As a 1 300 km longitudinally submerged extension of Madagascar, the Madagascar Ridge is a significant feature in the Southwest Indian Ocean. The ridge rises from abyssal depths to around 2000 m with several shallow (750-20m) seamounts occupying the northern and southern regions. Certainly the Walters Shoal is becoming recognised as a biological hotspot. The orientation of the ridge means that it traverses the background westward re-circulation of the Southwest Indian Ocean sub-gyre that apparently contributes to the Agulhas Current on the east coast of southern Africa. Because of its remoteness, little is known of the ridge's role in the local and regional oceanography and biology-although tuna fisheries operate in the north and around the Walters Shoal farther south. This remoteness has led to few in situ measurements having been made in the vicinity of the Madagascar Ridge. In this context, our study provides the first detailed description of the wind field and ocean properties along the Madagascar Ridge. In particular the study aims to provide a backdrop for several new investigations that focus on an unnamed seamount (260 m) on the northern sector of the ridge and the Walters Shoal (20 m) in the south. Spatial fields and along-ridge gradients of surface wind, SST, MLD, EKE, Heat Flux and Chl-a are produced to help biologists understand biological ramifications. These reveal contrasting environments between the northern and southern regions of the ridge. To gain an understanding of the circulation dynamics along the 1300 km long ridge and connectivity in the region-a novel approach of 'virtual moorings' is used. These comprised selected positions along the ridge with the compilation of 4year long time series of satellite-derived geostrophic currents. It is shown that currents (and EKE) decrease from north to south with greater variability. The northern ridge is highly dynamic due to the presence of the East Madagascar Current and its retroflection into the East Madagascar Return Current. By comparison, Walters Shoal in the south is in a quiescent zone.

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The composition and spatial variability of ichthyoplankton assemblages were investigated at three shallow seamounts between latitudes 19°S and 33°S in the South West Indian Ocean (SWIO) -La Pérouse (60 m), an unnamed pinnacle south of Madagascar, referred to hereafter as MAD-Ridge (240 m), and the Walters Shoal seamount (18 m). In all, 299 larvae (23 families, 54 species) were present at La Pérouse, 964 larvae (58 families and 127 species) at MAD-Ridge, and 129 larvae (9 families, 24 species) at the Walters Shoal. Larvae of mesopelagic fish in the families Myctophidae and Gonostomatidae were the most dominant at all three seamounts. All developmental stages were present at each seamount, suggesting the larval pelagic phase of certain species occurs at the seamounts. A 'seamount effect' was detected only at MAD-Ridge where larval fish densities were significantly higher at summit stations. Overall, MAD-Ridge had much higher densities of fish larvae (157.0 larvae 100 m−3) than La Pérouse (31.1 larvae 100 m−3) and the Walters Shoal (9.6 larvae 100 m−3). Our study demonstrates that ichthyoplankton communities at shallow seamounts in the SWIO are more influenced by their location relative to a landmass, and to oceanographic features such as currents, mesoscale eddies and water masses than the seamount latitude and topography itself.

Based on satellite and in situ data, the dynamic characteristics and vertical structure of a surface intensified mesoscale dipole recently expelled from the South East Madagascar Current (SEMC) is described for the first time. The dipole was surveyed 250 nautical miles south of Madagascar between 14 and 23 November 2016, during west-east and south-north transects carried out over the northern Madagascar Ridge. The dipole consisted of two counter-rotating vortices of similar size (100 km) and intensity (0.7 f), and an intense southwestward jet (150 cm s-1) in the frontal region between the two eddies. The cyclonic eddy was lying on the western side of the anticyclonic eddy. With azimuthal velocities reaching 100 cm s-1 at the surface and decreasing slowly with depth (40 cm s-1 at -600 m), this MAD-Ridge dipole was defined as a highly non-linear (Ro∼0.7) isolated eddy-type structure (cβ ∼ 11 cm s -1 and U/cβ ∼ 0.7) capable of trapping and advecting water masses over large distances. The enhanced concentration of chlorophyll-a found in the cyclone relative to the anticyclone could be tracked back to the spin-up phase of the two eddies and attributed to eddy-pumping. The eddy cores were located above the pycnocline (1026.4 kg m-3), within the upper 600 m, and consisted of varieties of Subtropical Underwater (STUW) found within the SEMC. The STUW found in the anticyclone was more saline and oxygenated than in the cyclone, highlighting mixing with the inshore shelf waters from the southeastern coastal upwelling cell off Madagascar. Observations suggest that the dipole interacted strongly with the chaotic bathymetry of the region, characterized by a group of five seamounts lying between -240 m and -1200 m. The bathymetry blocked its westward advection, trapping it in the vicinity of the seamount for more than 4 weeks, so enhancing the role of the eddy-induced velocities in stirring the surrounding water masses. Squeezed between the southern Madagascan shelf and the northern flank of the anticyclone, two filament-like dynamic features with very different water-mass properties could be observed on the southnorth transect: i) one filament highly concentrated in chlorophyll-a demonstrating the capacity of the eddy to export shelf water offshore; ii) intrusions of a more southern-type of STUW generally found south of the South Indian Counter Current (SICC) recirculating on the external flanks of the anticyclone. Although the observed circulation and hydrography were largely constrained by the presence of the mesoscale eddy 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.dipole, unmistakable fine-scale dynamics were also observed in the vicinity of the MAD-Ridge seamount, superimposed onto the mesoscale eddy flow.