellowship support by the Spanish Agency for International Development Cooperation (AECID), Ministry of Foreign Affairs and Cooperation. APT acknowledges pre-doctoral FPI Fellowship support from the Regional Government of the Balearic Islands, Conselleria d'Educació, Cultura i Universitats, selected as part of an operational program cofinanced by the Fondo Social Europeo. DSS and PA acknowledge partial support from research project CTM2010-22218-C02

22 pages, 11 figures, 7 tables.-- Data availability statement: Publicly available datasets were analyzed in this study. This data can be found here: Olivar et al. (2022) https://doi.org/10.1594/PANGAEA.947631 and Sarmiento-Lezcano et al. (2023) https://doi.org/10.1594/PANGAEA.95494, Non-migratory bristlemouth fishes (Cyclothone spp.) are the most abundant vertebrates on Earth and play an important role in the biological carbon pump by remineralizing organic carbon in deep ecosystems. Acoustic data and net sampling are often used in combination to estimate fish and zooplankton biomass, but this procedure may be subject to several sources of error when applied to mesopelagic species. For instance, the allocation of echoes to species has often been biased by not considering Cyclothone spp. due to the use of nets targeting larger fish. Furthermore, the acoustic properties of the target organisms must be well understood to convert acoustic density into numerical density. The characteristics of a fish’s swimbladder are the most relevant features necessary to assess its acoustic properties. This study provides information on the swimbladder properties of six Cyclothone species inhabiting the meso- and bathypelagic layers in the North Atlantic Ocean and Mediterranean Sea, including swimbladder location within the body, fat tissue content, morphology, morphometry (only available for C. braueri and C. pseudopallida), and fish body-mass density (only available for C. braueri, C. pseudopallida, C. pallida, and C. pygmaea). The studied species showed a functional physoclistous swimbladder, with well-developed gas glands and rete mirabile and numerous capillaries in the case of the shallower species C. braueri and C. pseudopallida (mainly distributed from 400 to 600 m depth), and a fat-invested swimbladder in species with deeper vertical distribution (C. livida, C. microdon, C. pallida, and C. pygmaea). The fat content in the swimbladder (C. pallida and C. microdon) increased with depth and latitude, reducing the space in the swimbladder that could contain gas. Changes in swimbladder size and volume during growth were analyzed for shallower species, where swimbladder volume and equivalent radius followed negative allometric growth in relation to body length. Finally, values of body-mass density (ρ) and gas content required for neutral buoyancy (VG) were estimated for C. braueri and C. pygmaea collected between 350 and 550 m (ρ = 1.052–1.072 g·cm−3, VG = 2%–4%; ρ = 1.052–1.062 g·cm−3, VG = 3.6%), and for C. pallida and C. pseudopallida sampled in the 450–700 m layer (ρ = 1.052–1.062 g·cm−3, VG = 2.6–3.1%; ρ = 1.052–1.062 g·cm−3, VG = 2.8%–3.25%). Results in this study highlight the change in scattering behavior of Cyclothone species from gas-bearing organisms (those that contain gas in their swimbladder) in the upper mesopelagic zone to the fluid-like scattering (with fat-filled swimbladders) of the deeper and northern individuals. The data presented in this manuscript are important for parametrizing acoustic backscattering models built to estimate the echo of Cyclothone species, although further work is needed, particularly for individuals with partially invested swimbladders with an irregular fat-free shape, This research was funded by projects Bathypelagic (CTM2016-78853-R), Ideados (CTM2007-65844-C02-01-02-03), and Desafío (PID2020-118118RB-I00) from the Spanish Ministry of Science and Technology, “Sustainable Management of Mesopelagic Resources” (SUMMER, 817806) from European Union Horizon 2020 Research and Innovation Programme, and the “South and Tropical Atlantic Climate-Based Marine Ecosystem Prediction for Sustainable Management” (TRIATLAS, 817578) from European Commission. AS-L was supported by a postgraduate grant (BES-2017-082540) from the Spanish Ministry of Science and Innovation, With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S)

Special issue Technological Oceanography.-- 10 pages, 6 figures, 1 table.-- Data Availability Statement: No new data were created or analyzed in this study. Data sharing is not applicable to this article, Different types and systems to discriminate plankton samples at different strata in the water column have been developed in recent decades. For sampling at sufficient depth, opening and closing zooplankton multinets are ideal because there is no contamination of one sample with organisms of the previous one. However, for bigger nets, such as those used to collect micronektonic organism, it is difficult to use multiple net units, and multiple cod ends are preferred because of their simplicity, but with the problem of sample contamination from having a common net passageway. We present here a cod-end Multisampler design, VERDA, that uses a carrousel-like system. Similar to some sediment traps, the system works like a revolver with six or eight compartments whose turning mechanism is triggered when the net arrives to a programmed depth level. This prototype was built with inexpensive and recycled components and electronics similar to Arduino® and Teensy PCB to carry out electronic control. The net we used for testing the equipment was a mid-size midwater trawl of ca. 30 m2 and total length of 58 m that works with a single towing cable and no doors. The overall system is useful for all type of ships, due to the relatively easy deployment operations and because the Multisampler does not need electrical cable or acoustics. In our case, we used a Marport® (Reykjavik Iceland) and Scanmar (Åsgårdstrand, Norway) sensors for real-time depth monitoring and opening distance, This research was funded by the projects “Migrants and Active Flux in the Atlantic Ocean” (MAFIA, CTM2012-39587-C04) and “Biomass and Active Flux in the Bathypelagic Zone” (Bathypelagic, CTM2016-78853-R) from the Spanish Ministry of Economy and Competitiveness, and SUMMER (grant agreement number 817806) from the European Union (Horizon 2020 Research and Innovation Programme), and the support through the “Severo Ochoa Centre of Excellence” accreditation (CEX2019-000928-S)

Aim Water column acoustic backscatter is regularly registered during oceanographic surveys, providing valuable information on the composition and distribution of pelagic life in the ocean. We propose an objective approach based on functional data analysis to classify these acoustic seascapes into biogeographical regions. Location Tropical South Atlantic Ocean off northeastern Brazil. Taxon Sound-scattering pelagic fauna detected with acoustic echosounders, principally small fish, crustaceans, squid and diverse gelatinous life-forms. Methods We use acoustic backscatter as a function of depth, simultaneously at three frequencies, to numerically describe the vertical distribution and composition of sound-scattering organisms in the water column. This information is used to classify the acoustic seascape through functional principal component analysis. The analysis routine is tested and illustrated with data collected at 38, 70 and 120 kHz in waters affected by contrasting environmental conditions. Results Acoustic seascape partitioning mirrored the distribution of current systems, fronts and taxonomically based regionalization. The study area was divided between slope-boundary and open-ocean waters, and between spring and fall hydrological regimes. Main Conclusions The acoustic seascape consistency and the spatiotemporal coherence of the regions classified show that the method is efficient at identifying homogeneous and cohesive sound-scattering communities. Comparisons against hydrological and biological regionalization prove that the method is reliable at delineating distinct pelagic ecosystems in a cost-efficient and non-intrusive way.

International audience; Mesopelagic communities are characterized by a large biomass of diverse macrozooplankton and micronekton (MM) performing diel vertical migration (DVM) connecting the surface to the deeper ocean and contributing to biogeochemical fluxes. In the Southern Ocean, a prominent High Nutrient Low Chlorophyll (HNLC) and low carbon export region, the contribution of MM to the vertical carbon flux of the biological pump remains largely unknown. Furthermore, few studies have investigated MM communities and vertical flux in naturally iron fertilized areas associated with shallow bathymetry. In this study, we assessed the MM community diversity, abundance and biomass in the Kerguelen Island region, including two stations in the HNLC region upstream of the islands, and two stations in naturally iron fertilized areas, one on the Plateau, and one downstream of the Plateau. The MM community was examined using a combination of trawl sampling and acoustic measurements at 18 and 38 kHz from the surface to 800 m. A conspicuous three-layer vertical system was observed in all areas - a shallow scattering layer, SSL, between 10 and 200 m; mid-depth scattering layer, MSL, between 200 and 500 m; deep scattering layer, DSL, between 500 and 800 m - but communities differing among stations. While salps (Salpa thompsoni) dominated the biomass at the productive Kerguelen Plateau and the downstream station, they were scarce in the HNLC upstream area. In addition, crustaceans (mainly Euphausia vallentini and Themisto gaudichaudii) were particularly abundant over the Plateau, representing a large, although varying, carbon stock in the 0–500 m water layer. Mesopelagic fish were prominent below 400 m where they formed permanent or migrant layers accounting for the main source of carbon biomass. Through these spatial and temporal sources of variability, complex patterns of the MM vertical distribution and associated carbon content were identified. The total carbon flux mediated by migratory myctophids at the four stations was quantified. While this flux was likely underestimated, this study identified the main components and mechanisms of active carbon export in the region and how they are modulated by complex topography and land mass effects.

International audience; Biophysical interactions are influential in determining the scale of key ecological processes within marine ecosystems. For oceanic predators, this means foraging behaviour is influenced by processes shaping the distribution of prey. However, oceanic prey is difficult to observe and its abundance and distribution is regionally generalised. We use a spatiotemporally resolved simulation model to describe mid‐trophic prey distribution within the Southern Ocean and demonstrate insights that this modelled prey field provides into the foraging behaviour of a widely distributed marine predator, the southern elephant seal.From a five‐year simulation of prey biomass, we computed climatologies of mean prey biomass (average prey conditions) and prey biomass variability (meso‐scale variability). We also compiled spatially gridded metrics of seal density and diving behaviour from 13 yr of tracking data. We statistically modelled these metrics as non‐linear functions of prey biomass (both mean and variability) and used these to predict seal distribution and behaviour. Our predictions were consistent with observations (R2adj = 0.23), indicating that seals aggregate in regions of high mesoscale activity where eddies concentrate prey. Here, seals dived deeper (R2marg = 0.12, R2cond = 0.51) and spent less time hunting (R2marg = 0.05, R2cond = 0.56), likely targeting deep but profitable prey patches. Seals generally avoided areas of low eddy activity where prey was likely dispersed. Most seals foraged south of the Subantarctic Front, despite north of the front exhibiting consistently high simulated prey biomasses. This likely reflects seal prey or habitat preferences, but also emphasises the importance of mesoscale prey biomass variability relative to regionally high mean biomass. This work demonstrates the value of coupling mechanistic representations of prey biomass with predator observations to provide insight into how biophysical processes combine to shape species distributions. This will be increasingly important for the robust prediction of species’ responses to rapid system change.

8 pages, 6 figures, 4 tables, Cephalopods are a key component of the marine food webs. Nevertheless, the deep-sea cephalopods are still poorly studied worldwide. The distribution and composition of the mesopelagic cephalopod’s community in different deep scattering layers from the Canary Islands (North-eastern Atlantic) are described here. The results of a mesopelagic fishing survey (CETOBAPH) at the western slopes of three islands of the Canary archipelago (El Hierro, La Palma and Tenerife) are reported. A total of 3,717 specimens of 17 families were caught at different acoustic scattering layers previously detected in depth. The pelagic families Pyroteuthidae, Enoploteuthidae, Onychoteuthidae and Cranchiidae comprised 91% of the total cephalopod catch. Species belonging to these families were responsible for the differences found in the cephalopod community assembly between the shallow sound scattering layers, situated at night in the epipelagic zone and deep sound scattering layers in the mesopelagic zone. No differences were observed in the cephalopod community composition among the three sampled islands. The species richness among islands were similar with 32, 30 and 31 species collected for El Hierro, La Palma and Tenerife, respectively. These results suggest the existence of vertical but no horizontal segregation of small cephalopod species at the mesoscale level in the Canary Islands, This work has been partially funded by the Spanish government via the Nationals Plans of Research, projects: CETOBAPH (CGL2009-13112) and DeepCom (CTM2017-88686-P). Alejandro Escánez is financed by “Margarita Salas” post-doctoral research contract funded by the Next Generation EU (NGEU) fund of the European Union and the “Plan de Recuperación Transformación y Resiliencia” of the Government of Spain

Understanding the ecological mechanisms underpinning distribution patterns is vital in managing populations of mobile marine species. This study is a first step towards an integrated description of the habitats and spatial distributions of marine predators in the Natural Park of the Coral Sea, one of the world’s largest marine-protected areas at about 1.3 million km2, covering the entirety of New Caledonia’s pelagic waters. The study aims to quantify the benefit of including a proxy for prey abundance in predator niche modelling, relative to other marine physical variables. Spatial distributions and relationships with environmental data were analysed using catch per unit of effort data for three fish species (albacore tuna, yellowfin tuna and dolphinfish), sightings collected from aerial surveys for three cetacean guilds (Delphininae, Globicephalinae and Ziphiidae) and foraging locations identified from bio-tracking for three seabird species (wedge-tailed shearwater, Tahiti petrel and red-footed booby). Predator distributions were modelled as a function of a static covariate (bathymetry), oceanographic covariates (sea surface temperature, chlorophyll-a concentration and 20 °C-isotherm depth) and an acoustically derived micronekton preyscape covariate. While distributions were mostly linked to bathymetry for seabirds, and chlorophyll and temperature for fish and cetaceans, acoustically derived prey abundance proxies slightly improved distribution models for all fishes and seabirds except the Tahiti petrel, but not for the cetaceans. Predicted spatial distributions showed that pelagic habitats occupied by predator fishes did not spatially overlap. Finally, predicted habitats and the use of the preyscapes in predator habitat modelling were discussed.

Unusual environmental events can push marine animals outside their physiological tolerances through changes in trophic and/or thermal conditions. Such events typically increase the risk of stranding. Rescue Centers offer a unique opportunity to report animals in distress and satellite track rehabilitated individuals to identify potential new habitats and support an effective conservation of these endangered species. By combining sightings (1988–2020) and tracking data (2008–2020) collected along the French Atlantic and English Channel coasts, our study assessed if the Bay of Biscay is an ecological trap or a favorable habitat for immature sea turtles. The largest tracked individuals migrated westward to pelagic waters, likely toward their natal beaches, while smaller individuals remained within the Bay of Biscay (BoB) and crossed colder (mean: 17.8 ± 3.0°C) but more productive waters. The turtles’ directions differed from the ones of ocean currents, excluding a passive advection to these unexpected habitats. Although the BoB might be thermally unsuitable in winter, the higher micronekton biomass predicted in this region could offer a productive foraging habitat for immature turtles. However, the majority of the sightings referred to individuals stranded alive (75%), suggesting this area could also act as an ecological trap for the smallest individuals that are mostly reported in winter suffering cold-stunning. Assumed to be outside the species range, our results reveal a potential foraging ground in the North-East Atlantic for these young turtles, confirming the crucial role of the rehabilitation centers and the need to continue prioritizing conservation of these endangered species, particularly vulnerable at this stage and at such temperate latitudes.

9 pages, 3 figures, 1 table, supplementary data https://doi.org/10.1093/icesjms/fsab189.-- Data availability statement: The original data on sample location, individual fish characteristics and stable isotope composition, including amino acids, can be accessed through the PANGAEA repository (Bode et al., 2021b). Similarly, the original data for copepods can be found in Bode et al. (2018b), The trophic position (TP) of fishes determines their importance in terms of energy flows within food webs. However, accurate estimations of TP are often prevented because of the difficulties in tracing all food sources. This is particularly challenging for omnivorous fishes, such as those from the Order Stomiiformes. In this study, we applied recent developments in stable isotope analysis of amino acids to untangle the contributions of microbial vs. metazoan food webs in 13 species of Stomiiformes. The inclusion of the microbial food web reduced the differences between TP estimates using stable isotopes and those derived from stomach content analysis. In addition, the new estimates allowed to quantify the relative contribution of the microbial food web to each species (6–21%), highlighting the importance of detritus consumption even in piscivorous species (e.g. Stomias boa and Chauliodus danae). The comparison of TP estimates obtained with selected amino acids in fish muscle allowed for the detection of the microbial influence integrated at time scales relevant for net fish growth, even when trophic exchanges in the microbial food web occur at much shorter time scales. The assessment of TP considering the differential contribution of microbial and metazoan food webs challenges our current understanding of marine food webs; yet provides a new quantitative tool for the analysis of their structure and function, This research was funded by projects MAFIA (CTM2012-39587-C04), BATHYPELAGIC (CTM2016-78853-R), and QLOCKS (PID2020-115620RB-100) from the Plan Estatal de I+D+I (Spain), projects SUMMER (grant agreement number 817806) and TRIATLAS (grant agreement number 817578), from the European Union (Horizon 2020 Research and Innovation Programme), and the support through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S)

The Clarion-Clipperton Zone (CCZ) is a 4 million km2 area in the eastern Central Pacific Ocean exhibiting large variability in environmental parameters, particularly oxygen and primary production, that is being targeted for deep-sea polymetallic nodule mining. This remote region’s pelagic biology is very poorly sampled, including for micronekton and zooplankton that provide essential ecosystem services such as carbon flux and support for commercial fisheries. We built a baseline of deep scattering layer (DSL) depths and vertical migration behaviors, proxies for mesopelagic micronekton and zooplankton communities, using shipboard acoustic Doppler current profiler datasets. Acoustic data (38 kHz, 75 kHz) were compiled from research cruises passing near or through the CCZ (2004–2019), and environmental data (mean midwater oxygen partial pressure, surface chlorophyll-a, and sea surface height anomaly) were assembled from the World Ocean Atlas and satellite oceanographic datasets. Our results suggest that midwater oxygen, associated with the Eastern Tropical Pacific Oxygen Minimum Zone (OMZ), is the strongest predictor of daytime DSL depths and the proportions of midwater populations that undergo vertical migration in this region. We used these relationships to predict micronekton and zooplankton behaviors across the CCZ, including licensed mining exploration areas and no-mining reserves. While the OMZ encompasses most licensed exploration areas, the current network of reserves lies outside of the core OMZ and ultimately may not represent or protect the pelagic OMZ fauna at highest risk from mining impacts. This research will further assist in developing resource exploitation regulations by the International Seabed Authority, and will provide mesopelagic baseline information for monitoring changes that may occur in the CCZ once industrial-scale mining begins.

Vertical migrations into shallower waters at night are beneficial for migrators as they reduce predation risk and allow migrators to encounter a higher density of prey. Nevertheless, ocean acoustics data and trawl data have shown that a portion of some vertically migrating populations remain at depth and do not migrate. One hypothesis for this phenomenon is the Hunger-Satiation hypothesis, which in part states that the non-migrating portion of the migrating species-assemblage refrains from migrating if they have full or partially full stomachs from daytime or nocturnal feeding. However, stomach fullness of the non-migrating subpopulation compared to the migrating portion has rarely been studied, due to the difficulty in obtaining sufficient samples. The stomach fullness levels of numerically abundant crustacean and fish species with well-known depth distributions were quantified in the present study. Animals were captured during night trawls from discrete-depth intervals between 0 and 1,500 m. Stomach fullness indices were assigned from 0 to 5 and compared between migratory taxa caught in shallow and deep waters. Data from the crustaceansAcanthephyra purpurea, Gardinerosergia splendens, Plesionika richardi, andSystellaspsis debilis, as well as the fishesLampanyctus alatus, Lepidophanes guentheri, andNotolychnus valdiviae, provided support for the Hunger-Satiation hypothesis, while data from the crustaceansGennadas capensisandGennadas valensand the fishBenthosema suborbitaledid not. These findings suggest that stomach vacancy may be just one of several factors regulating the dynamics of vertical migration in those species whose behavioral plasticity suggests daily “choices” in whether or not to vertically migrate.

The pelagic Gulf of Mexico (GoM) is a complex system of dynamic physical oceanography (western boundary current, mesoscale eddies), high biological diversity, and community integration via diel vertical migration and lateral advection. Humans also heavily utilize this system, including its deep-sea components, for resource extraction, shipping, tourism, and other commercial activity. This utilization has had impacts, some with disastrous consequences. The Deepwater Horizon oil spill (DWHOS) occurred at a depth of ∼1500 m (Macondo wellhead), creating a persistent and toxic mixture of hydrocarbons and dispersant in the deep-pelagic (water column below 200 m depth) habitat. In order to assess the impacts of the DWHOS on this habitat, two large-scale research programs, described herein, were designed and executed. These programs, ONSAP and DEEPEND, aimed to quantitatively characterize the oceanic ecosystem of the northern GoM and to establish a time-series with which natural and anthropogenic changes could be detected. The approach was multi-disciplinary in nature and included in situ sampling, acoustic sensing, water column profiling and sampling, satellite remote sensing, AUV sensing, numerical modeling, genetic sequencing, and biogeochemical analyses. The synergy of these methodologies has provided new and unprecedented perspectives of an oceanic ecosystem with respect to composition, connectivity, drivers, and variability.

To better understand spatiotemporal variation in the trophic structure of deep-pelagic species, we examined the isotope values of particulate organic matter (POM) (isotopic baseline) and seven deep-pelagic fishes with similar diet compositions but contrasting vertical distributions across mesoscale features in the Gulf of Mexico using stable isotope and amino acid compound-specific isotope analyses. Species examined included four migratory (Benthosema suborbitale, Lepidophanes guentheri, Melamphaes simus, Sigmops elongatus) and three non-migratory zooplanktivorous fishes (Argyropelecus hemigymnus, Cyclothone obscura, Sternoptyx pseudobscura). Isotopic values of POM increased with depth, with meso- and bathypelagic samples characterized by higher δ13C and δ15N values relative to epipelagic samples. Despite similar diet compositions, mean δ15N values of fishes spanned 3.43‰ resulting in mean trophic position estimates among species varying by 1.09 trophic levels. Interspecific differences in δ15N were driven by higher δ15N values in the non-migratory and deepest dwelling C. obscura (10.61‰) and lower δ15N values in the migratory and shallowest dwelling L. guentheri (7.18‰) and B. suborbitale (8.11‰). Similarly, fish δ15NsourceAA values were correlated with depth, with the lowest values occurring in the migratory L. guentheri and B. suborbitale and highest values occurring in the non-migratory C. obscura. Our data suggest that depth-related trends in fish δ15N and δ15NsourceAA values are driven by shallower dwelling species feeding within epipelagic food webs supported by POM with lower δ15N values, while deeper dwelling, non-migratory species increasingly use food webs at depth supported by POM with elevated δ15N values. Horizontal isotopic variation was observed across a large mesoscale oceanographic feature (Loop Current), with POM, three migratory, and one non-migratory species characterized by higher δ13C and lower δ15N values in the anticyclonic Loop Current relative to surrounding water masses. Our results demonstrate that isotopic values of POM can vary significantly over relatively small horizontal and vertical scales and that baseline variation can be conserved in the signatures of higher-order consumers. By gaining a more thorough understanding of the sources contributing to isotopic variation of deep-pelagic fishes, this paper will inform the design and interpretation of future feeding studies in the pelagic realm and advances our knowledge of deep-pelagic food web structure.

In the original article, there was a mistake in the legend for Table S-3 as published. Units were not included in the original table legend. The correct legend appears below. Table S-3. Length to weight relationships used to calculate carbon weight (CW; in mg) for micronekton captured in the MIDOC. Lengths are reported as either total length (TL) or standard length (SL) in millimeters. Additionally, there was a mistake in Table S-3 as published.We have re-configured some of the equations within the table to add clarity for those that wish to apply these equations with their own data. In the original table the wet weight to carbon conversions on some of the equations were improperly placed. The corrected Table S-3 appears below. The authors apologize for these errors and state that they do not change the scientific conclusions of the article in any way. The original article has been updated.

WOS:000551093900001; International audience; Mesoscale eddies play a key role in structuring open ocean ecosystems, affecting the entire trophic web from primary producers to large pelagic predators including sharks and elephant seals. Recent advances in the tracking of pelagic predators have revealed that these animals forage in the mesopelagic and the depth and duration of their foraging dives are affected by the presence of eddies. The ways in which eddies impact the distribution of mesopelagic micronekton, however, remain largely unknown. During a multi-seasonal experiment we used a shipboard scientific echosounder transmitting at 38 kHz to observe the distribution of acoustic backscattering in the energetic mesoscale eddy field of the northwestern Atlantic. Observations were collected at 24 stations with 6 located in anticyclonic and 7 in cyclonic eddies. The sampled anticyclonic eddies are characterized by intense acoustic backscattering in the mesopelagic and changes in the intensity of acoustic backscattering layers match gradients of surface properties. Furthermore, mesopelagic daytime backscattering is positively correlated with sea level anomaly. These results suggest that anticyclonic eddies in the northwestern Atlantic impact the distribution of mesopelagic micronekton and may have the potential to locally enhance or structure spatially mesopelagic communities.

International audience; An investigation of the vertical and horizontal distributions of micronekton, as influenced by mesoscale eddies, the Madagascar shelf and shallow seamounts, was undertaken using acoustic data collected during two research cruises at an unnamed pinnacle (summit depth ~240 m) thereafter named “MAD-Ridge”, and at La Pérouse seamount (~60 m) in the south-western Indian Ocean. MAD-Ridge is located to the south of Madagascar, in an “eddy corridor”, known both for its high mesoscale activity and high primary productivity. In contrast, La Pérouse is located on the outskirts of the Indian South Subtropical Gyre (ISSG) province, characterised by low mesoscale activity and low primary productivity. During the MAD-Ridge cruise, a dipole was located in the vicinity of the seamount, with the anticyclone being almost stationary on the pinnacle. Total micronekton acoustic densities were greater at MAD-Ridge than at La Pérouse. Micronekton acoustic densities of the total water column were lower within the anticyclone than within the cyclone during MAD-Ridge. Micronekton followed the usual diel vertical migration (DVM) pattern, except within the cyclone during MAD-Ridge where greater acoustic densities were recorded in the daytime surface layer. The backscatter intensities were stronger at the 38 kHz than at the 70 and 120 kHz frequencies in the daytime surface layer at MAD-Ridge cyclonic stations. These backscatter intensities likely correspond to gas-filled swimbladders of epi- and mesopelagic fish actively swimming and feeding within the cyclone or gelatinous organisms with gas inclusions. Our findings evidenced that the distributions of micronekton and DVM patterns are complex and are influenced significantly by physical processes within mesoscale eddies. The mesoscale eddies’ effects were dominant over any potential seamount effects at the highly dynamic environment prevailing at MAD-Ridge during the cruise. No significant increase in total micronekton acoustic densities was observed over either seamount, but dense aggregations of biological scatterers were observed on their summits during both day and night.

International audience; The La Pérouse seamount (60 m depth) has so far been poorly studied despite it being a short distance (160 km) from Réunion Island. As part of the MADRidge project, a multidisciplinary cruise was conducted to evaluate the effect of this shallow seamount on the local hydrology and ecology. Current measurements, temperature and chlorophyll-a profiles, and mesozooplankton and micronekton samples were collected between the summit and 35 km away. Micronekton data were supplemented with stomach content of pelagic top predators as well as fisheries statistics from the domestic longline fleet operating from Réunion. Vertical current profiles revealed distinct patterns between the offshore and seamount-flanked stations, giving evidence of topographical induced flow instabilities, notably on its leeward side (west) relative to the east flank. Distinct patterns in temperature and chlorophyll-a vertical profiles suggest the formation of convergent and divergent circulation cells as a result of the irregular and crescent-like summit topography. Spatial differences in zooplankton abundance were detected with higher biovolumes on the leeward flank. The overall acoustic backscatter for micronekton over the summit was weaker than offshore, but highly concentrated in the upper layer. Albacore tuna and swordfish dominate the longline catch west of Réunion, seemingly in association with a deep (900 m) topographic feature. Yet the largest catch is not directly associated with La Pérouse which would be too shallow for top predators to aggregate around in the long term. Enhanced levels of phytoplankton or zooplankton enrichment at La Pérouse were not demonstrated in this study, nor was there notable diversity of micronekton species. This might explain the relatively limited importance of this seamount to the tuna fisheries in this region.

International audience; Acoustic data are an invaluable source of information for characterizing the distribution and abundance of mid-trophic level organisms (MTLOs) in the ocean. These organisms play a key role in the ecosystem as prey of top predators and as predators of lower trophic level organisms, as well as in carbon export from the surface into deeper waters. This study used 38 kHz-EK60 acoustic echosounder data from six cruises spanning 2011–2017 to explore the seasonal and spatial variability in the vertical distribution of MTLOs’ from 10 to 600 m in the New Caledonian (South Pacific) Exclusive Economic Zone. A total of 16715 acoustic vertical profiles of acoustic backscattering strength were clustered into homogeneous groups. Two small shallow scattering layers (SSLs) between 0 and 100 m, and one large deep scattering layer (DSL) at around 550 m depth, characterized the mean vertical distribution of MTLOs. A machine-learning model (eXtreme Gradient tree Boosting algorithm, XGBoost) was fitted to explain the acoustic profile clusters with environmental variables as predictors. Sun inclination was the most important factor in structuring the vertical profile shapes due to the diel vertical migration signal, followed by the mean oxygen value of the top 600 m. Bathymetry, euphotic depth, 0–600 m mean temperature and SST were the next most significant variables. Isotherm depth, surface chlorophyll-a, wind, and mean salinity had a lower influence on the shape of the vertical profiles. The model was then used to construct vertical echograms at the scale of the New Caledonian EEZ, showing an accuracy up to 87% in cross validation. Across the EEZ, the shape of vertical acoustic profiles were comparable, though layer echo intensities varied spatially with a marked north-south gradient that remained relatively constant seasonally. The vertically-averaged acoustic values were characterized by a maximum to the south of the EEZ in summer, mainly driven by high oxygen values as well as shallow euphotic depth. We also estimated a migrant proportion between day DSL and night SSL of about 78%. Our methodology offers a promising approach for analyzing the control of the environment on the vertical distribution of MTLOs for other oceanic provinces, while also providing a framework to investigate the corresponding trophic interactions between MTLOs and their predators feeding at different depths and times. Moreover, our findings stress the need to consolidate knowledge on species composition to optimize acoustic data interpretation.

WOS:000529921100010; International audience; Acoustic data are invaluable information sources for characterizing the distribution and abundance of mid-trophic-level organisms (micronekton). These organisms play a pivotal role in the ecosystem as prey of top predators and as predators of low-trophic-level organisms. Although shipboard-ADCP (acoustic Doppler current profiler) acoustic backscatter signal intensity cannot provide an absolute biomass estimate, it may be a useful proxy to investigate variability in the distribution and relative density of micro-nekton. This study used acoustic recordings data spread across 19 years (1999-2017) from 54 ADCP cruises in New Caledonia's subtropical EEZ (exclusive economic zone) to assess seasonal and interannual variabilities and spatial distribution of micro-nekton. The dataset was composed of two different ADCPs: 150 kHz for the first period, followed by 75 kHz for more recent years. We examined the 20-120 m averaged scattering layer. Using the few cruises with concurrent EK60 measurements, we proposed that the backscatter from the ADCPs and 70 kHz EK60 were sufficiently closely linked to allow the use of the backscatter signal from the ADCPs in a combined dataset over the full time series. We then designed a GAMM (generalized additive mixed model) model that takes into account the two ADCP devices as well as temporal variability. After accounting for the effect of the devices, we showed that the acoustic signal was mainly driven by diel vertical migration, season, year, and ENSO (El Nino-Southern Oscillation). In a second step, a consensus model between two statistical approaches (GAMM and SVM) (support vector machine) was constructed, linking the nighttime 20-120 m backscatter to the oceanographic and geographic environment. This model showed that sea surface temperature was the main factor driving backscatter variability in the EEZ, with intensified backscatter during the austral summer (December to May) in the northern part of the EEZ. We showed that acoustic density differed significantly, spatially and temporally from micronekton biomass predicted for the same period by the SEAPODYM-MTL (mid-trophic level) ecosystem model. The seasonal cycle given by ADCP data lagged behind the SEAPODYM-MTL seasonal cycle by around three months. Reasons to explain these differences and further needs in observation and modeling were explored in the discussion. In addition to providing new insights for micronekton dynamics in this EEZ (i.e., the science needed for ecosystem-based fisheries management), the data should help improve our ability to model this key trophic component.

The lanternfishes (Myctophidae) are a highly speciose, globally-distributed family of fishes that constitute a dominant component of the global pelagic fauna. As a vertically-migrating taxon of oceanic micronekton, myctophids play vital ecological roles in the biological carbon pump and as an important prey group for several commercially-important species. However, our knowledge of the ecology of this taxon remains incomplete, and as anthropogenic impacts continue to develop and extend into deeper waters, there is a clear need for a better understanding of its ecological role and assemblage dynamics. The aim of the present study was to examine the distribution patterns of the myctophid assemblage within a 200 × 700 km grid of the northern Gulf of Mexico (GoM) in relation to major mesoscale hydrographic features. The 22 dominant myctophid species (> 0.05% by relative abundance) were analysed from a total of 302 trawl samples collected between January and September 2011, from 0 – 1000 m depth. Redundancy analysis (RDA) indicated that measured environmental variables and spatial patterning explained an average of 12% (range 0 – 27%) of the observed variance in the myctophid assemblage. Distance-based Moran’s Eigenvector Mapping (dbMEM) and trend analysis (RDA) indicated limited significant spatial coherence within the assemblage at the scales considered. Local contribution to beta diversity (LCBD) scores corroborated these findings, indicating that the majority of samples were not significantly different from the mean assemblage structure. Taken together, these results suggest that the myctophid assemblage in the northern GoM is well-mixed and highly dispersed at the sub-basin scale (at least), likely the result of the interaction between vertical migration and depth-specific lateral advection. Findings such as these inform our approach to assessing impacts in a large, dynamic, pelagic ecosystems. It is essential to know over what spatial scales assessments of pelagic faunal impacts, and potential recoveries, must be based. In cases of large, spatially integrated pelagic assemblages, high dispersal rates may serve to either ameliorate the effects of a disturbance through immigration or spread the effects across a wider spatial area than the disturbance phenomenon footprint itself.

Mesozooplankton (June 2015 and September 2017) and micronekton (September 2017) were sampled along the eastern coast of Australia. Depth stratified mesozooplankton and micronekton were collected using a Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS) and an International Young Gadoid Pelagic Trawl (IYGPT) equipped with an opening/closing codend. Sampling was undertaken at the center and edge of a frontal cold-core eddy (F-CCE Center and Edge) in 2015, and at the center of a cold-core eddy (B-CCE) and two warmcore eddies (R-WCE and WCE) in 2017. We assess the diel vertical structure, biomass, and downward active carbon transport by mesozooplankton and micronekton in eddies. Total water column mesozooplankton and micronekton biomass did not vary substantially across water masses, while the extent and depth of diel vertical migration did. Using in situ measurements of temperature and measurements of mesozooplankton and micronekton abundance and biomass, we estimated the contribution of respiration, dissolved organic carbon (DOC) excretion, gut flux, and mortality to total downward active carbon transport in each water mass. Overall, active carbon transport by mesozooplankton and micronekton below the mixed layer varied substantially across water masses. We corrected estimates of micronekton migratory biomass and active carbon transport assuming 50% net efficiency. In the R-WCE mesozooplankton remained within the mixed layer during the day and night; only 50% of the total micronekton population migrated below the mixed layer contributing to carbon transport, equating to 2.89 mg C m(-2) d(-1). Mesozooplankton actively transported 16.1 and 8.0 mg C m(-2) d(-1) at the F-CCE Center and Edge, respectively. Mesozooplankton and micronekton active carbon transport in the B-CCE were 5.4 and 0.74 mg C m(-2) d(-1), and in the WCE 88 and 13.4 mg C m(-2) d(-1). Differences in carbon export were dependent on food availability, temperature, time spent migrating, and mixed layer depth. These findings suggest that under certain conditions mesoscale eddies can act as important carbon sinks.

From March 18 to April 2, 2017, the PUFFAlis expedition took place aboard research vessel Alis of IRD. This campaign was a continuation of previous oceanographic expeditions concerned with the feeding areas of seabirds (MOMAlis) as well as on land on the Gouaro Deva shearwater colony. It was also the extension of the NECTAlis campaigns that were dedicated at studying the oceanic trophic chain that leads to tunas. Six scientists embarked to sample the different levels of the oceanic trophic chain of the New Caledonia basin off Pindai. In the middle of the expedition, a day was devoted to communicating with the high school students of Pouembout and the services of the environment and fisheries of the Northern Province. The expedition’s logbook is preceded by the press release issued on this occasion., Du 18 mars au 02 avril 2017 s’est déroulée la mission PUFFAlis à bord du navire océanographique Alis de l’IRD. Cette campagne s’inscrit dans la continuité d’expéditions océanographiques centrées sur les zones d’alimentation des oiseaux marins, effectuées précédemment en mer (MOMAlis) comme à terre sur la colonie de puffins de Gouaro Deva. Elle est aussi la prolongation des campagnes NECTAlis destinées à étudier la chaîne trophique océanique qui conduit aux thons. Six scientifiques ont embarqué pour échantillonner les différents niveaux de la chaîne trophique océanique du bassin de Nouvelle-Calédonie au large de Pindaï. En milieu de mission, une journée a été consacrée à la communication avec les lycéens de Pouembout et les services de l’environnement et des pêches de la Province nord. Le journal de bord de la mission est précédé du communiqué de presse diffusé à cette occasion.

International audience; The ecology of leptocephali remains poorly known but they appear to feed on marine snow that can vary spatially and temporally according to the food web dynamics. This study provided new information about the position of leptocephali within the functional structure of microbial plankton and other food web components of the western South Pacific (WSP) region at a large geographic scale including the New Caledonia, Fiji, and Samoa islands. The hydrographic structure varied with latitude, and nutrient levels were generally low but somewhat variable. Stable isotopic signatures were examined in relation to the 3 current zones of the eastward flowing South Equatorial Countercurrent (SECC; north), the westward South Equatorial Current (SEC; mid-latitudes), and the eastward South Tropical Countercurrent (STCC; south), and all zones were found to be primarily based on a heterotrophic planktonic functioning that were co-limited by inorganic nitrogen and phosphorus, with biomasses of planktonic groups varying with depth. Isotopic signatures of leptocephali were compared to the signatures of other mesozoplankton, micronekton, and Trichodesmium components of the food web, and in relation to the signatures of particulate organic matter (POM) that varied among the 3 collection depths. The isotopic signatures of six taxa of leptocephali, other taxonomic groups and POM showed interesting variability according to latitude and among some stations. The presence of Trichodesmium at the surface in the STCC zone influenced the isotopic signatures of POM and thus the signatures of leptocephali. The signatures of leptocephali were therefore linked with the overall food web and were consistent with the larvae feeding on marine snow components of POM. The two apparent groups of leptocephali with different isotopic signatures that have also been observed in other oceanic areas may be explained by feeding behavior at different depths or on different types of marine snow.

Understanding the relationship between sound-scattering layers (SSLs) and pelagic habitat characteristics is a substantial step to apprehend ecosystem dynamics. SSLs are detected on echo sounders representing aggregated marine pelagic organisms. In this study, SSL characteristics of zooplankton and micronekton were identified during an upwelling event in two contrasting areas of the Senegalese continental shelf. Here a cold upwelling-influenced inshore area was sharply separated by a strong thermal boundary from a deeper, warmer, stratified offshore area. Mean SSL thickness and SSL vertical depth increased with the shelf depth. The thickest and deepest SSLs were observed in the offshore part of the shelf. Hence, zooplankton and micronekton seem to occur more frequently in stratified water conditions rather than in fresh upwelled water. Diel vertical and horizontal migrations of SSLs were observed in the study area. Diel period and physicochemical water characteristics influenced SSL depth and SSL thickness. Although chlorophyll-a concentration insignificantly affected SSL characteristics, the peak of chlorophyll a was always located above or in the middle of the SSLs, regularly matching with the peak of SSL biomass. Such observations indicate trophic relationships, suggesting SSLs to be mainly composed of phytoplanktivorous zooplankton and micronekton. Despite local hypoxia, below 30 m depth, distribution patterns of SSLs indicate no vertical migration boundary. The results increase the understanding of the spatial organization of mid-trophic species and migration patterns of zooplankton and micronekton, and they will also improve dispersal models for organisms in upwelling regions.

International audience; Micronekton distributions and assemblages were investigated at two shallow seamounts of the south-western Indian Ocean using a combination of trawl data and multi-frequency acoustic visualisation techniques. La Pérouse (∼60 m) seamount is located on the outskirts of the oligotrophic Indian South Subtropical Gyre province with weak mesoscale activities and low primary productivity all year round. The “MAD-Ridge” seamount (thus termed in this study; ∼240 m) is located in the productive East African Coastal (EAFR) province with high mesoscale activities to the south of Madagascar. This resulted in higher micronekton species richness at MAD-Ridge compared to La Pérouse. Resulting productivity at MAD-Ridge seamount was likely due to the action of mesoscale eddies advecting larvae and productivity from the Madagascar shelf rather than local dynamic processes such as Taylor column formation. Mean micronekton abundance/biomass, as estimated from mesopelagic trawl catches, were lower over the summit compared to the vicinity of the seamounts, due to net selectivity and catchability and depth gradient on micronekton assemblages. Mean acoustic densities in the night shallow scattering layer (SSL: 10-200 m) over the summit were not significantly different compared to the vicinity (within 14 nautical miles) of MAD-Ridge. At La Pérouse and MAD-Ridge, the night and day SSL were dominated by common diel vertical migrant and non-migrant micronekton species respectively. While seamount-associated mesopelagic fishes such as Diaphus suborbitalis (La Pérouse and MAD-Ridge) and Benthosema fibulatum performed diel vertical migrations (DVM) along the seamounts’ flanks, seamount-resident benthopelagic fishes, including Cookeolus japonicus (MAD-Ridge), were aggregated over MAD-Ridge summit both during the day and night. Before sunrise, mid-water migrants initiated the first vertical migration event from the intermediate to the deep scattering layer (DSL, La Pérouse: 500-650m; MAD-Ridge: 400-700 m) or deeper. During sunrise, the other taxa contributing to the night SSL exhibited a successive series of vertical migration events from the surface to the DSL or deeper. Some scatterers were blocked in their upward and downward migrations due to the seamount topography, more commonly known as the sound scattering layer interception/topographic blockage hypothesis. Possible mechanisms leading to the observed patterns in micronekton vertical and horizontal distributions are discussed. This study contributes to a better understanding of how seamounts influence the diel vertical migration, horizontal distribution and community composition of micronekton and seamount-associated/resident species at two poorly studied shallow topographic features in the south-western Indian Ocean.

International audience; The Kerguelen Plateau is a region of natural iron fertilization in the Southern Ocean, within the typically iron limited High Nutrient Low Chlorophyll (HNLC) waters of the eastward flowing Antarctic Circumpolar Current. Between 26 February and 19 March 2018, the MOBYDICK expedition investigated pelagic ecosystem dynamics in the Kerguelen Island region on the northern plateau during the post-phytoplankton bloom period. The survey specifically targeted sampling at two stations in the HNLC waters to the west of the Kerguelen Plateau, and in the iron enriched waters on and to the east of the plateau. A combination of WP2, WP3 and Mesopelagos midwater trawl were used to sample the mesozooplankton (125 μm to ≤ 10 mm), macrozooplankton (10–30 mm) and micronekton (> 30 to 200 mm) communities. Stable carbon (δ13C) and nitrogen (δ15N) isotopes were measured across representative samples of taxa from all stations. Trophic positions was estimated using a Bayesian isotope mixing model (tRophicPosition), that uses both the δ13C and δ15N values of baselines and consumers. Meso/macrozooplankton trophic positions (TPs) were ~ 2.4 on the plateau and > 0.6 TPs higher at the upstream HNLC stations. This provides empirical evidence for shorter food chains on the diatom dominated plateau and longer food chains in the HNLC region dominated by pico and nanophytopklankon. Meso/macrozooplankton TPs were also elevated downstream of the plateau, though to a lesser extent than in the upstream region, likely reflecting the effect of downstream iron transport, supporting elevated phytoplankton production in the downstream region. There was a consistent increase in trophic position between meso/macrozooplankton and micronekton of ~ 0.6 at all stations. Food chain length was therefore determined by the composition of and interactions between the trophic levels below micronekton. Similar trophic structure at the two upstream HNLC stations, on either side of the northern branch of the Polar Front, indicated that food chain length was robust to the differences in community composition, and supports the critical role of size structured trophic interactions in determining food web properties.

International audience; Despite expanding in-situ observations of marine ecosystems by new-generation sensors, information about intermediate trophic levels remains sparse. Indeed, mid-trophic levels, while encompassing a broad range of zooplankton and micronekton organisms that represent a key component of marine ecosystems and sustain large and diverse communities of marine predators, are challenging to sample and identify. In this study, we examined whether an animal-borne miniature active echosounder can provide information on the distribution and movements of mid-trophic level organisms. If so, such a sonar tag, harnessing the persistent diving behaviour of far-ranging marine mammals, could greatly increase the density of data on this under-studied biome. High-frequency (1.5 MHz) sonar tags were deployed simultaneously with oceanographic tags on two southern elephant seals (Mirounga leonina), at the Kerguelen Islands and Valdés Peninsula (Argentina), and recorded acoustic backscatter while the seals foraged respectively in the Indian and the Atlantic sectors of the Southern Ocean. The backscatter varied widely over time and space, and the seals attempted to capture only a small fraction of the insonified targets. Diel vertical migration patterns were clearly identifiable in the data, reinforcing our confidence in the ability of the sonar tags to detect living mid-trophic organisms along with possibly sinking biological detritus. Moreover, CTD tags attached to the same animals indicated how the abundance, size distribution, and diel migration behaviour of acoustic targets varied with water bodies. These preliminary results demonstrate the potential for animal-borne sonars to provide detailed in-situ information. Further validation effort will make it a valuable tool to refine the estimation of carbon export fluxes as well as for assessing the variation of mid-trophic level biomass according to oceanographic domains and seasons.

WOS:000660310900002; International audience; Ecosystem models forced by future climate simulations outputs from the Coupled Model Intercomparison Project Phase 5 (CMIP5) simulate a substantial decline of tropical marine animal biomass over the course of the 21st century. Regional projections are however far more uncertain because of well-known biases common to most CMIP5 historical simulations that propagate within the food web. Moreover, the model outputs for high trophic levels marine fauna suffer from lack of validation based on in situ data. In this study, we implement a "bias-mitigation" strategy to reduce the physical oceanography and biogeochemical biases simulated by three CMIP5 models under the future RCP8.5 scenario. We force two very different micronekton models with these "bias-mitigated" outputs to infer the future micronekton changes in the Coral Sea: a 3-D deterministic population dynamics model; and a 3-D statistical model based on in situ hydro-acoustic data. These two models forecast a consistent pattern of micronekton abundance changes in the epipelagic layer (0-150 m) by 2100 for three different climate forcing used, with a marked decrease south of 22 degrees S and a smaller increase further north mostly related to temperature and chlorophyll changes. In contrast, changes in the vertical patterns of micronekton predicted by the two models considerably differ in the upper mesopelagic layers (150-450 m) and lower mesopelagic layer (450-1000 m), highlighting the structural sensitivity in model type. Since micronekton are prey of all larger marine predators, those discrepancies in vertical structures of micronekton may hamper our potential to predict how top predators may evolve in the future.

20 pages, 11 figures, 3 tables, supplemental material https://doi.org/10.3389/fmars.2019.00535, Quantification of the actual amount of carbon export to the mesopelagic layer by both zooplankton and micronekton is at present a gap in the knowledge of the biological pump. These organisms perform diel vertical migrations exporting carbon through respiration, excretion, mortality, and egestion during their residence at depth. The role of zooplankton in active flux is nowadays partially assessed. However, micronekton active flux is scarcely known and only a few studies addressed this downward transport. Even less is known about the capacity of both communities to export carbon in the ocean. Here, we show the results of zooplankton and micronekton active flux across a productivity gradient in the tropical and subtropical Atlantic Ocean. Biomass vertical distribution from the surface up to 800 m depth by day and night was studied during April 2015 in a transect from 9°S to 25°N, covering from the quite oligotrophic zone off Brazil to the meso- and eutrophic areas of the equator, Guinea Dome, and the oceanic upwelling off Northwest Africa. Zooplankton and micronekton migrant biomass was estimated from day and night catches at different layers of the water column using MOCNESS-1 (1 m2 mouth area) and Mesopelagos (35 m2) nets, respectively. Respiratory flux was assessed by measuring the enzymatic activity of the electron transfer system (ETS) of organisms at depth. Results showed a close relationship between migrant biomass and respiratory flux in zooplankton and micronekton as expected. Using a rather conservative 50% of efficiency for the net used to capture micronekton, respiratory flux resulted in similar values for both communities. Gravitational (passive) flux measured using sediment traps increased from the oligotrophic toward the meso- and eutrophic zones. Total active flux (including respiration and estimated mortality, excretion, and gut flux) by zooplankton and micronekton accounted for about 25% of total flux (passive plus active) in oligotrophic zones. Total active flux also increased toward meso- and eutrophic zones, reaching about 80% of total flux and being at least twofold higher than passive flux. These results alert about an important underestimation of the ocean biological pump using only passive flux measurements, This work was supported by projects “Migrants and Active Flux in the Atlantic Ocean” (MAFIA, CTM2012-39587-C04), and “Biomass and Active Flux in the Bathypelagic Zone” (Bathypelagic, CTM2016-78853-R) from the Spanish Ministry of Economy and Competitiveness

Micronekton are a key component of the pelagic food web of the Chatham Rise east of New Zealand. The Chatham Rise is an important fishing area for hoki (Macruronus novaezelandiae), New Zealand’s largest finfish fishery, and a predator on mesopelagic fish. Four fisheries oceanographic voyages provided multi-frequency acoustic data (18, 38, 70, 120, and 200 kHz) and midwater trawls, which were used to define a classification tree to separate micronektonic organisms. We carried out validation and sensitivity analyses that showed that we were able to classify pearlside (Maurolicus australis) and euphausiids. Other mesopelagic targets (mainly myctophids) were classified together based on their acoustic frequency response. Using scripting in the open-source software ESP3, we applied our classification tree to an independent time series of acoustic data from trawl surveys on the Chatham Rise between 2009 and 2018, that was not used for model development or validation. Our methodology allowed us to study temporal and spatial patterns of M. australis, euphausiids, and total backscatter in the water column. Total backscatter associated with micronekton has varied over the last 10 years, with no clear trend. The abundance of euphausiids showed a significant decreasing trend over the last 10 years. Abundance of M. australis also decreased since 2012, though this was not significant. This work contributes to on-going efforts to monitor and detect changes in the pelagic ecosystems.

The community structure of pelagic zooplankton and micronekton may be a sensitive indicator of changes in environmental conditions within the California Current ecosystem. Substantial oceanographic changes in 2015 and 2016, due to the anomalously warm ocean conditions associated with a large-scale marine heatwave perturbation, resulted in onshore and northward advection of warmer and more stratified surface waters resulting in reduced upwelling. Here we quantify changes in the macrozooplankton and micronekton community composition and structure based on five highly contrasting ocean conditions. Data from fine-mesh pelagic trawl surveys conducted off Oregon and Washington during early summer of 2011 and 2013–2016 were examined for interannual changes in spatial distribution and abundance of fish and invertebrate taxa. Overall species diversity was highest in 2015 and lowest in 2011, but 2016 was similar to the other years, although the evenness was somewhat lower. The community of taxa in both 2015 and 2016 was significantly different from the previously sampled years. Crustacean plankton densities (especially Euphausiidae) were extremely low in both of these years, and the invertebrate composition became dominated mostly by gelatinous zooplankton. Fishes and cephalopods showed mixed trends overall, but some species such as age-0 Pacific hake were found in relatively high abundances mainly along the shelf break in 2015 and 2016. These results suggest dramatically different pelagic communities were present during the recent warm years with a greater contribution from offshore taxa, especially gelatinous taxa, during 2015 and 2016. The substantial reorganization of the pelagic community has the potential to lead to major alterations in trophic functioning in this normally productive ecosystem.

The New Caledonian pelagic ecosystem in the southwest Pacific contains a high diversity of predators, such as seabirds and cetaceans. The recent creation of the Natural Park of the Coral Sea created a need for solid information on the functioning of this remarkable ecosystem, particularly on the dynamics of the micronecton (organisms between 1 and 20 cm constituting food of top predators) and its central role in food webs, especially since is the most poorly understood. In this context, this PhD showed a strong influence of environment on its spatial and seasonal distribution with on average more micronekton in the south than in the north, and more in winter than in summer. The species composing the micronecton are riche in crustaceans in the north and in cephalopods and fish in the south. The micronecton abundance would tend to decrease under the influence of climate change. Finally, we showed that the spatial distribution of this group influenced the presence of six groups of predators: yellowfin tuna, albacore tuna, dolphin, dolphin, shearwater and red-footed body., L'écosystème océanique Néo-Calédonien dans le Pacifique sud-ouest contient une forte diversité de prédateurs, comme les oiseaux marins ou les cétacés. La récente création du Parc naturel de la mer de corail a ouvert un besoin d'informations solides sur le fonctionnement de cet écosystème remarquable, notamment sur la dynamique du micronecton (organismes entre 1 et 20cm constituant la nourriture des prédateurs supérieurs) et de son rôle central dans les réseaux alimentaires, d'autant plus que ce maillon est le plus méconnu. Dans ce contexte, cette thèse a montré une forte influence de l’environnement sur sa distribution spatiale et saisonnière avec en moyenne plus de micronecton dans le sud que dans le nord, et plus en hiver qu’en été. Les espèces composant le micronecton sont plus riches en crustacés au nord en céphalopodes et poissons au sud. L’abondance de micronecton aurait tendance à diminuer sous l’influence du changement climatique. Enfin, nous avons montré que la répartition spatiale de ce groupe influençait la présence de six groupes de prédateurs : thon jaune, thon germon, dorade coryphène, dauphins, puffin et fou à pieds rouges.

The stable carbon (delta C-13) and nitrogen (delta N-15) isotope values of soft tissues of micronekton (crustaceans, squid, mesopelagic fish) and zooplankton were measured from organisms collected on the RV Antea at two seamounts located in the south-western Indian Ocean: La Perouse (summit depth similar to 60 m) and "MAD-Ridge" (thus named in this study; summit depth similar to 240 m). Surface particulate organic matter (POM-Surf) showed higher delta C-13 at the more productive MAD-Ridge than at the oligotrophic La Perouse seamount. Particulate organic matter and zooplankton were depleted in N-15 at La Pemuse pinnacle compared with MAD-Ridge. Gelatinous organisms and crustaceans occupied the lowest and intermediate tmphic levels (TL similar to 2 and 3 respectively) at both seamounts. Mesopelagic fish and smaller-sized squid sampled at both seamounts occupied TL similar to 3 to 4, whereas the large nektonic squid, Ommastrephes bartramii, collected at MAD-Ridge only, exhibited a TL of similar to 5. The delta N-15 values of common open-water mesopelagic taxa were strongly influenced by specimen size and feeding habits at both seamounts, with an increase in delta N-15 values with increasing size. Carnivorous fish species sampled exclusively over the seamounts' flanks and summits exhibited TL values of similar to 4, irrespective of their wide size ranges. The work could not demonstrate any differences in delta C-13 values of mesopelagic fish between the seamounts and the surrounding oceanic areas. The study segregated clusters of mesopelagic organisms according to their delta C-13 and delta N-15 values, with variations in stable isotope values reflecting a complex range of processes possibly linked to productivity as well as biological and ecological traits of the species (size and feeding mode).

Spatial distribution, community composition and trophic roles of micronekton (crustaceans, fishes and squids) were investigated in the Indian South Subtropical Gyre (ISSG) province and the East African Coastal province (EAFR), by combining acoustic surveys, mid-water trawls and stable isotope analyses from scientific cruises conducted in 2009 and 2010. Mesopelagic micronekton performed diel vertical migrations in both provinces, from deep (400–740 m) to surface (0–200 m) layers at dusk and in the opposite direction at dawn, with some species migrating below 740 m. The EAFR province was more dynamic than the oligotrophic ISSG province, with enhanced eddy activity and enhanced yearly productivity. The active enrichment mechanisms in the EAFR, in terms of available primary production, led to high micronekton acoustic density (as a proxy of micronekton abundance) and large micronekton weight and abundance estimates from trawl data. Particulate organic matter in the EAFR exhibited greater enrichment in 13 C and 15 N compared to the ISSG and, consequently, tissues of selected micronekton organisms in the EAFR were more enriched in 15 N (higher δ 15 N values). In both provinces, micronekton encompassed a wide range of isotopic niches, with large overlaps between species. Micronekton and swordfish in the EAFR had an overlapping range of δ 15 N values, contrasting with the ISSG province where swordfish were two trophic levels higher than the sampled micronekton. Our results provide some evidence that the combined action of riverine input and the dynamics of eddies might influence productivity in the EAFR, and hence the abundance of micronekton and the enrichment of tissues in 15 N, compared to the oligotrophic ISSG province.

Scattering structures, including deep (>200 m) scattering layers are common in most oceans, but have not previously been properly documented in the Arctic Ocean. In this work, we combine acoustic data for distribution and abundance estimation of zooplankton and fish with biological sampling from the region west and north of Svalbard, to examine high latitude meso- and epipelagic scattering layers and their biological constituents. Our results show that typically, there was strong patchy scattering in the upper part of the epipelagic zone ( 440 m) in the Arctic habitat north of Svalbard compared to those south in the Fram Strait west of Svalbard (WMD ~400 m). The surface integrated backscatter [Nautical Area-Scattering Coefficient, NASC, sA (m2 nmi−2)] was considerably lower in the waters around Svalbard compared to the more southern regions (62–69°N). Also, the integrated DSL nautical area scattering coefficient was a factor of ~6–10 lower around Svalbard compared to the areas in the south-eastern part of the Norwegian Sea ~62°30′N. The documented patterns and structures, particularly the DSL and its constituents, will be key reference points for understanding and quantifying future changes in the pelagic ecosystem at the entrance to the Arctic Ocean.

Knowledge of the dynamics of micronekton at seamounts is critical to understanding the ecological role of these ecosystems. Active acoustic techniques are an effective tool to monitor the distribution and movements of pelagic organisms. We carried out several day- and nighttime active acoustic surveys over a 3-year period (2009-2011) to characterize the spatial and temporal distribution of micronekton backscatter on two seamounts (Condor and Gigante) in the Azores and in the surrounding open-waters. The highest mean volume backscattering strength (MVBS) was consistently found in the water column over the seamount summits, regardless of the season and diel period. MVBS over the summits was 14 to 26 times higher than over the slopes, and 10 times higher than in open-waters. Diel variations in backscatter intensity were more pronounced in open-waters and in Gigante seamount, with higher values during the day in open-waters, and at night over the summits and slopes of Gigante. Over Condor seamount, diel changes in backscatter intensity were small, but MVBS was generally higher at night than during the day, as in Gigante. Persistence of strong acoustic backscatter over the summits of Condor and Gigante seamounts is a key finding of this study and may be explained by the presence of a seamount-associated micronekton community and by the retention of vertically migrating micronekton. The latter hypothesis is consistent with observed day-night differences in backscatter, suggesting that nocturnal migrants may be passively transported or actively swim above seamount summits and slopes. Possible physical mechanisms leading to the observed patterns in micronekton distribution are discussed. This study contributes to a better understanding of how seamounts may influence the spatial and temporal dynamics of micronekton assemblages.

From March 18 to April 2, 2017, the PUFFAlis expedition took place aboard research vessel Alis of IRD. This campaign was a continuation of previous oceanographic expeditions concerned with the feeding areas of seabirds (MOMAlis) as well as on land on the Gouaro Deva shearwater colony. It was also the extension of the NECTAlis campaigns that were dedicated at studying the oceanic trophic chain that leads to tunas. Six scientists embarked to sample the different levels of the oceanic trophic chain of the New Caledonia basin off Pindai. In the middle of the expedition, a day was devoted to communicating with the high school students of Pouembout and the services of the environment and fisheries of the Northern Province. The expedition's logbook is preceded by the press release issued on this occasion.; Du 18 mars au 02 avril 2017 s'est déroulée la mission PUFFAlis à bord du navire océanographique Alis de l'IRD. Cette campagne s'inscrit dans la continuité d'expéditions océanographiques centrées sur les zones d'alimentation des oiseaux marins, effectuées précédemment en mer (MOMAlis) comme à terre sur la colonie de puffins de Gouaro Deva. Elle est aussi la prolongation des campagnes NECTAlis destinées à étudier la chaîne trophique océanique qui conduit aux thons. Six scientifiques ont embarqué pour échantillonner les différents niveaux de la chaîne trophique océanique du bassin de Nouvelle-Calédonie au large de Pindaï. En milieu de mission, une journée a été consacrée à la communication avec les lycéens de Pouembout et les services de l'environnement et des pêches de la Province nord. Le journal de bord de la mission est précédé du communiqué de presse diffusé à cette occasion.

We use zooplankton and ichthyoplankton data from the ~ 60-year CalCOFI time series to examine relationships of mesopelagic (. i.e. midwater) fishes in the California Current System with midwater predators, potential competitors (epipelagic planktivorous fishes) and zooplankton prey, within the context of local and basin-scale oceanography. Equilibrium-based near-steady state models and the "wasp-waist" paradigm for eastern boundary currents predict tightly-coupled trophic interactions, with negative correlations between the abundance of planktivorous competitors and between dominant planktivores and their prey. Testing these hypotheses with the CalCOFI time series, we found them to be generally invalid. Potential competitors within the mesopelagic community (planktivorous vertical migrators (VMs) and non-migrators (NMs)) were highly positively correlated, as were these groups with the mesopelagic piscivores (. e.g. dragonfishes) that prey on them. In addition, the abundance of VMs was mostly positively correlated with that of epipelagic planktivores, such as anchovy, mackerels and hake. The VMs and epipelagic planktivores were negatively correlated with key potential planktonic prey groups, indicating a lack of bottom-up forcing. However, neither do these negative correlations appear to signify top-down forcing, since they seem to be mediated through correlations with key environmental drivers, such as the Pacific Decadal Oscillation (PDO), sea surface temperature, and the relative strength of the California Current. We suggest that the web of correlations linking key meso- and epipelagic planktivores, their predators and prey is mediated through common links with basin-scale oceanographic drivers, such as the PDO and ENSO cycles. Thus, the abundance of mesopelagic fishes in the California Current is closely tied to variation in the oxygen minimum zone, whose dynamics have been linked to the PDO. The PDO and other drivers are also linked to the transport of the California Current System, which influences the abundance of many dominant taxa off southern California that have broad biogeographic distributions linked to water masses that extend to the north (Transition Zone/sub-Arctic faunas) or the south (tropical/subtropical faunas). © 2013 Elsevier B.V.

14 pages, 9 figures, 5 tables, Surface waters are an attractive foraging ground for small fish in the open ocean. This study aims to determine the importance of vertically migrating species in the neuston of oceanic waters across the Atlantic, Indian and Pacific Oceans and to ascertain the influence of environmental variables on their distribution patterns. Neustonic fish assemblages were primarily controlled by light. They were dominated by late-larvae and juveniles of Exocoetidae, Hemiramphidae and Scomberesocidae during the day. At night, the vertical migration of mesopelagic species changed the dominance pattern in favour of Myctophidae and Scomberesocidae. The neustonic families’ distribution was primarily related to sea surface temperatures, whereas environmental variables at deeper layers were related to mesopelagic migrating families. Canonical correspondence analysis showed a low but statistically significant contribution of several environmental variables to myctophid species composition (10%), with minimum oxygen concentrations ranking first in variance explanation followed by maximum fluorescence, sea surface temperature and 400-m temperature. Spatial autocorrelation also explained 17% of the variance, indicating the influence of other factors such as historical, demographic and dispersal constraints. The low number of myctophid species in the North Pacific Equatorial Countercurrent appears to be related to the low oxygen concentrations observed in this province, This study was financed by the Spanish Ministry of Economy and Competitiveness (Consolider-Ingenio 2010, CSD2008-00077 and CTM2012–39587-C04–03)

Zooplankton and micronecton are the first marine trophic levels. Different by their size (200μm to 2mm for zooplankton, 2 to 20cm for micronekton), this two groups undergo diel vertical migration from depth by day to the surface during the night. These migrations create major organic matter fluxes between the deep ocean and the surface. Biogeochemical cycles are of great importance for climate change studies. These studies are conducted with ocean global circulation model and biogeochemical model. The way to go is develop low and mid-trophic level modelling approaches. SEAPODYM ensemble of models are three parsimonious model of biomass at diverse level of the trophic chain, from zooplankton to top predators. This thesis introduce the zooplankton biomass model SEAPODYM-LTL (lower trophic level) and a forcing fields sensitivity analysis. Indeed, these model are forced off line by currents, temperature and primary production fields produced by other models. SEAPODYM-LTL has also been compared to PISCES (NPZD) and both have similar performance score in this study. In order to improve SEAPODYM-MTL (mid trophic level) predictions, a data assimilation framework has been developed to find a better parameterisation. 38kHz active acoustic data have been used to improve the model. This methodology has been develop thanks to a test case that we present in this thesis. The gathered acoustic dataset permitted to show the need of a better definition of vertical layer depths. It has been developed using the acoustic dataset. The related study is presented in this thesis., Le zooplancton et le micronecton sont les deux premiers échelons animaux de la chaine trophique marine. Bien que de tailles très différentes (200μm à 2mm pour le zooplancton, 2 à 20cm pour le micronecton), ces deux groupes d'espèces variées partagent un comportement singulier : les migrations nycthémérales. Ces migrations journalières entre la profondeur de jour et la surface de nuit induisent des flux de matière organique très importants entre les différentes profondeurs de l'océan. L'étude des cycles biogéochimiques océaniques a une grande importance pour l'étude du changement climatique. Cette étude est notamment conduite à travers le développement de modèles globaux de circulation océanique et de biogéochimie. La suite logique de ces développements est donc la modélisation du zooplancton et du micronecton. La gamme de modèles SEAPODYM modélise avec parcimonie la chaine trophique depuis le zooplancton jusqu'aux prédateurs supérieurs à l'aide de trois modèles. Cette thèse présente le modèle de biomasse de zooplancton SEAPODYM-LTL (pour lower trophic level, niveau trophique bas), ainsi qu'une analyse de sa sensibilité aux forçages. En effet, la particularité de ces modèles est leur forçage offline par des champs de courants, température et production primaire produits par d'autres modèles. Le modèle SEAPODYM-LTL est également comparé au modèle PISCES (NPZD), et présente des performances similaires à ce dernier dans le cas testé. Afin d'améliorer les prédictions du modèle SEAPODYM-MTL (mid-trophic level, i.e. le modèle de biomasse de micronecton), une méthodologie d'assimilation de données a été mise en place pour affiner la paramétrisation utilisée. Des données d'acoustique active (38kHz) sont donc utilisées pour enrichir le modèle. Cette méthodologie a été conçue autour d'un cas test présenté dans cette thèse. L'extension du jeu de données acoustiques assimilées au modèle a permis de mettre en évidence le besoin de mieux modéliser les profondeurs des couches verticales de SEAPODYM. Cela a été réalisé à l'aide du jeu de données acoustiques évoqué précédemment. Cette étude est également présentée dans cette thèse.

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.

10 pages, 6 figures, 4 tables, Food web structure of the macroplankton/micronekton fauna on the continental slope of the Catalan Sea (Balearic basin, NW Mediterranean) was investigated using carbon and nitrogen stable isotope tracers on a total of 34 taxa. Samples were collected close to Barcelona, Spain, on the middle slope, at a seasonal scale. Mean δ13C values ranged from −22.1‰ (Salpa maxima) to −16.9‰ (the mysid Eucopia hanseni). Values of δ15N ranged from 2.5‰ (the hyperiid Vibilia armata) to 9.8‰ (the pelagic polychaete Tomopteris sp.). The stable isotope ratios of this fauna displayed a continuum of values over the δ15N range of 7‰, confirming a wide spectrum of feeding strategies (from filter feeders to predators). High annual mean δ15N values were found among carnivorous large zooplankton and micronekton, including species that prey on gelatinous plankton (i.e. salps, siphonophores), euphausiids, natantian decapod crustaceans and fish (i.e. myctophids and stomiiformes). In agreement with the available information on diets of planktonic taxa, the lowest isotope ratios were found for filter feeders (V. armata, S. maxima, the pteropods Cymbulia peroni and Cavolinia inflexa, ostracods and the thaliacean Pyrosoma atlanticum), all of which feed on particulate organic matter. We found three trophic levels in macroplankton/micronekton food webs based on a 15N-enrichment factor of ~2.5‰per level. The range of δ13C was particularly wide among carnivores (−20.7‰ to −16.6‰), suggesting predation on a variety of prey from gelatinous zooplankton (which displayed more depleted δ13C signatures) to small fishes and decapods. Correlation between δ13C–δ15N was generally weak, likely due to the consumption of different kinds of sinking particles (e.g. marine snow, phytodetritus), some constituted of multiply recycled particulate organic matter (POM). However, higher δ13C–δ15N correlations were observed during winter and spring, periods of water column homogenization, suggesting that the planktonic community assimilates pulses of new production from the photic zone (peaking in January–February). Low correlations were observed during periods of water column stratification, particularly in summer, when production is especially low, suggesting that in this period macroplankton–micronekton community rely on sources other than surface primary production such as POM derived from river discharge, This study was carried out within the framework of the Spanish funded BIOMARE projects (ref. CTM2006-13508-CO2-02/MAR: Identification of BIOmarkers of the anthropogenic impact on MARine communities: an Ecosystemic approach) and ANTROMARE (ref. CTM2009-12214-C02-01/MAR)

In this review we focus on research performed by Italian scientists on pelagic communities, from microzooplankton to micronekton, mainly in the Italian Seas. We considered published data, mostly as grey literature, and unpublished ones. Firstly we describe data collected over a time span of more than 30 years, during several cruises all around the Italian peninsula on zooplankton composition and distribution. We identified rare vs. common species, which enhanced biodiversity of the pelagic ecosystem. Time series, some also very long, allowed us to describe seasonal recurrent patterns, interannual fluctuations and recent shifts driven by climatic changes. More recently Italian researches were processes oriented and we analyzed results obtained on the impact of predation of both micro- and mesozooplankton on both autotrophic and heterotrophic preys. Carbon fluxes through zooplankton components were variable in space and time, but accounted for important phytoplankton losses, and when this resource became scarce they relied on heterotrophic production. Through respiration measurements of mesozooplankton another aspect of the C flux was estimated showing an increase in C demand in the most oligotrophic area. Egg production by copepods appeared to be mostly controlled by temperature and quantity/quality of available food.

Oceanic mesoscale circulation is a crucial structuring force in the marine environment. Dynamic processes associated with eddies, such as eddy-induced upwelling or eddy-eddy interac- tion, drive the transport and distribution of nutrients that support the whole food chain, presumably through bottom-up processes. Eddies can shape the distribution of organisms at both low (phyto- plankton, zooplankton and fish larvae) and high trophic levels (top fish predators, seabirds or turtles), but the impact of mesoscale features on intermediate trophic levels (micronekton) remains poorly understood. We analysed the influence of eddies on the distribution of micronekton aggregations in the Mozambique Channel by combining data from acoustic surveys and satellite sea topography. We demonstrated that large aggregations of micronekton occurred mainly in areas where the local horizontal gradient of sea level anomalies is strong, i.e. at the periphery of eddies. We observed that, in this region, eddies running along the coast advect coastal nutrient-rich waters at their edges, which support the base of the food chain. We propose that eddies can shape the distribution and the aggregation patterns of the prey of marine top predators through bottom-up processes.

Because of logistic difficulties and technical complexities, experimental study on the metabolism of deep-sea zooplankton had been lagged far behind those of epipelagic zooplankton until the observation of lower oxygen consumption rates of deeper-living mysids, euphausiids, decapods and micronektonic fishes in early 1970s. Since then, many studies followed and confirmed the pattern. In situ observations with submersibles revealed that many deep-sea pelagic fishes and large crustaceans are "lethargic", which was postulated to be due to extremely higher water contents or lower muscle (protein or nitrogen) contents in their bodies. According to "Visual Interactions Hypothesis (VIH)", reduction in metabolism seen in deep-sea pelagic crustaceans, fishes and cephalopods with image-forming eyes are a result from the relaxation of selection for strong swimming abilities (which accompany with high metabolic rates) for both predators and prey as the visual reactive distances shorten with increasing depth. For zooplank-ton taxa lack image-forming eyes (copepods, chaetognaths, medusae etc.) the VIH predicts neither depth-related changes in the metabolism nor chemical composition. However, recent studies on metabolism, chemical composition (N contents) and protein synthesis activity (RNA: DNA ratios) of pelagic copepods revealed the depth-related pattern similar to that of taxa with image-forming eyes, leading an alternative "Predation-Mediated Selection Hypothesis (PMSH)". PMSH explains depth-related reduction in metabolism to be a result of lowered selective pressure of high activity toward depth in all taxa with and without eyes. Differences between VIH and PMSH are discussed in the light of rational of the definition of the depth of occurrence of zooplankton ("the minimum depth of occurrence" or "mid-capture depth") and expression of metabolic rates (wet mass specific rate or N-mass specific rate). In terms of metabolic rate of animals standardized to a body mass of 1 mg body Nat 1°C (assumed Q10=2), oxygen consumption rates of diverse pelagic taxa including large crustaceans, fishes, cephalopods and copepods from 500-2,000 m depth are shown to converge at 2-3μ1 O2 mg N^-0.8 h^-1. As an extension of PMSH, a global-bathymetric model for oxygen consumption by copepods with four parameters (body mass, temperature, depth, oxygen saturation) was shown to be superior to casual global model for general epipelagic zooplankton with two parameters (body mass, temperature) to predict observed oxygen consump-tion rates of diverse deep-sea zooplankton species. Considering large regional variations in the deep-sea environments of the world oceans, the data of deep-sea zooplankton without eyes presently available represent only from two regions (off south California and Oyashio region, western subarctic Pacific). For the validation of VIH or PMSH, there is an urgent need to collect more metabolic data on zooplankton from diverse deep-sea environments in future study.

During three Antarctic expeditions (2004, ANT XXI-4 and XXII-2; 2006, ANT XXIII-6) with the German research icebreaker R/V Polarstern, six different amphipod species were recorded under the pack ice of the Weddell Sea and the Lazarev Sea. These cruises covered Austral autumn (April), summer (December) and winter (August) situations, respectively. Five of the amphipod species recorded here belong to the family Eusiridae (Eusirus antarcticus, E. laticarpus, E. microps, E. perdentatus and E. tridentatus), while the last belongs to the Lysianassidea, genus Cheirimedon (cf. femoratus). Sampling was performed by a specially designed under-ice trawl in the Lazarev Sea, whereas in the Weddell Sea sampling was done by scuba divers and deployment of baited traps. In the Weddell Sea, individuals of E. antarcticus and E. tridentatus were repeatedly observed in situ during under-ice dives, and single individuals were even found in the infiltration layer. Also in aquarium observations, individuals of E. antarcticus and E tridentatus attached themselves readily to sea ice. Feeding experiments on E antarcticus and E. tridentatus indicated a carnivorous diet. Individuals of the Lysianassoid Cheirimedon were only collected in baited traps there. Repeated conventional zooplankton hauls performed in parallel to this study did not record any of these amphipods from the water column. In the Lazarev Sea, E. microps, E. perdentatus and E laticarpus were regularly found in under-ice trawls. We discuss the origin and possible sympagic life style of these amphipods. (C) 2008 Elsevier Ltd. All rights reserved.

Fisheries acoustic data are continuously collected during ecosystemic surveys to detect fish schools of commercial species and to quantify the fish abundance. These multifrequency data also contain information about smaller target aggregations producing ubiquitous Sound Scattering Layers (SSLs) on the echograms, with quite diverse shape, frequency responses and taxonomic compositions. In this work, we focus on the SSLs observed in spring in the Bay of Biscay during the PELGAS surveys. The objectives are: i) to classify SSLs, ii) to describe their spatial meso-scale distribution, iii) to investigate their relationships with other ecosystem components, and iv) to investigate the taxonomic composition of resonant SSLs at fine scale. A Multiple Correspondence Analysis based method for non supervised echoes classification is applied to PELGAS multifrequency data to define constrasted and spatially structured “acoustic landscapes”. Acoustic landscapes are dominated by frequency responses of gas-bearing organisms, whose gas inclusion might resonate at the fisheries acoustic frequencies. No correlation is found between the spatial distributions of acoustic landscapes and those of other pelagic ecosystem components indices. The biological composition of resonant SSLs is investigated by jointly applying forward and inverse approaches to fine scale acoustic and biological data collected in the same dense SSL. Results suggest that the dense SSLs may be comprised of fish larvae (e.g. Sardina pilchardus) and larger micronektonic gas-bearing organims that are so far very poorly sampled by non-acoustic devices., Des données acoustiques sont collectées en continu lors de campagnes écosystémiques pour détecter les bancs de poissons d’espèces commerciales et en quantifier l’abondance. Ces données multifréquences contiennent également des informations sur des agrégations de cibles plus petites qui se présentent sous forme de couches diffusantes. Il existe une grande diversité de couches diffusantes dont l’aspect sur les échogrammes, la réponse fréquentielle et la composition spécifique varient. L'objectif de cette thèse est de définir les types de couches diffusantes rencontrées lors de la campagne PELGAS, de décrire leurs distributions spatiales à méso-échelle, de les comparer à celles des autres compartiments de l'écosystème, et d'examiner la composition taxonomique des couches diffusantes résonantes à fine échelle. Une méthode de classification non supervisée des échos appliquée aux données PELGAS permet de définir des « paysages acoustiques » structurés spatialement. Ces paysages acoustiques sont dominés par des réflecteurs porteurs de bulles de gaz résonnant aux fréquences halieutiques et ne sont corrélés à aucun compartiment de l'écosystème pélagique issus de PELGAS2013. La composition biologique de couches diffusantes denses est étudiée à fine échelle en combinant par des approches directe et inverse les résultats de leur échantillonnage acoustique et biologique. Les résultats montrent que la réponse acoustique des couches diffusantes denses est dominée par celle de bulles de gaz incluses dans des larves de poissons (e.g. Sardina pilchardus}), et dans des organismes micronectoniques, qui sont peu ou pas échantillonnés avec les outils de collecte biologique.

International audience; We review present understanding of the spatial and temporal diet variability (trophodynamics) of a range of pelagic marine top predators, at both early and adult life history stages. We begin with a review of methodologies used to advance our understanding of the trophodynamics of marine top predators, particularly in relation to climate change. We then explore how these developments are informing our understanding of the major trophic groups in food webs leading to, and including, marine top predators. We examine through specific examples how the impacts of ocean warming may affect pelagic food web relationships from both top-down and bottom-up perspectives. We examine the potential, in the absence of long-term data sets, of using large-scale spatial studies to examine how potential changes in biological oceanography could impact the biomass and composition of prey species, particularly the role of phytoplankton size spectra. We focus on examples from regions where biotic change with respect to climate change is likely. In particular, we detail the effects of climate change on oceanographic and bathymetric "hotspots" and provide the example involving seabirds in the Benguela Current system. We end by urging the development of international collaborations and databases to facilitate comprehensive ocean-scale understanding of climate impacts on marine top predators.

International audience; This study investigated the food web structure of the oligotrophic picophytoplankton-dominated pelagic ecosystem in the vicinity of New Caledonia, within the Archipelagic Deep Basin (ARCH) province of the southwest sub-tropical Pacific. Nitrogen stable isotope (delta N-15) data were collected for mesozooplankton (0.2-2 mm), macrozooplankton (2-20 mm), micronelcton (20-200 mm) and nekton ( >200 mm) during 2002-2004 and 2011. Using a coupled delta N-15 size-spectrum approach, we estimated (1) organism trophic level (TL); (2) food chain length (FCL); (3) predator prey mass ratio (PPMR); and (4) transfer efficiency (TE). The role of phytoplankton size structure in determining these parameters was investigated. Applying atrophic enrichment factor (TEF) of 3.4, maximum TL was calculated at similar to 5. The number of TLs spanned by each length class was 1.97 for mesozooplankton, 2.07 for macrozooplankton, 2.75 for micronekton, and 2.21 for nekton. Estimated PPMR was 10,099:1 for mesozooplankton, 3683:1 for macrozooplankton/micronekton, and 2.44 x 10(5):1 for nekton, corresponding to TEs of 6.3%, 8.5% and 2.4%, respectively. PPMR and TE were strongly influenced by the TEF used, and TEF 3.4 likely over and underestimated PPMR and TE, respectively, for mesozooplankton and macrozooplankton/micronekton. Comparatively low PPMR for mesozooplankton and macrozooplankton/micronekton indicated longer food chains and higher connectivity within these groups than for the nekton. Conversely, the high PPMR yet high trophic niche width for the nekton indicated that they prey primarily on macrozooplankton/micronekton, with a relatively high degree of dietary specialisation. Our results are discussed in the context of other marine food webs. The ARCH food chain was found to be 1-1.5 trophic levels longer than the eutrophic micro-/nanophytoplankton-dominated Californian upwelling system, providing empirical support for the role of phytoplankton size in determining FCL Group specific PPMR estimates demonstrated that it is changes in trophic pathways across the mesozooplankton/macrozooplankton/ micronekton groups that are primarily responsible for higher FCL under oligotrophic conditions. Finally, we discuss consistently low delta N-15 values to the east of New Caledonia, and implications for the contribution of diazotroph nitrogen to the pelagic food web in this region.