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

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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.

Introduction

Features such as mesoscale cyclonic and anticyclonic eddies, upwelling events, tidal fronts, shelves, seamounts and river runoff play a significant role in regional ecosystems (Bakun, 2006; Mann and Lazier, 2006; Benitez-Nelson and McGillicuddy, 2008). Mesoscale cyclonic and anticyclonic eddies are ubiquitous in the world’s oceans (Chelton et al., 2011). They have time-scales of approximately 10–30 days and horizontal scales between 10 and 100 km (Mann and Lazier, 2006; Chelton et al., 2011). In oligotrophic systems, eddies are important features because they provide mechanisms whereby the physical energy of the ocean is converted to trophic energy to support biological processes (Bakun, 2006; Godø et al., 2012). Cyclonic eddies, through upwelling of nutrients in their centres from deeper layers to the euphotic zone, are usually known to enhance local productivity (Owen, 1980, 1981; McGillicuddy and Robinson, 1997; McGillicuddy et al., 1998; Klein and Lapeyre, 2009; Huggett, 2014; Singh et al., 2015). Anticyclonic eddies may promote the development of frontal structures (Bakun, 2006). In anticyclones, highly productive waters may be entrained laterally from nearby regions to the eddy periphery or upwelling of nutrients may occur along the eddy boundary (McGillicuddy, 2016). At the frontier between eddies, smaller-scale or submesoscale features (elongated filaments with a 10-km width) have been reported to enhance nutrient supply and primary productivity in oligotrophic conditions (Lévy et al., 2001, 2018; Klein and Lapeyre, 2009). Biological responses to eddies, however, are complex and depend on a range of factors including seasonal modulation of the mixed layer depth (Dufois et al., 2014), timing, magnitude and duration of nutrient input and also on eddy properties such as the formation, intensity, age and eddy-induced Ekman pumping (Benitez-Nelson and McGillicuddy, 2008).

Continental shelves and seamounts are also features that may lead to enhanced productivity when certain conditions are met. Upwelling regions south of Madagascar have been observed to be biological hotspots with increased productivity (Raj et al., 2010; Ramanantsoa et al., 2018) and increased acoustic biomass estimates of pelagic fish and whale sightings (Pripp et al., 2014). Phytoplankton types may also differ between continental shelves and ocean basins, with shelf areas exhibiting larger phytoplankton cells because of the processes leading to high nutrient concentrations in the euphotic zone and cells rapidly take up nutrients (Nishino et al., 2011). Seamounts are ubiquitous features of the world’s oceans and have been reported to influence the prevailing ocean currents (Royer, 1978; White et al., 2007), creating various local dynamic responses such as formation of a Taylor column, isopycnal doming (Mohn and Beckmann, 2002), enclosed circulation cell (White et al., 2007), upwelling, vertical mixing of nutrient-rich waters and enhanced productivity (Boehlert and Genin, 1987; Genin, 2004). In a nutrient-limited environment like the south-western Indian Ocean, processes injecting nutrients into the euphotic zone (such as mesoscale features, seamounts, coastal upwelling events and river runoff) are likely to modulate the chlorophyll a signature by increasing phytoplankton growth, attracting a range of secondary and tertiary consumers such as zooplankton and micronekton.

Mesopelagic micronekton are actively swimming organisms that typically range in size from 2 to 20 cm. They include diverse taxonomic groups (De Forest and Drazen, 2009) such as crustaceans (adult euphausiids, pelagic decapods and mysids), cephalopods (small species and juvenile stages of large oceanic species) and fish (mainly mesopelagic species and juveniles of other fish) (Brodeur et al., 2005; Brodeur and Yamamura, 2005; Ménard et al., 2014). Gelatinous organisms are under-represented components of the mesopelagic community (Lehodey et al., 2010; Kloser et al., 2016). Micronekton are important in the energy transfer to higher trophic levels because they are preyed upon by various top marine predators (Guinet et al., 1996; Bertrand et al., 2002; Potier et al., 2007; Cherel et al., 2010; Danckwerts et al., 2014; Jaquemet et al., 2014). They also transport energy to deeper regions of the ocean via respiration, excretion and natural mortality (Hidaka et al., 2001; Catul et al., 2011; Bianchi et al., 2013). This energy transport is made possible by the extensive diel vertical migration (DVM) patterns of some micronekton species, with the organisms migrating to the upper 200 m of the water column at dusk and below 400 m at dawn (Lebourges-Dhaussy et al., 2000; Béhagle et al., 2014; Annasawmy et al., 2018). Diel vertical migration is believed to result from a compromise between the need to feed and to avoid predation (Heywood, 1996), with light being the main controlling factor in initiating ascent and descent (Heywood, 1996; Andersen et al., 1998; Brierley, 2014). The distribution of micronekton communities across ocean basins is not uniform (Judkins and Haedrich, 2018). Some studies have reported higher biomasses of micronekton scattering layers at seamount flanks and summits relative to the surrounding ocean, e.g. the Emperor (265m, Boehlert, 1988) and Cross seamounts in the Pacific (330 m, Johnston et al., 2008); Condor (182–214 m) and Gigante (161 m) seamounts in the Azores (Cascão et al., 2017).

At the ocean-basin scale, the western side of the oligotrophic Indian South Subtropical Gyre (ISSG) biogeochemical province (Longhurst, 2007) holds reduced micronekton abundances and acoustic densities relative to the dynamic and more productive East African Coastal (EAFR) province (Annasawmy et al., 2018). Within the ISSG and EAFR provinces, features such as eddies, coastal upwelling at the Madagascar shelf and seamounts may further impact the local productivity, resulting in significant variability in micronekton distributions via bottom-up processes. This paper investigates the influence of mesoscale eddies, the South Madagascar shelf and two shallow seamounts, La Pérouse and an unnamed pinnacle on the Madagascar Ridge, hereafter called “MAD-Ridge”, in shaping micronekton vertical and horizontal distributions by combining data from ship-based platforms (acoustics, current profiler and CTD) and satellite altimetry.

Section snippets

Cruises

Two research surveys were carried out on board the RV Antea at La Pérouse (19°43’S and 54°10’E) and MAD-Ridge seamounts (27°29’S and 46°16’E). La Pérouse (summit depth ~60 m) is located along the north-western boundary of the ISSG province and MAD-Ridge (summit depth ~240 m) is located on the southern boundary of the EAFR (Fig. 1a). The La Pérouse cruise (DOI: 10.17600/16004500) investigated the area within 10–18 km around the seamount from the 15 to 30 September 2016 (Fig. 1b). The MAD-Ridge

Synoptic ocean circulation during the MAD-Ridge cruise

A cyclonic/anticyclonic eddy dipole was encountered along the West-East transect (hydrographic stations 1–15) of Leg 1 of the MAD-Ridge cruise, whereas the South-North transect (hydrographic stations 16–31) was mostly located inside the anticyclonic eddy and reached the Madagascar shelf (Fig. 2). Along the West-East transect, at hydrographic station 5, a sharp front was observed in the sea surface temperature and salinity data collected from the ship-mounted thermosalinograph, indicating the

Oceanographic conditions during the MAD-Ridge and La Pérouse cruises

This study demonstrated the strong influence of mesoscale cyclonic and anticyclonic eddies on the physical and biological properties at MAD-Ridge seamount. The doming of isotherms and shallowing of the Fmax depth was observed within the cyclonic eddy during the MAD-Ridge cruise. Such processes are associated with eddy-induced pumping and upwelling of cool, nutrient-rich waters, triggering an increase in primary production in the photic layer (McGillicuddy and Robinson, 1997; McGillicuddy et

Concluding remarks

This study has suggested a link between the physical processes leading to enhanced productivity and the biological response of micronekton. Two main processes were identified to have a positive effect on the observed productivity: 1) the influence of the cyclonic eddy through the enrichment of surface waters, 2) the advection of shelf waters with high chlorophyll a concentrations. La Pérouse and MAD-Ridge seamounts did not show any enhanced biomass of micronekton, as reported to be the case for

CRediT authorship contribution statement

Pavanee Annasawmy: Conceptualization, Methodology, Software, Formal analysis, Investigation, Writing - original draft, Writing - review & editing, Visualization. Jean-François Ternon: Conceptualization, Writing - review & editing, Supervision, Funding acquisition, Project administration. Anne Lebourges-Dhaussy: Software, Resources. Gildas Roudaut: Software, Resources. Pascal Cotel: Software. Steven Herbette: Writing - review & editing, Formal analysis. Frédéric Ménard: Conceptualization,

Acknowledgements

We acknowledge the work carried out by the non-scientific and scientific staff on board the RV Antea in taking part in the data acquisition and data processing. The study was mainly supported by the Flotte Océanographique Française (French Oceanographic Fleet) and the Institut de Recherche pour le Développement (IRD) in relation to the logistics of the RV Antea. Additional funding was received from the Conseil Régional de la Reunion (Réunion Regional Council, Bop 123/2016) for the La Pérouse

References (112)

  • J.F. Dower et al.

    “Seamount effects” in the zooplankton community near Cobb Seamount

    Deep-Sea Res. I

    (1996)
  • J. Dower et al.

    A strong biological response to oceanic flow past Cobb Seamount

    Deep-Sea Res.

    (1992)
  • J.C. Drazen et al.

    Micronekton abundance and biomass in Hawaiian waters as influenced by seamounts, eddies and the moon

    Deep-Sea Res. I

    (2011)
  • A. Genin

    Bio-physical coupling in the formation of zooplankton and fish aggregations over adrupt topographies

    J. Mar. Sci.

    (2004)
  • I. Halo et al.

    Eddy properties in the Mozambique Channel: a comparison between observations and two numerical ocean circulation models

    Deep-Sea Res. II

    (2014)
  • K. Hidaka et al.

    Downward transport of organic carbon by diel migratory micronekton in the western equatorial Pacific: its quantitative and qualitative importance

    Deep-Sea Res. I

    (2001)
  • J.A. Huggett

    Mesoscale distribution and community composition of zooplankton in the Mozambique Channel

    Deep-Sea Res. II

    (2014)
  • J. Isern-Fontanet et al.

    Spatial structure of anticyclonic eddies in the Algerian basin (Mediterranean Sea) analyzed using the Okubo-Weiss parameter

    Deep-Sea Res. II

    (2004)
  • S. Jaquemet et al.

    Contrasted structuring effects of mesoscale features on the seabird community in the Mozambique Channel

    Deep-Sea Res. II

    (2014)
  • B. Jena et al.

    Investigation of the biophysical processes over the oligotrophic waters of South Indian Ocean subtropical gyre, triggered by cyclone Edzani

    Int. J. Appl. Earth Obs. Geoinf.

    (2012)
  • B. Jena et al.

    Observation of oligotrophic gyre variability in the south Indian Ocean: environmental forcing and biological response

    Deep-Sea Res. I

    (2013)
  • Y.S. José et al.

    Influence of mesoscale eddies biological production in the Mozambique Channel: several contrasted examples from a coupled ocean-biogeochemistry model

    Deep-Sea Res. II

    (2014)
  • D.C. Judkins et al.

    The deep scattering layer micronektonic fish faunas of the Atlantic mesopelagic ecoregions with comparison of the corresponding decapod shrimp faunas

    Deep-Sea Res. I

    (2018)
  • T. Lamont et al.

    Characterisation of mesoscale features and phytoplankton variability in the Mozambique Channel

    Deep Sea Res. Part II Top. Stud. Oceanogr.

    (2014)
  • A. Lebourges-Dhaussy et al.

    Vinciguerria nimbaria (micronekton), environmetn and tuna: their relationships in the Eastern Tropical Atlantic

    Oceanol. Acta

    (2000)
  • P. Lehodey et al.

    Bridging the gap from ocean models to population dynamics of large marine predators: a model of mid-trophic functional groups

    Prog. Oceanogr.

    (2010)
  • D.J. McGillicuddy et al.

    Eddy-induced nutrient supply and new production in Sargasso Sea

    Deep-Sea Res. I

    (1997)
  • F. Ménard et al.

    Stable isotope patterns in micronekton from the Mozambique Channel

    Deep-Sea Res II

    (2014)
  • C.E. Menkes et al.

    Seasonal oceanography from physics to micronekton in the south-west Pacific

    Deep-Sea Res. II

    (2015)
  • A. Okubo

    Horizontal dispersion of floatable particles in the vicinity of velocity singularities such as convergences

    Deep Sea Res.

    (1970)
  • R. Pollard et al.

    Circulation, stratification and seamounts in the southwest Indian ocean

    Deep-Sea Res. II

    (2017)
  • M. Potier et al.

    Forage fauna in the diet of three large pelagic fishes (lancetfish, swordfish and yellowfin tuna) in the western equatorial Indian Ocean

    Fish. Res.

    (2007)
  • T. Pripp et al.

    Physical influence on biological production along the western shelf of Madagascar

    Deep-Sea Res. II

    (2014)
  • R.P. Raj et al.

    Oceanic and atmospheric influences on the variability of phytoplankton bloom in the Southwestern Indian Ocean

    J. Mar. Syst.

    (2010)
  • F.G. Alverson

    Daylight surface occurrence of myctophid fishes off the coast of Central America

    Pac. Sci.

    (1961)
  • P. Annasawmy et al.

    Micronekton distribution and assemblages at two shallow seamounts in the south-western Indian Ocean: insights from acoustics and mesopelagic trawl data

    Prog. Oceanogr

    (2019)
  • A. Bakun

    Fronts and eddies as key structures in the habitat of marine fish larvae: opportunity, adaptive response and competitive advantage

    Sci. Mar.

    (2006)
  • B.M. Baliño et al.

    Winter distribution and migration of the sound scattering layers, zooplankton and micronekton in Masfjorden, western Norway

    Mar. Ecol. Prog. Ser.

    (1993)
  • A. Bertrand et al.

    Tuna food habits related to the micronekton distribution in French Polynesia

    Mar. Biol.

    (2002)
  • A. Bertrand et al.

    Acoustic characterisation of micronekton distribution in French Polynesia

    Mar. Ecol. Prog. Ser.

    (1999)
  • D. Bianchi et al.

    Global patterns of diel vertical migration times and velocities from acoustic data

    Limnol. Oceanogr.

    (2016)
  • D. Bianchi et al.

    Diel vertical migration: ecological controls and impacts on the biological pump in a one-dimensional ocean model

    Global Biogeochem. Cycles

    (2013)
  • G.W. Boehlert

    Current-topography interactions at mid-ocean seamounts and the impact on pelagic ecosystems

    Geojournal

    (1988)
  • G.W. Boehlert et al.
  • G.W. Boehlert et al.

    Populations of the sternoptychid fish Maurolicus muelleri on seamounts in the central North pacific

    Pac. Sci.

    (1994)
  • R.D. Brodeur et al.

    PICES Scientific Report No. 30 Micronekton of the North Pacific

    (2005)
  • R.D. Brodeur et al.

    Micronekton-What are they and why are they important?

    PICES Press

    (2005)
  • I. Cascão et al.

    Persistent enhancement of micronekton backscatter at the summits of seamounts in the Azores

    Front. Mar. Sci.

    (2017)
  • V. Catul et al.

    A review of mesopelagic fishes belonging to family Mcytophidae

    Rev. Fish Biol. Fish.

    (2011)
  • D.C. Chapman et al.

    Formation of Taylor Caps over a tall isolated seamount in a stratified ocean

    Geophys. Astrophys. Fluid Dynam.

    (1992)
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