Effect of environmental variables on the structure of micronectonic layers over the Senegalese continental shelf [résumé]

2019

Colloque

Diogoul N., Brehmer Patrice, Perrot Yannick, Tiedemann M., Thiam A., El Ayoubi S., Mouget Anne, Migayrou C., Sadio Oumar, Sarré A.

Brehmer Patrice (ed.), Diogoul N. (collab.), Zenk C. (collab.), Vareilles M. de (collab.), Keenlyside N. (collab.), Nascimento J.M. (collab.), Ramos V. M. (collab.), Sow B.A. (collab.), Fock H. (collab.), Schmidt J. (collab.), Ekau W. (collab.), Mbaye A. (collab.), Fall. A. (collab.), Monteiro I. (collab.), Kouassi A.M. (collab.), Silva O. (collab.), Brochier Timothée (collab.), Sall. M. (collab.), Mayif M. (collab.), Koné V. (collab.), Gorgues Thomas (collab.), Ferreira Santos C. (collab.), Bamy I.L. (collab.), Barry I. (collab.), Sidibe M. (collab.), Diadhou H. (collab.) International conference ICAWA 2017 and 2018 : extended book of abstract : the AWA project : ecosystem approach to the management of fisheries and the marine environment in West African waters

[s.l.] : SRFC/CSRP ; IRD, 2019, p. 51 ISBN 978-9553602-0-06

ICAWA : International Conference AWA, 4., Mindelo (CPV), 2017/11/13-17

The micronectonic organisms aggregate at specific depths and occur as scattering layer on echosounder records. They constitute an important component in the marine food web in direct contact with primary producers. We characterized the Senegalese water masses of the 'Petite côte' on physicochemical and biological criteria using an in situ data set collected during an acoustics survey. Then we described at fine scale spatial and temporal variation of micronektonic layers in relation with their environment. Two areas with different characteristics have been discriminated: the upwelling's cell area and the upwelling's offshore area more stratified, warm and sharply separated from the other area by a strong thermal boundary. The spatio-temporal variation of scattering layer's thickness of micronectonic is strongly influenced by depth and the time of the day. The continental shelf scattering layer's thickness increases with depth, but no variation is reported in longitudinal plane. In both areas nocturnal layers are thicker and deeper than diurnal ones. The hydrological structure of the water column also influence the micronectonic scattering layer. The scattering layer requires "stable" physical conditions which support vertical stratification. In the upwelling's area cell, the chlorophyll-a (CHL) concentration is correlated to scattering layer thickness during night time. In the upwelling's offshore area, sea temperature, water density and oxygen have a significant effect on the scattering layer's thickness during the nightime. However, during the daytime, CHL has a significant effect on the scattering layer's thickness. This correlation between CHL and scattering layer thickness in the upwelling's offshore area during day could be explained by an inverse diel vertical migration of a micronectonic group. On this basis we assume that trophic relationship between phytoplankton and micronecton operate during the day at the surface in this area.

Sciences du milieu [021] ; Limnologie physique / Océanographie physique [032] ; Limnologie biologique / Océanographie biologique [034] ; Ressources halieutiques [040]

Fonds IRD [F A010078243]

fdi:010078253