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Auteur Annasawmy, P.; Ternon, J.-F.; Lebourges-Dhaussy, A.; Roudaut, G.; Cotel, P.; Herbette, S.; Menard, F.; Marsac, F.
Titre Micronekton distribution as influenced by mesoscale eddies, Madagascar shelf and shallow seamounts in the south-western Indian Ocean: an acoustic approach Type Article scientifique
Année 2020 Publication Revue Abrégée Deep-Sea Res. Part II-Top. Stud. Oceanogr.
Volume (down) 176 Numéro Pages 104812
Mots-Clés biological production; circulation; community composition; deep-scattering layer; diel vertical migration; Diel vertical migration; dynamics; Madagascar shelf; mesopelagic fish; Mesoscale eddies; Micronekton; mozambique channel; Multi-frequency; myctophid fishes; Seamount; South-western indian ocean; zooplankton
Résumé 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 similar to 240 m) thereafter named “MAD-Ridge”, and at La Perouse seamount (similar to 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 Perouse 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 Perouse. 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.
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Numéro d'Appel MARBEC @ isabelle.vidal-ayouba @ collection 2836
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Auteur Demarcq, H.; Noyon, M.; Roberts, M.J.
Titre Satellite observations of phytoplankton enrichments around seamounts in the South West Indian Ocean with a special focus on the Walters Shoal Type Article scientifique
Année 2020 Publication Revue Abrégée Deep-Sea Res. Part II-Top. Stud. Oceanogr.
Volume (down) 176 Numéro Pages 104800
Mots-Clés chlorophyll; Chlorophyll-a; currents; Deep chlorophyll maximum; ecology; flow; Geostrophic currents; model; MODIS-Aqua; Remote sensing; swig; zooplankton
Résumé Spatial anomalies associated with seamounts and islands have been detected in the South West Indian Ocean using a new surface chlorophyll-a based enrichment index (EI). Calculated from daily L3 4-km MODIS chl-a data from 2003 to 2018, the EI was used to quantify recurrent (seasonal) surface enrichment associated with these features. Most notable were the shallow Walters Shoal (18 m) and nearby deeper WS-2 seamount (480 m), both located on the southern Madagascar Ridge, which showed high EI values of 40% and 15% local enhancement. Tmmelin Island, east of Madagascar, exhibited more moderate values of 15% local increase. Other shallower seamounts including La Perouse (60 m) and MAD-Ridge (240 m) exhibited sporadic or no measurable surface maxima. Regions of strong mesoscale activity such as south of Madagascar where the South East Madagascar Current detaches from the continent also revealed high EI values. A marked seasonality of the EI was observed over the Walters Shoal, with higher values during the oligotrophic season than in the austral winter. The seasonal variability of the MLD in combination with the seamount bathymetry appears to induce the thermal and biological anomalies observed there, possibly favoured by the presence of a Taylor cap at the summit. Ship-collected in situ measurements showed the satellite observed enrichment to be associated with shallowing of the deep chlorophyll maximum, as well as a shift of phytoplankton groups towards diatoms and small flagellates. No net increase of integrated chl-a was measured but a potential doubling in primary productivity is expected. The EI developed in this study, designed to reveal sporadic, small localised chl-a maxima, is likely applicable to other areas in the ocean where there is local enrichments.
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Numéro d'Appel MARBEC @ isabelle.vidal-ayouba @ collection 2841
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Auteur Marsac, F.; Annasawmy, P.; Noyon, M.; Demarcq, H.; Soria, M.; Rabearisoa, N.; Bach, P.; Cherel, Y.; Grelet, J.; Romanov, E.
Titre Seamount effect on circulation and distribution of ocean taxa in the vicinity of La Perouse, a shallow seamount in the southwestern Indian Ocean Type Article scientifique
Année 2020 Publication Revue Abrégée Deep-Sea Res. Part II-Top. Stud. Oceanogr.
Volume (down) 176 Numéro Pages 104806
Mots-Clés acoustics; Bathymetry; behavior; chlorophyll; diet; humpback whales; l-adcp; Mesoscale eddies; Mesozooplankton; micronekton; Micronekton; migration; Pelagic longline fisheries; Stomach contents; swordfish xiphias-gladius; tuna; zooplankton
Résumé The La Perouse seamount (60 m depth) has so far been poorly studied despite it being a short distance (160 km) from Reunion 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 Reunion. 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 Reunion, seemingly in association with a deep (900 m) topographic feature. Yet the largest catch is not directly associated with La Perouse which would be too shallow for top predators to aggregate around in the long term. Enhanced levels of phytoplankton or zooplankton enrichment at La Perouse 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.
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ISSN 0967-0645 ISBN Médium
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Notes WOS:000556810400009 Approuvé pas de
Numéro d'Appel MARBEC @ isabelle.vidal-ayouba @ collection 2850
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Auteur Noyon, M.; Rasoloarijao, Z.; Huggett, J.; Ternon, J.-F.; Roberts, M.
Titre Comparison of mesozooplankton communities at three shallow seamounts in the South West Indian Ocean Type Article scientifique
Année 2020 Publication Revue Abrégée Deep-Sea Res. Part II-Top. Stud. Oceanogr.
Volume (down) 176 Numéro Pages 104759
Mots-Clés biomass size spectrum; chlorophyll-a; cyclonic eddy; eddies; leeuwin current; Mesoscale eddies; Normalised biovolume size spectrum (NBSS); north pacific; Oligotrophic environment; patterns; Pelagic ecology; plankton; temperature; Topography; zooplankton distribution
Résumé Seamounts are recognised as hotspots of biodiversity, attracting large numbers of top predators, but the underlying mechanisms are still unclear. We studied mesozooplankton abundance and size distribution at three shallow seamounts (60 m, 240 m and 18 m deep) in the South West Indian Ocean, along a latitudinal gradient (19 degrees S, 27 degrees S and 33 degrees S). Samples were analysed using a ZooScan, allowing the use of a size-based approach. Differences were observed between seamount areas, but overall zooplankton communities did not seem to be affected by the changes in topography. Only in the lee of La Perouse seamount was the zooplankton community slightly more concentrated than upstream, suggesting that zooplankton were flushed downstream of the seamount. The southernmost and shallowest seamount, Walters Shoal, had low abundance and its size spectrum differed greatly from the two other seamounts further north. These differences were attributed to seasonality and mesozooplankton population dynamics, whereas the other two seamounts exhibited a more “typical” oligotrophic pelagic ecosystem, at equilibrium and dominated by small organisms. At the time of sampling, the unnamed seamount south of Madagascar was influenced by a mesoscale dipole that impacted the zooplankton distribution, potentially masking any seamount effect. The normalised biomass size spectrum approach contributed to a better understanding of the ecosystem dynamics (i.e. equilibrium vs. non-steady state) but revealed little variability within a stable oligotrophic environment.
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Langue English Langue du Résumé Titre Original
Éditeur de collection Titre de collection Titre de collection Abrégé
Volume de collection Numéro de collection Edition
ISSN 0967-0645 ISBN Médium
Région Expédition Conférence
Notes WOS:000556810400008 Approuvé pas de
Numéro d'Appel MARBEC @ isabelle.vidal-ayouba @ collection 2853
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Auteur Arones, K.; Grados, D.; Ayon, P.; Bertrand, A.
Titre Spatio-temporal trends in zooplankton biomass in the northern Humboldt current system off Peru from 1961-2012 Type Article scientifique
Année 2019 Publication Revue Abrégée Deep-Sea Res. Part II-Top. Stud. Oceanogr.
Volume (down) 169 Numéro Pages Unsp-104656
Mots-Clés anchovy engraulis-ringens; chile; current ecosystem; Decadal trends; diet; Macrozooplankton; Mesozooplankton; Regime-shift; sardine; Secondary production; Spatiotemporal distribution; volume; zone
Résumé Anchovy (Engraulis ringens) is the most important exploited fish species in the Northern Humboldt Current System (NHCS) off Peru. This species, as well as most other pelagic resources, mainly forage on zooplankton. The NHCS is bottom-up controlled at a variety of scales. Therefore, fish biomass is driven by the abundance of their prey. In this context, we studied the spatiotemporal patterns of zooplankton biomass in the NHCS from 1961-2012. Data were collected with Hensen net all along the Peruvian coast. To transform zooplankton biovolume into biomass we used a regression that was calibrated from 145 zooplankton samples collected during four surveys and, for which, precise information was available on both biovolume and wet weight. The regression model was then applied on a time-series encompassing 158 cruises performed by the Peruvian Institute of the Sea (IMARPE) between 1961 and 2012. We observed a clear multidecadal pattern and two regime shifts, in 1973 and 1992. Maximum biomass occurred between 1961 and 1973 (61.5 g m(-2)). The lowest biomass (17.8 g m(-2)) occurred between 1974 and 1992. Finally, the biomass increased after 1993 (26.6 g m(-2)) but without reaching the levels observed before 1973. A seasonal pattern was observed with significantly more biomass in spring than in other seasons. Spatially, zooplankton biomass was higher offshore and in northern and southern Peru. Interestingly, the zooplankton sampling was performed using classic zooplankton net that are well fitted to mesozooplankton and are known to underestimate the macrozooplankton; however, the spatiotemporal patterns we observed are consistent with those of macrozooplankton, in particular euphausiids. This suggests that in the NHCS, when and where macrozooplankton dominates it also dominates the biomass obtained using classic zooplankton net samples. Finally, until now, in the NHCS only time-series on zooplankton biovolume were available. The biomass data we provide are more directly usable in trophic or end-to-end models.
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Editeur Lieu de Publication Éditeur
Langue English Langue du Résumé Titre Original
Éditeur de collection Titre de collection Titre de collection Abrégé
Volume de collection Numéro de collection Edition
ISSN 0967-0645 ISBN Médium
Région Expédition Conférence
Notes WOS:000504782200009 Approuvé pas de
Numéro d'Appel MARBEC @ isabelle.vidal-ayouba @ collection 2693
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