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

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.Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site.

Micronekton distributions and assemblages at two shallow seamounts of the south-western Indian Ocean: Insights from acoustics and mesopelagic trawl data

“…Local productivity at MAD-Ridge did not result from a Taylor column (Annasawmy et al, 2019c) since the latter may form transiently, under specific conditions of summit depth, water column stratification and current speeds (Owens & Hogg, 1980, Freeland, 1994, Mohn et al, 2009, Wagawa et al, 2012Bashmachnikov et al, 2013). concluded that while, Taylor columns may theoretically be formed over several seamounts of the Madagascar Ridge, the relatively strong currents associated with mesoscale eddies may prevent their formation or sweep away any incipient Taylor cap before settlement.…”

“…Eddies are well known in influencing local water properties, such as the trapping of anomalous water masses (Swart et al, 2010;Pollard & Read, 2015), or the advection of coastal waters with high phytoplankton biomass from the coast to open waters (Quartly & Srokosz, 2004;Tew-Kai & Marsac, 2009;Kolasinski et al, 2012). Coastally upwelled waters of high biological productivity over the southern Madagascar shelf (Ramanantsoa et al, 2018) may be trapped by mesoscale features that propagate over MAD-Ridge (Annasawmy et al, 2019c).…”

“…Some micronekton taxa undergo diel vertical migration (DVM) from deep to shallow layers (upper 200 m) at dusk and from shallow to deep layers (below 400 m) at dawn (Lebourges-Dhaussy et al, 2000;Béhagle et al, 2014;Annasawmy et al, 2018). Other taxa were shown to exhibit delayed migration (Watanabe et al, 2002), reverse migration (Alverson, 1961;Gjosaeter, 1978Gjosaeter, , 1984Marchal & Lebourges-Dhaussy, 1996;Lebourges-Dhaussy, 2000;Annasawmy et al, 2019c), mid-water migration or non-migration (Annasawmy et al, 2018).…”

“…It is thought that the spatial/horizontal distribution of micronekton communities is also not uniform across the Atlantic (Judkins & Haedrich, 2018) and south-western Indian Ocean (Annasawmy et al, 2018). Various physical processes such as mesoscale eddies, proximity to continental shelves and landmasses, and seamounts may influence micronekton's distributions (Annasawmy et al, 2019c). Seamounts, by introducing irregularities into the pelagic environment, such as a hard substrate, disrupted current flows, increased primary productivity, entrapment of plankton, and presence of benthic predators, are thought to influence the abundance, biomass, diversity and taxonomic composition of micronekton (Genin 2004;Wilson & Boehlert, 2004;De Forest & Drazen, 2009).…”

“…These eddies may become trapped over the seamount and have an influence on the assemblages and DVM patterns of micronekton communities (Annasawmy et al, 2019c).…”