Constancy and change in coral reef habitats along depth gradients at Curaçao.

The cover of the main components of the substratum, their spatial relations as well as mortality of the most important living component (Scleractinia) were studied at the leeward reef of Curaçao, Netherlands Antilles. We used a point intercept method to analyse cover as well as change in spatial arrangement in sets of photographs of the same 12 quadrats (3mx3m) taken in 1973 and 1978. Four quadrats were situated, along each of three transects, on the reef slope at depth of 10, 20, 30 and 40 m.Cover was very constant in both living and non-living components over the study period. There was a small but significant change in coral cover caused by a decrease at 10 and 20 m.Spatial arrangement of substratum components was subjected to changes equally large in living and non-living components. There was a significant difference in the magnitude of such changes between the shallower (10, 20 m) and the deeper quadrats (30, 40 m), the spatial rearrangement being much greater in the shallower habitats. In addition, there are important variations in the relative spatial change of the different coral species. The observed patterns of species that are more and less mobile through time, such as Agaricia agaricites and Montastrea spp. respectively, are related to life history phenomena such as recruitment and mortality.Mortality of corals was studied using interval (8-10 months) sets of photographs. We found mortality to be high in colonies (≧30 cm diameter) of A. agaricites and low in A. lamarcki and Montastrea spp. Mortality of coral colonies in this size class is often catastrophic in character.Our evidence indicates that community organization in deep coral reefs, both along the depth gradient and along the coast, is more influenced by spatial rearrangement of the substrata than has previously been recognized.