Micronekton distribution as influenced by mesoscale eddies, Madagascar shelf and shallow seamounts in the south-western Indian Ocean: An acoustic approach

Type Article
Date 2020-06
Language English
Author(s) Annasawmy Pavanee1, 2, Ternon Jean-Francois1, Lebourges-Dhaussy Anne3, Roudaut Gildas3, Cotel Pascal1, Herbette Steven4, Ménard Frédéric5, Marsac Francis1, 2
Affiliation(s) 1 : MARBEC, Univ. Montpellier, CNRS, Ifremer, IRD, Sète, France
2 : Department of Biological Sciences and Marine Research Institute/ICEMASA, University of Cape Town, Cape Town, South Africa
3 : LEMAR, IRD, UBO, CNRS, Ifremer, Plouzané, France
4 : Laboratoire d’Océanographie Physique et Spatiale (LOPS), IUEM, Univ. Brest, CNRS, Ifremer, IRD, Brest, France
5 : Aix Marseille Univ., Université de Toulon, CNRS, IRD, MIO, Marseille, France
Source Deep-sea Research Part Ii-topical Studies In Oceanography (0967-0645) (Elsevier BV), 2020-06 , Vol. 176 , P. 104812 (15p.)
DOI 10.1016/j.dsr2.2020.104812
Keyword(s) Micronekton, Diel vertical migration, Mesoscale eddies, Madagascar shelf, Seamount, South-western indian ocean, Multi-frequency
Abstract

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 ∼240 m) thereafter named “MAD-Ridge”, and at La Pérouse seamount (∼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 Pérouse 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 Pérouse. 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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Annasawmy Pavanee, Ternon Jean-Francois, Lebourges-Dhaussy Anne, Roudaut Gildas, Cotel Pascal, Herbette Steven, Ménard Frédéric, Marsac Francis (2020). Micronekton distribution as influenced by mesoscale eddies, Madagascar shelf and shallow seamounts in the south-western Indian Ocean: An acoustic approach. Deep-sea Research Part Ii-topical Studies In Oceanography, 176, 104812 (15p.). Publisher's official version : https://doi.org/10.1016/j.dsr2.2020.104812 , Open Access version : https://archimer.ifremer.fr/doc/00634/74579/