Wednesday, 15 October 2008

What are we doing here?

A little bit about the Mascarene Plateau

The Mascarene Plateau, in the Southwestern Indian Ocean, is about 2200 km in
length, running from the Seychelles Bank at 4°S to the island of Mauritius at
20°S. As you can see on the left its oriented roughly north–south similar to a crescent. It is characterised by a series of islands, banks and shoals separated by deeper ridges and channels. The main banks are called the Seychelles Plateau, the Saya de Malha Bank, the Nazareth Bank and the Cargados-Carajos Bank. These are typically 20-100 m deep, coral topped and sometimes break the surface to form small islands – in fact we will anchor off Coco island later today! The Plateau is surrounded by steeply descending slopes so that water depths rapidly increase to 3000-4000 m on either side of the plateau - as you can see from the figure below. The nature of this long fractured plateau is thought to form a barrier to the surrounding ocean circulation. Some of the shoals are more than 250 km wide. The Mascarene Plateau is a rare example of an extensive shallow-shelf sea completely detached from land boundaries and is, except for the Shoals of Capricorn Marine Programme and Darwin Initiative in 2000 – 2001, a largely unexplored marine ecosystem.

The red line in the picture above relates to the bathymetry figure on your left which shows the steep rise associated with the Mascarene Plateau. We think that the flow moves up onto the plateau in the east bringing with it (up and over the plateau) an increase in nutrients.

So what do we hope to see?

The most northerly part of the study area is under the influence of the northeast
monsoon from December to February, but during the remainder of the year the whole area is under the influence of the Southeast Trades and the South Equatorial Current (SEC). Recent studies in this area suggest that the shallow Mascarene Plateau acts as a barrier to the predominantly westward zonal flow of the SEC causing it to split into a number of tributaries, which then become channelled through the deep gaps separating the shoals. This creates some interesting questions as to the effect this channeling has on the plateau's ecosystem.

The figure left shows the position of the banks and the velocity of the surface circulation. What we can see really clearly here is how these deep gaps influence the surface circulation as the flow associated with the South Equatorial Current is channelled through each gap...

Key Questions and a cruise track!

1. What is the influence of the South Equatorial Current on the waters and ecosystem over the Mascarene Plateau?

2. In what way is the flow of the South Equatorial Current affected by the gaps in the Mascarene Plateau?

3. Is the Mascarene Plateau characterised by an increased diversity in habitats and biota?

4. What are the main components in the Mascarene Plateau pelagic ecosystem, its distribution and abundance?

5. What are the biodiversity of the pelagic ecosystem, and the main fauna of the demersal fish community?

6. Can the Mascarene Plateau be considered a Large Marine Ecosystem on its own?

....and that’s what we are doing here.. to answer the questions we are deploying a network of CTD stations, multinet stations, bongos and demersal trawls similar to what you see in the track above....

Keep posted for some answers!

posted by Isabelle Ansorge - here at 59°S, 16°30' E

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