The groove and spur structures of Chagos Atolls and their coral zonation

The groove and spur formations of two atolls of the Chagos Archipelago, Indian Ocean are described and the coral zonations on and immediately to seaward of them outlined. A gradation of spur and algal ridge structures are traced, from very rudimentary calcareous spurs in one lagoon through to full development of 75 m long spurs integral with a broad, high ridge on some seaward reefs. Spurs may exist without a ridge in areas of low wave energy, while a ridge never existed without spurs. Unusual structures existed in a lagoon pass where two superimposed sets of grooves and spurs intersect at right angles, and on seaward shores that lack a reef flat. The corals from low water to _ 5 m are zoned in relation to depth and distance from the algal ridge. On seaward reefs two categories of coral community exist; one dominated by encrusting Acropora palifera and one that almost entirely lacks this species. The former are the most common, and have a coral cover of 50% or more. A. palifera-absent communities occur on the longest and most exposed spurs only and have a coral cover of 5_10%. Other species occur in approximately the same order in both cases, though in different proportions, and at least five species are characteristic of this turbulent area.
Sheppard Charles .
marine algae, spurs, calcareous sediments, algae, coral atolls, zonation, coral reefs