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Annasawmy, P., Cherel, Y., Romanov, E., Le Loc'h, F., Menard, F., Ternon, J. - F., et al. (2020). Stable isotope patterns of mesopelagic communities over two shallow seamounts of the south-western Indian Ocean. Deep-Sea Res. Part II-Top. Stud. Oceanogr., 176, 104804.
Résumé: 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).
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Annasawmy, P., Ternon, J. F., Marsac, F., Cherel, Y., Behagle, N., Roudaut, G., et al. (2018). Micronekton diel migration, community composition and trophic position within two biogeochemical provinces of the South West Indian Ocean: Insight from acoustics and stable isotopes. Deep-Sea Res. Part I-Oceanogr. Res. Pap., 138, 85–97.
Résumé: 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 C-13 and N-15 compared to the ISSG and, consequently, tissues of selected micronekton organisms in the EAFR were more enriched in N-15 (higher delta N-15 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 delta N-15 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 N-15, compared to the oligotrophic ISSG province.
Mots-Clés: Diel vertical migration; East African Coastal province; equatorial atlantic; feeding ecology; Indian South Subtropical Gyre; large pelagic fishes; mesopelagic fishes; mesoscale features; Micronekton; mozambique channel; myctophid fishes; north-atlantic ocean; respiratory carbon; Trophic level; vertical-distribution
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Chouvelon, T., Brach-Papa, C., Auger, D., Bodin, N., Bruzac, S., Crochet, S., et al. (2017). Chemical contaminants (trace metals, persistent organic pollutants) in albacore tuna from western Indian and south-eastern Atlantic Oceans: Trophic influence and potential as tracers of populations. Sci. Total Environ., 596, 481–495.
Résumé: Albacore tuna (Thunnus alalunga) is a highly commercial fish species harvested in the world's Oceans. Identifying the potential links between populations is one of the key tools that can improve the current management across fisheries areas. In addition to characterising populations' contamination state, chemical compounds can help refine foraging areas, individual flows and populations' structure, especially when combined with other intrinsic biogeochemical (trophic) markers such as carbon and nitrogen stable isotopes. This study investigated the bioaccumulation of seven selected trace metals – chromium, nickel, copper (Cu), zinc (Zn), cadmium (Cd), mercury (Hg) and lead – in the muscle of 443 albacore tunas, collected over two seasons and/or years in the western Indian Ocean (WIO: Reunion Island and Seychelles) and in the south-eastern Atlantic Ocean (SEAO: South Africa). The main factor that explained metal concentration variability was the geographic origin of fish, rather than the size and the sex of individuals, or the season/year of sampling. The elements Cu, Zn, Cd and Hg indicated a segregation of the geographic groups most clearly. For similar sized-individuals, tunas from SEAO had significantly higher concentrations in Cu, Zn and Cd, but lower Hg concentrations than those from WIO. Information inferred from the analysis of trophic markers (delta C-13, delta N-15) and selected persistent organic pollutants, as well as information on stomach contents, corroborated the geographical differences obtained by trace metals. It also highlighted the influence of trophic ecology on metal bioaccumulation. Finally, this study evidenced the potential of metals and chemical contaminants in general as tracers, by segregating groups of individuals using different food webs or habitats, to better understand spatial connectivity at the population scale. Limited flows of individuals between the SEAO and the WIO are suggested. Albacore as predatory fish also provided some information on environmental and food web chemical contamination in the different study areas. (C) 2017 Elsevier B.V. All rights reserved.
Mots-Clés: Bioaccumulation; biscay northeast atlantic; enhanced bioaccumulation; feeding ecology; Inorganic elements; Intrinsic markers; marine food webs; mercury concentrations; merluccius-merluccius; Organic contaminants; organochlorine compounds; polychlorinated-biphenyls; stable-isotope analysis; Stable isotopes; thunnus-alalunga; Top predator
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Dupuy, C., Pagano, M., Got, P., Domaizon, I., Chappuis, A., Marchessaux, G., et al. (2016). Trophic relationships between metazooplankton communities and their plankton food sources in the Iles Eparses (Western Indian Ocean). Marine Environmental Research, 116, 18–31.
Résumé: Coral reef and atoll lagoons are among the most diversified marine ecosystems but also the most affected by the combined effects of climate change and human activities. The Iles Eparses (Scattered Islands) in the Western Indian Ocean have been little affected by human pressure and can be considered to be “pristine” ecosystems. Metazooplankton plays a major role in the functioning and productivity of aquatic ecosystems, and this study was undertaken: (i) to determine the spatial abundance, distribution and species composition of metazooplankton, (ii) to assess the effect of metazooplankton grazing on pico- and nanophytoplankton and (iii) to analyze the trophic positions of metazooplankton by using the stable isotope signatures of a wide variety of taxa and particulate organic matter from the Iles Eparses and Mayotte. Tromelin Island (which is not located in the Mozambique Channel) had the lowest metazooplankton abundance with no cyanobacteria Trichodesmium spp. or mollusks (pteropods) presence, and with δ15N signatures of organisms that were higher than for the islands in the Mozambique Channel. Trichodesmium spp. was found in the Mozambique Channel and the plankton food web was probably based preferentially on these cyanobacteria with lower δ15N signatures indicating direct or indirect trophic transfer of diazotrophic nitrogen to metazooplankton. Three of the islands were distinct: Europa had the highest proportion of copepods, with oithonids being dominant, which is typical of rich mangrove systems, while Juan de Nova and Mayotte seemed to be the sites most affected by human activity with a high abundance of appendicularians and distinct particulate organic matter ∂13C signatures. Grazing experiments showed that food could be a limiting factor for metazooplankton in the Iles Eparses. However, the effect of metazooplankton grazing on phytoplankton appeared to be very low (0.01–2.32% of the total phytoplankton per day).
Mots-Clés: feeding ecology; grazing effect; Iles Eparses; Mayotte; Metazooplankton; spatial distribution
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Eduardo Nole, L., Lucena-Fredou, F., Mincarone, M. M., Soares, A., Le Loc'h, F., Fredou, T., et al. (2020). Trophic ecology, habitat, and migratory behaviour of the viperfish Chauliodus sloani reveal a key mesopelagic player. Sci Rep, 10(1), 20996.
Résumé: Mesopelagic fishes are numerically the most important vertebrate group of all world's oceans. While these species are increasingly threatened by anthropogenic activities, basic biological knowledge is still lacking. For instance, major uncertainties remain on the behaviour, ecology, and thus functional roles of mesopelagic micronektivores, particularly regarding their interactions with physicochemical features. Here, we examine the trophic ecology, habitat, and migratory behaviour of the viperfish (Chauliodus sloani)-a poorly known and abundant deep-sea species-to further understand the ecology and thus functional role of mesopelagic micronektivores. Moreover, we explore how physical drivers may affect these features and how these relationships are likely to change over large oceanic areas. The viperfish heavily preys on epipelagic migrant species, especially myctophids, and presents spatial and trophic ontogenetic shifts. Temperature restricts its vertical distribution. Therefore, its trophodynamics, migratory behaviour, and functional roles are expected to be modulated by the latitudinal change in temperature. For instance, in most tropical regions the viperfish stay full-time feeding, excreting, and serving as prey (e.g. for bathypelagic predators) at deep layers. On the contrary, in temperate regions, the viperfish ascend to superficial waters where they trophically interact with epipelagic predators and may release carbon where its remineralization is the greatest.
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