A SeaWifs satellite image showing 'ocean colour' - a measure of the amount of chlorophyll in the water.  The ocean around Chagos is very low in chlorophyll and nutrients (deep blue).  The Chagos archipelago (and others in the tropical area) are centered in patches of higher productivity (light blue) caused partly by the shallow surfaces providing substrate for algae, and partly, it is suspected, because the giant undersea mountains which support the atolls generate mild upwelling which brings nutrients to the surface, causing plankton to bloom.  Another theory sugests that the productivity from the shallow water 'leaks' nutrients to the surrounding area, and it is this which raises plankton growth.   Either way, the area of raised productivity extends across a much greater area than that actually occupied by the shallow atolls.  Currents generally cause a drift from Indonesia to Africa.  This is seen in the large tendrils extending westwards.  In this scene, these miss Chagos and cross the Maldives.  It is probable that the Maldive-Chagos chain, and the Seychelles-Mascarene chain, provide vital shallow 'stepping stones' for the transport of larvae across this ocean.  The patches of red sea are very high productivity, caused mostly by sewage pollution.  Medium levels of productivity are seen around Arabia (Arabian upwelling system) and in the band of cold water around Antarctica.
