Ocean currents and gradients of surface layer properties in the vicinity of the Madagascar Ridge (including seamounts) in the South West Indian Ocean

Abstract

This work is part of the MADRidge Project special issue which aims to describe pelagic ecosystems in the vicinity of three prominent shallow seamounts in the South West Indian Ocean: one here named MAD-Ridge (240 m below the surface) plus Walters Shoal (18 m) on the Madagascar Ridge, and La Pérouse (60 m) on the abyssal plain east of Madagascar. The three span latitudes 20°S and 33°S, some 1500 km. The study provides the background oceanography for the once-off, multidisciplinary snapshot cruise studies around the seamounts. As life on seamounts is determined by factors such as summit depth, proximity to the light layers of the ocean, and the ambient circulation, a first description of regional spatial-field climatologies (16–22 years) and monthly along-ridge gradients of surface wind (driving force), water column properties of sea surface temperature, mixed layer depth, chlorophyll-a and eddy kinetic energy, plus ocean currents is provided. Being relevant to many applications in the study domain, these properties in particular reveal contrasting environments along the Madagascar Ridge and between the three seamounts that should drive biological differences. Relative to the other two seamounts, MAD-Ridge is in the more extreme situation, being at the end of the East Madagascar Current, where it experiences sturdy, albeit variable, currents and the frequent passing of mesoscale eddies.

Introduction

Most of the world's extensive seafloor consists of a deep muddy plain inhabited by molluscs, worms and echinoderms (Thistle, 2003). Bathypelagic fish are few. It is only sharp ridges and seamounts that provide a deep-sea relief, rocky and sediment-free, that can support abundant diverse and distinctive fauna. Seamounts are of volcanic origin, and tend to be on the oceanic crust near mid-ocean ridges, mantle plumes (hotspots) and island arcs (Keating et al., 1987). They range from isolated landmarks to clusters and chains (Roberts et al., 2020). Estimates of the number of seamounts in the global ocean vary between 10 000 and 100 000 (Kitchingman et al., 2007; Harris et al., 2014). The total area of these features is equivalent to about 30% of the global shelf region, making them significant platforms in sustaining life in the vast, deep ocean. With structure for animals to settle and live on, and currents supplying nutrients and food, the variety of life at seamounts is often great. This situation has been noticed by fishers who, in many cases, have plundered species of commercial value such as the deep-living orange roughy (Hoplostethus atlanticus) and Patagonian toothfish (Dissostichus eleginoides) at the base of seamounts, and various tuna species and swordfish around the summits.

However, given the large number of seamounts globally and their remote locations, much about the structure, function and connectivity of seamount ecosystems remains unexplored and unknown. Threats such as fishing and seabed mining are creating an unprecedented demand for research to inform conservation and management strategies. Clark et al. (2012) in particular called for renewed and intensive scientific effort to enhance the physical and biological information on seamount location, physical characteristics, comprehensiveness of biodiversity inventories, and the understanding of seamount connectivity and faunal dispersal.

Most seamounts lie deep beneath the ocean surface, rising <1500 m from the abyssal plains. A few reach the photic zone, and even fewer protrude above the ocean surface to form islands. Their volcanic nature and steep sides mean that bottom sediment loads tend to be low, providing islands of exposed hard substratum in the wider ocean. However, there are also other factors that determine the nature of the benthic and pelagic ecosystems that evolve around seamounts. Shape and summit depth determine the interactions with the surrounding oceanography, especially currents. Current velocities are strongest at the surface but decrease rapidly with depth. A protruding seamount disturbs the water flow, causing eddies of varying size (sub-mesoscale to mesoscale), internal waves, turbulent mixing and upwelling. A specific seamount-associated eddy is known as a Taylor column (or sometimes Taylor cap), which is anticyclonic and stationary over the summit, and it tends to retain small particles such as plankton and larvae. Such physical processes induce vertical movement of deeper, nutrient-rich water towards the sea surface which, given sufficient light, can enhance local plankton populations, and result in seamounts being aggregation areas for micronekton feeding (Lavelle and Mohn, 2010). The small fish in turn fall prey to predation by cephalopods, tuna, sharks and marine mammals, plus seabirds when the features are notably shallow. Other factors including water temperature, the depth of the upper mixed layer (and wind) and the extent of stratification, all play a role in shaping the benthic and pelagic ecosystems around seamounts.

The clear water conditions in the open ocean allow photosynthesis at greater depths than around continental shelves, so coralline algae for example can live at depths of 270 m (Littler et al., 1986). However, as depth increases beneath the upper sunlit layers of the ocean, current velocities and temperatures decrease, creating a different environment for the benthos. Evolving over millions of years, seamounts seem to have become isolated habitats that support communities high in endemism (Rogers, 1994; Tyler et al., 1995; Parin et al., 1997; Richer de Forges et al., 2000). Their isolated benthic and pelagic ecosystems make seamounts biological hotspots, stopping points for migrating animals such as whales, and stepping stones for the dispersion of biota across ocean basins. The position of a seamount relative to current systems, landmasses and latitude will influence dispersion, connectivity and ambient water properties.

As stated by Roberts et al. (2020), most seamounts in the Indian Ocean are on the western side of the basin, notably along the South West Indian Ridge (SWIR). Fishers for decades have explored and fished the numerous seamounts on this ridge, first Soviet fleets, then French and Asian fleets in the 1970s and 1980s (Clark et al., 2007; Rogers et al., 2017). More recently, since 1990, the tuna longline fishery, mostly working from Réunion Island (Evano and Bourjea, 2012), has focused on a region south and east of Madagascar, where productivity seems to be enriched, a statement supported by studies on seabirds (Pinet et al., 2012). The region south of Madagascar overlies the Madagascar Ridge, an impressive bathymetric feature that to date has received little scientific attention because of its remoteness.

Aligned longitudinally, the Madagascar Ridge extends south of the Madagascar landmass for some 1300 km (~10 degrees of latitude; Fig. 1, Fig. 2) with a width of ~400 km. Water depths over much of the ridge are between 2 and 3 km. The southern half of the ridge rises to the prominent Walters Shoal, one of a group of several deeper seamounts, but which itself comes within 18 m of the surface (Fig. 2b). Its flat summit is rather bare and covered with massive blocks of calcareous coraline algae (Bouchet P., pers. comm.). The northern part of the Ridge likewise consists of a cluster of seamounts shallower than 750 m. One of these, referred to in this study as the MAD-Ridge seamount, rises to a depth of 240 m below the sea surface (27.5°S, 46.25°E; Roberts et al., 2020). The western side of the Madagascar Ridge has a steep scarp that runs down into the 5-km-deep Mozambique Basin. The slope of the eastern flank is gentler, leading into the 5–6-km-deep Madagascar Basin. South of Walters Shoal, the water depth increases rapidly to more than 3000 m, whereupon, the 4000 m isobath joins the SWIR. Even with the northern and southern seamount clusters, the ridge is mostly flat-topped and covered by 0.5–1.0 km of undisturbed sediments (Goslin et al., 1980).

The Madagascar Ridge lies in a region of the southern Indian Ocean where the Subtropical Anticyclonic Gyre forms the general background circulation (Fig. 1a; Stramma and Lutjeharms, 1997; Lutjeharms, 2006), circulation portrayed by the baroclinic volume flux field over the upper 1000 m. Closure of the Subtropical Gyre has not yet been completely resolved (Pollard and Read, 2017). The gyre includes the powerful Agulhas Current (AC) as the major western boundary current (Fig. 1b). This flow retroflects at the southern tip of Africa, and mostly flows back east as the Agulhas Return Current (ARC), undergoing a series of semi-permanent meanders between 37°S and 41°S just north of the Subtropical Front (STF). The ARC weakens towards the east as transport peels off to the north (Stramma and Lutjeharms, 1997; Lutjeharms, 2007), then turns west to close the anticyclonic gyre. Northward leakage from the ARC also occurs in the form of cyclonic eddies that regularly form and break away, moving west (Pollard and Read, 2017). The westward recirculation within the Southwest Indian subgyre flows across the Madagascar Ridge.

The circulation south of Madagascar is complex and dominated by the strong South East Madagascar Current (S-EMC; Nauw et al., 2008). Similar to the situation for the AC, Lutjeharms (2007) suggested that the S-EMC undergoes an eastward retroflection once it becomes a free jet south of the landmass. Mesoscale eddies are formed there, which then propagate west towards southern Africa, where they merge with the upper reaches of the AC (Halo et al., 2014; Braby et al., 2016; de Ruijter et al., 2004). Ridderinkhof et al. (2013) demonstrated that much of this propagating turbulence is in the form of dipoles. Quartly et al. (2006) suggested that the retroflection at the end of the S-EMC is not a permanent feature. Siedler et al., 2006, Siedler et al., 2009, on the basis of climatological altimetry data, proposed that the South Indian Ocean Countercurrent (SICC) was an eastward extension of the S-EMC retroflection. Palastanga et al. (2007) observed the SICC to extend to 100°E, and Siedler et al. (2009) suggested that up to 40% of SICC waters originate in the S-EMC.

Not much is known of the circulation south of the S-EMC retroflection area. Read and Pollard (2017) suggested that integrated westward transport between Madagascar and 37°S accounts for 50 Sv, which added to 25 Sv from the S-EMC, is sufficient to account for the total AC transport of 70 ± 21 Sv, i.e. that it is a slow background flow. Observations from altimetry data show, superimposed on this background, that the southern part of the Madagascar Ridge is regularly affected by low-intensity eddies that propagate westwards (Read and Pollard, 2017).

The MADRidge Project (Roberts et al., 2020) was established in 2016 to further understand the pelagic ecosystems potentially supporting the productivity observed over the Madagascar Ridge and east of Madagascar. Three prominent shallow seamounts were selected for scientific investigation: two on the Madagascar Ridge, the Walters Shoal (18 m below the surface) and MAD-Ridge (240 m), and one east of Madagascar, La Pérouse (60 m), which rises from the abyssal plain. With the wish to help answer the question as to what shapes life on seamounts (see above), the present study was designed to provide a regional view of surface wind as a driver of upwelling and mixing, sea surface temperature (SST), mixed layer depth (MLD), and chlorophyll-a (hereafter chl-a) as key water column properties, and eddy kinetic energy (EKE) and currents as drivers of seamount processes and dispersion/connectivity. We attempt to provide a backdrop to the more-detailed ship-based, satellite and modelling investigations that supplement this study in the MADRidge Project special issue.