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The Saya de Malha Banks

submitted by Marjo Vierros

Figure 1. The location of the Saya de Malha Banks in the Indian Ocean between Seychelles and Mauritius.

The Saya de Malha Banks are the largest submerged banks in the world containing a unique seagrass biotope in the open ocean. Due to their remoteness, the Saya de Malha Banks are host to some of the least explored shallow tropical marine ecosystems globally, completely detached from land boundaries, and providing an ecologically important oasis of high productivity in the Indian Ocean.

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Scientific Background

The Saya de Malha Banks are unique and significant in at least three distinct ways: through their geology, high productivity in an area that is at least partly a biological desert, and through their status as the largest seagrass meadow in the open ocean.

Geologically, the Saya de Malha Banks are a unique formation comprising a large shallow area in the middle of the Indian Ocean, east of the northern tip of Madagascar, southeast of the Seychelles, and north of Nazareth Bank and the island of Mauritius. The Saya de Malha Banks are the largest submerged banks in the world, covering an area of 40,808 km². Composed of two separate structures, the smaller North Bank and the much larger South Bank, The Saya de Malha Banks are part of the underwater Mascarene Plateau. Due to their remote location, the Banks are among the least-studied shallow marine ecosystems on the planet.

The Mascarene Plateau was formed by volcanic activity from the Reunion hot spot between 20 and 40 million years ago (Shor and Pollard, 1963). The mountainous volcanic islands formed by the hot spot eventually sank below the ocean surface, possibly as recently as 18,000 - 6,000 years ago. Today, the Saya de Malha Banks consist of a series of narrow underwater shoals, with depths from 8 to 150 metres, covered with seagrasses and interspersed with small coral reefs (Goreau, 2002). These banks are surrounded by fairly sharp drop-offs to 2000m especially off the northwestern and northeastern portions of the South Bank.

Although the role of the oceanographic and meteorological factors in the area are not completely understood (Payet, 2005), the Saya de Malha Banks are considered to be an isolated area of high productivity relative to the surrounding, nutrient-poor ocean. As the Banks plunge rapidly into deep ocean basins on all sides, the upwelling of deep, cold, nutrient rich waters is forced to the surface (Goreau, 2002). These deep nutrients are thought to fuel the elevated productivity of the area, which is thought to be highest along the eastern edges of the Banks. Additionally, the entire 2,200km long Mascarene Plateau forms a barrier modifying the predominantly westward passage of the South Equatorial Current, causing upwelling, nutrient enrichment and enhanced chlorophyll and secondary production. This secondary production is evident in the form of the diverse fish communities present on the Banks (Smythe-Wright et al, 2005), as well as in the operation of an intensive commercial hook and line fishery in the area (Grandcourt, 2003).

 

How the area containing unique, rare species and unique, rare or distinct habitats or ecosystems was identified

Both satellite data and field measurements support the hypothesis that the Saya de Malha Banks form an area of high productivity. Enhanced chlorophyll levels (associated with relatively higher biomass of plant material, such as phytoplankton) are visible in satellite imagery (New et al, 2005) and field measurements collected by a research cruise in 2008 also found higher chlorophyll-a levels around the Saya de Malha Banks than in the surrounding area (figure 3).

Figure 2

Figure 2: Data collected by a research cruise undertaken in 2008 by the Agulhas and Somali Current Large Marine Ecosystems Project shows relatively higher chlorophyll-a concentrations around Saya de Malha Banks (ASCLME, 2008). Chlorophyll-a is indicative of higher plant biomass in the area.

Due to their size and shallowness, the Saya de Malha Banks represent one of the largest shallow tropical marine ecosystems on Earth, and they may contain the most extensive seagrass area in the world, potentially covering much of the over 40,000 square kilometre area of the Banks. According to a 2002 research expedition, seagrass covered roughly 80-90% of the bottom, with a diverse range of coral species covering around 10-20%, and sandy areas less than 5% (Goreau, 2002).  Seagrass species present in the area include Thalassodendron ciliatum, Halophila decipiens and Enhalus acoroides, some of which were found growing at a deeper depth here than elsewhere in the Indian Ocean (Milchakova et al, 2005). The Banks may serve as a significant sink of atmospheric carbon dioxide and a source of oxygen, since a large part of the seagrass organic production is swept by the currents into deep waters, where some of it is buried in sediments of the ocean bottom (Goreau, 2002).


The shallow water marine ecosystem on Saya de Malha Banks provides feeding habitat for the green turtle (Chelonia mydas), as well as breeding grounds for blue whales of the subspecies Balaenoptera musculus brevicauda, referred to as the pygmy blue whale and resident particularly to the western Indian Ocean (Reilly et al, 2008).The Banks may play a role in the maintenance of the straddling fish stocks that supply much of the catch in neighbouring waters of Seychelles and Mauritius. Because the Banks are remote and not well explored, new species continue to be discovered in the area by research expeditions (Richards, 1992; Kim and Amaoka, 2001).

The ecological importance of the Banks may extend to providing a potentially important stepping stone in the migration of shallow water species across the Indian Ocean. The Banks may have played a critical role in the colonization of the shores of East Africa and Western Indian Ocean islands by species originating from the Indonesian global marine biodiversity maximum. This is an important consideration in the face of climate change, as the Banks’ unique conditions and remoteness from direct sources of anthropogenic stress may make them a crucial reservoir for the maintenance of biodiversity in the surrounding islands and coastal areas (Goreau, 2002).

This unique ecosystem was identified based on the opinion of scientific experts and on a review of available literature. Scientific experts taking part in the High Seas Gems project (IUCN and MCBI, 2008) and the World Heritage Marine Biodiversity Workshop in Hanoi, Vietnam (2002) identified the Saya de Malha Banks as an ecologically and biologically significant area.

 

Sources of data

Data relating to the Saya de Malha Banks are based on a number of expeditions to the area. The first survey of the Banks was undertaken by Captain Robert Moresby of the British Royal Navy in 1838. The Mascarene Plateau was described in detail by Fisher et al. (1967) and has been studied by 26 Russian fisheries expeditions between the 1960s and 1989. These expeditions included dives with submersibles. More recently, the Global Coral Reef Alliance conducted two research expeditions, in 1997 and 2002, describing the biology, ecology, bathymetry and oceanography of the Banks. From 2000 to 2003, the UK Royal Geographic Society undertook the Shoals of Capricorn project, which concentrated on zooplankton ecology of the Mascarene Plateau. One of the participants in this project, the Southampton Oceanography Centre also conducted a research cruise into the area in 2002 as part of the SCIPIO (Satellite Calibration and Interior Physics of the Indian Ocean) project. In addition, the Japan Marine Fishery Resource Research Centre has conducted trawling surveys in the area (Kim and Amaoka, 2001), and the Albion Fisheries Research Centre (Mauritius) and the Seychelles Fishing Authority have conducted fishery-related studies. Most recently, in 2008, the Agulhas and Somali Current Large Marine Ecosystem (ASCLME) Project has undertaken a research cruise to the area, collecting data on bathymetry, acoustics, and physical and biological oceanography.

 

Important considerations

While there is sufficient scientific information to demonstrate the uniqueness of the Saya de Malha Banks, further research into its ecology and biology is desirable as a basis for developing appropriate management measures. Research should also be undertaken in the deep waters immediately surrounding the Banks, which are the presumed source of the nutrients fueling the area’s high productivity. Deep diving sperm and beaked whales are also expected to occur in these areas, as well as various pelagic dolphin species (Taylor et al. 2008). Most of the Saya de Malha Banks is located in marine areas beyond the limits of national jurisdiction, though parts of the Banks straddle the EEZs and the extended continental shelves of Mauritius and Seychelles.


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