Micronektonic acoustic density variations in Guinea Current Large Marine Ecosystem continental shelf from 1999 to 2006 [résumé de poster]

2019

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Kouassi A.M., Behagle Nolwenn, Koné V., Perrot Yannick, Tiedemann M., Mouget Anne, Migayrou C., Krakstad J.O., Kanga D.K., Kouadio B., Diogoul N., Brehmer Patrice

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. 144-145 ISBN 978-9553602-0-06

ICAWA : International Conference AWA, 5., Lanzarote (ESP), 2018/04/17-20

The Guinea Current Large Marine Ecosystem (GCLME) extends from Bissagos Island (Guinea Bissau) in the north (11°N, 16°W) to Cape Lopez (Gabon) in the south (0°S, 8°E) and includes the study zone, from 4°N, 8°W to 6°N, 3°E. Acoustic data were recorded with a 38 kHz echosounder, from 10 to 500 m depth over 7 surveys, 6 were selected here, totalling 16 618 nmi from 1999 to 2006. To get homogenous data, (i) only off-upwelling season surveys (April to September) were studied and (ii) only continental shelf data were considered (10-150 m). The mean volume backscattering strength (Sv in dB) was used as a micronektonic biomass proxy to assess its spatial inter-annual variability. Diel transition periods were removed from analyses to avoid micronektonic density changes bias due to diel vertical migrations. Data were echointegrated at a spatial resolution of 0,1 nmi*1 m depth using Matecho tool (Perrot et al., 2018). (i) On horizontal dimension, the variability in annual micronektonic densities was assessed using the mean Sv value for each 0,1 nmi Elementary Sample Unit (ESU). (ii) On vertical dimension, the water column variation (%) filled by micronektonic acoustic layer (filling rate) across years was estimated using a linear regression and the change of micronektonic spatial structure between day and night was assessed using the mean Sv value for each 1 m depth step. GCLME have a narrow continental shelf vs. other African Atlantic LMEs. No significant change of micronektonic biomass proxy has been observed from 1999 to 2006 in this study (Fig. 1). As expected, a difference is observed in the vertical micronektonic acoustic density between day and night (Fig. 3). However, there is a paradoxical process, indeed there is an increase in Sv during nighttime. Two hypotheses are proposed: (i) the increase in density could be explained by an offshore horizontal diel migration or (ii) a very high contribution of the micronektonic density occurring in surface (0-10 m) is suspected, which corresponds to the blind zone of the research vessel. According to the new descriptor “water column filling rate”, a significant change in the system is reported. Indeed there is an increase in five years. Future investigations should focus on this interesting phenomenon, which could be link to an effect of global change. We have to take care that such increase could also inform on a major change in the trophic web in this part of the GCLME. These findings can be interpreted as an early warning signal and encourage for future study.

Sciences fondamentales / Techniques d'analyse et de recherche [020] ; Sciences du milieu [021] ; Limnologie physique / Océanographie physique [032] ; Limnologie biologique / Océanographie biologique [034]

Fonds IRD [F A010078243]

fdi:010078275