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1 - Laboratoire d’Océanographie de Villefranche (LOV, UMR 7093), Villefranche-sur-Mer

Macroplankton and micronekton in the Ligurian and Tyrrhenian Seas. Horizontal distribution in spring. Valérie Andersen 1 , Frédérique Carcaillet 2 , Eric Thiébaut 3 , Marc Picheral 1 , Marie-Dominique Pizay 1 and Paul Nival 1.

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1 - Laboratoire d’Océanographie de Villefranche (LOV, UMR 7093), Villefranche-sur-Mer

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  1. Macroplankton and micronekton in the Ligurian and Tyrrhenian Seas. Horizontal distribution in spring Valérie Andersen1, Frédérique Carcaillet2, Eric Thiébaut3, Marc Picheral1, Marie-Dominique Pizay1 and Paul Nival1 1 - Laboratoire d’Océanographie de Villefranche (LOV, UMR 7093), Villefranche-sur-Mer 2 – Ecosystèmes lagunaires (EL, UMR 5119), Montpellier 3 - Biologie des Organismes Marins et Ecosystèmes (BOME, UMR 5178), Paris Thanks to Jacques Sardou for his help in taxonomic analyses

  2. Sampling sites • Biomed 1 cruise • 11-27 April 1994 • * 5 transects • distance between stations • A, B, C, D: 10 nautical miles • E: 20 miles • * Hauls • 0-700 m (except coastal stations A1 & B1 and transect E) • - midday, midnight • (only midnight for B1, D2 & D3) • - multiple opening and closing net • (Bioness) • - 500 µm mesh • * CTD casts, bottle sampling • temperature, salinity • Chlorophyll a

  3. Vertical distribution  Organisms highly concentrated in the surface layer at night (species living in the surface layers by day and at night both, diel migrants) below 250 m depth: mainly Cyclothone braueri and C. pygmaea (fish) 19 stations sampled at night (16 stations by day)  Analyses based on numerical density (individuals per m²), night, 0-250 m Vertical distributions: Andersen et al. 1998 Oceanol. Acta

  4. Macroplankton and micronecton Total (0-250 m; night)  similar densities of the total community throughout the explored area  excepted in the Tyrrhenian Sea (Thalia democratica, juveniles of Meganyctiphanes norvegica and Nematoscelis megalops)  different patterns according to the species

  5. Specific composition 0-250 m : 22 to 34 species Vc = std X / mean X, with X = Log (ind/m²+1) 20 main species

  6. Meganyctiphanes norvegica adults  NW juveniles  Tyrrhenian Sea Tyrrhenian Sea = nursery?

  7. Comparison with historical data Meganyctiphanes norvegica Casanova, B., 1974. Thèse es Sciences Naturelles sampling between 1961 and 1968 (various cruises) essentially in autumn and winter adults (full symbols), larvae (open symbols)  Northern species, north of 40°N • particularly abundant in the central zone of the Ligurian Sea  Biomed results (in spring) in agreement with Casanova’s data 40° N

  8. Nematoscelis megalops adults  ubiquitous juveniles  Tyrrhenian Sea

  9. Nematoscelis megalops Casanova, B., 1974. Thèse es Sciences Naturelles sampling between 1961 and 1968 (various cruises) essentially in autumn and winter adults (black symbols), larvae (white symbols) • ubiquitous species • +/- homogeneously distributed in the western Mediterranean Sea  Biomed results (in spring) in agreement

  10. Euphausia brevis  Tyrrhenian Sea  low densities, irregular

  11. Euphausia brevis Casanova, B., 1974. Thèse es Sciences Naturelles sampling between 1961 and 1968 (various cruises) essentially in autumn and winter adults (black symbols), larvae (white symbols) • southern species (tropical and sub-tropical areas) • abundant in the Tyrrhenian Sea  Biomed results (in spring) in agreement

  12. Siphonophores Lensia conoidea  ubiquitous, irregular Chelophyes appendiculata SW

  13. Salps Thalia democratica  coastal + Tyrrhenian Sea (swarms) Salpa fusiformis  Ligurian Sea Pegea confoederata  SW (swarms)

  14. Hydrology • characteristics of • - coastal waters • - frontal zone • - central zone • (no data for D2 and D3)  Transect C : small eddy  Tyrrhenian Sea - higher temperature of MAW and LIW - North : running of freshwater from the Italian coast

  15. Chlorophyll a mg Chl a/m²; 0-200 m • high standing stocks in the frontal and central zones of the Ligurian Sea • Tyrrhenian Sea very poor in phytoplankton (high abundances of phytophagous zooplankton)

  16. Environmental conditions Ascending Hierarchical Classification (Euclidean distances) temperature, salinity: surface (MAW), 100 m (WIW), 250 m (LIW) Chlorophyll a: surface, 30 and 50 m (sub-superficial peak)

  17. similarity mixed group 4 groups 2 groups of Tyrrhenian stations group of 2 coastal stations Zooplankton – AHC and MDS - Ascending Hierarchical Classification - Multi-Dimensionnal Scaling based on - similarity index of Bray Curtis - Log (species density + 1) similarity

  18. similarity mixed group 4 groups 2 groups of Tyrrhenian stations Cavolinia inflexa +++ Euphausia hemigibba +++ Pegea confoederata: no Gennadas elegans: no Chelophyes appendiculata +++ Euphausia krohni +++ M. norvegica larvae +++ group of 2 coastal stations Thalia democratica +++ Zooplankton – AHC and MDS - Ascending Hierarchical Classification - Multi-Dimensionnal Scaling based on - similarity index of Bray Curtis - Log (species density) similarity

  19. Groups of macroplankton and micronekton and general surface circulation pattern

  20. Conclusions • Different patterns according to the species Nematoscelis megalops: ubiquitous Euphausia brevis: Tyrrhenian Sea Salpa fusiformis: Ligurian Sea, … but also irregular distribution of some species Swarms of Lensia conoidea, Pegea confoederata, … • Spatial structuring not entirely related to the hydrological structures high infuence of species swarms: scale at which the species aggregates?  Needs for data at smaller horizontal scales (and in the northern part of the Ligurian Sea) • Tyrrhenian Sea = nursery? Abundance of juveniles of M. norvegica and N. megalops  Influence of warmer temperatures?  Trophic conditions in the Tyrrhenian Sea Very low chlorophyll content High densities of phytophagous zooplankton (salps, euphausiid larvae)  top-down control?  Further analyses of these cruise data? Analysis of copepod community (shorter generation time) Analysis of samples collected during a similar cruise in autumn

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