Coral Cover, Zonation and Diversity on Reef Slopes of Chagos Atolls, and Population Structures of the Major Species

Substrate cover by corals, soft corals, sand, bare rock and algae was determined by several means for seaward reef slopes of the Central Indian Ocean atolls of Peros Banhos and Salomon, Chagos Archipelago, and in the lagoon of the former. Cover of hard substrate by coelenterates averages 80 O/O between 3 and 42 m, and calcareous algae dominate above and below these depths. Alcyonarian corals contribute about 15 "/,I of thls between 3 and 23 m; at least 5 '/U of rock at all sites is uncolonised. A quantitative method identifies species zones to 45 m in detail and to 60 m more briefly on seaward slopes, and to 25 m on lagoon reef slopes and knolls. On seaward slopes one typical pattern of zones exists of several species and one 'indominant' zone. Different typical zonations exist on the lagoon reef slopes and on knolls. Exceptions to typical zones are recorded. Species richness increases with depth on both seaward and lagoonal slopes to a peak at 20 m after which a steady decline occurs. The diversity curve on seaward slopes results from two overlapping species associations. Species richness has no general correlation with coral cover, or with species zones. Size-frequency histograms are drawn for 8 densely crowded and 5 more dispersed coral populations. The former and 2 of the latter show bimodal distributions; this is interpreted as reflecting a variable growth rate where colonies remain small for variable, sometimes long periods until opportunity allows accelerated growth into the adult canopy or mode. More dispersed species are either bimodal or unimodal with young that grow rapidly immediately. Population structures of zone-forming species reflect intrarather than interspecific competition for unpartitioned resources, and the canopy structure resembles those of phaeophytes and trees.