Astove Atoll

Astove Island, slightly oblique view from space.
Note the wide land area (dark) surrounding the whitish lagoon

Astove Island is part of the Aldabra Group of the Seychelles. It is 38 km SSE of Cosmoledo Atoll, located at (10°06′S 47°45′E). It is a raised coral island of most peculiar form: a single stretch of land, more than 1 km (nearly one mile) at the widest, almost entirely encloses a shallow lagoon. This has a maximum depth of 3 metres (10 ft), and the only exit is a winding passage in the southwest, called Gueule Bras Channel.

Astove Island measures nearly 6 km (3.7 miles) north to south and about 4 km (2.5 mi) at most east to west. The land area is 4.96 km², and the total area including the lagoon 9.5 km². The only settlement, on the western coast, has been abandoned since the 1980s. There is a grass airstrip on the north east point of the island, and remnants of a former coconut and sisal plantation. Today, the island is rarely visited - usually by scientists researching the lagoon's ecology - but the near-vertical drop-off from its outer reef edge is a popular location for diving cruises.

Geology and geography

Astove's unusual structure has raised questions about its formation. It was suggested that Astove is not a true raised atoll, but rather a reef flat, with the lagoon being washed out later. In any case, the lagoon indeed grows slowly, owing to rainwater, acidic from the soil's humic acids, dissolving the lagoon's carbonate rock bottom. This causes the unusual milky-white color of the lagoon's waters.

On the large land mass towards the western tip, exposed reef rock raises to some 5 m (15 ft) ASL. Elsewhere it is largely covered by gravelly debris. Dunes of up to 18 m (60 ft) line the windswept eastern rim of the island, and the eastern part of the lagoon is especially shallow due to the inblown dune sand. Astove Island's fringing reef is just about 180 m (300 ft) wide. As noted above, beyond the reef's outer edge the seafloor drops down steeply, with a depth of 550 m (300 fathoms) a mere 300 metres (900 ft) away from the shoreline.

The climate is dominated by the southeast trade winds which are most pronounced between April and November. Temperatures are around 28 °C (83°F) in the shade during that time, and slightly higher during the northwest monsoon season. At that time, tropical cyclones with torrential rain sometimes hit the island, but overall it is very arid.

Ecology

Flora

Astove Island has a very thin soil layer overlying its rocky core, which is pockmarked with caverns. Guano of nesting seabirds has accumulated in these, and the western part has been worked over and some of the guano was mined in the past. Large stretches of the reef rock were stripped bare of vegetation, but some Pisonia grandis and White Milkwood (Sideroxylon inerme) persisted. The general vegetation on much of the island's western side is herbaceous plants however, mainly the leadwort Plumbago aphylla, as well as Stachytarpheta species and the crowfoot grass Dactyloctenium pilosum. Coconut Palms (Cocos nucifera) remaining from the struggling plantation along the western shore, Sisal (Agave sisalana), and Wild Cotton (Gossypium) are also found here and there. Maize (Zea mays) was planted by the guano miners, but presumably this plant which depends on constant care has disappeared since.

The eastern dunes are overgrown with the dropseed grass Sporobolus virginicus near the sea, and on the higher parts bwa matlo (Bay Cedar, Suriana maritima) shrubs are found. In more sheltered places, a regular scrubland of vouloutye (Scaevola sericea) and Tree Heliotrope (Heliotropium foertherianum), with some Pisonia, occurs. Here, the main herbs are fimbries (Fimbristylis) and the parasitic vine Cassytha filiformis.

The flats around the lagoon show a mixture of grassland (mainly Pembagrass, Stenotaphrum dimidiatum) and Pemphis acidula scrub; in places, it is difficult to penetrate to the lagoon through the mass of Pemphis and bwa matlo. Small Grey Mangroves (Avicennia marina) line much of the southern half of the lagoon rim.

Fauna

Astove Island is home to four landbirds:

• Cosmoledo Abbott's Sunbird (Cinnyris sovimanga buchenorum or C. abbotti buchenorum) - only occurs here and on nearby Cosmoledo
• Madagascar Cisticola (Cisticola cherina)
• Pied Crow (Corvus albus)
• Common Madagascar White-eye (Zosterops maderaspatanus maderaspatanus)

There are few seabirds, probably due to the presence of rats and feral pigs, left behind when the settlement was abandoned. Unlike the other two atolls of the group, (Aldabra and Cosmoledo), there are no predator-free islets except for a few small sandbanks close to the lagoon entrance. At the lagoon, Caspian Terns (Hydroprogne caspia) will often be seen but it is not known whether or not they ever breed on the sandbanks. Indopacific Sooty Terns (Onychoprion fuscatus nubilosus) and Western Black-naped Terns (Sterna sumatrana mathewsi) have also been reported. Green turtles (Chelonia mydas) remain very common here despite years of exploitation and remarkably high numbers will be seen on dives or from tender trips to the lagoon entrance close to high tide.

References

• Piggott, C.J. (1961): Notes on some of the Seychelles Islands, Indian Ocean. Atoll Research Bulletin 83: 1-10. PDF fulltext