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'These three categories are the dominant substrata in the "framework", "pavement" and "sand" zones, respectively, of Figure 15.2 and Section 15.2.3. Varying proportions of one or both of the other two categories may be present.

'These three categories are the dominant substrata in the "framework", "pavement" and "sand" zones, respectively, of Figure 15.2 and Section 15.2.3. Varying proportions of one or both of the other two categories may be present.

major source of limestone sediments). In other words, carbon fluxes, sources, sinks and links, which if measured at the scale of a whole reef would be deemed to signify "degradation", often reflect "healthy" ecosystem function at the scalc of individual zones.

The passage of time is often accompanied by changes in the relative abundance of the different patch types, and the absolute abundance of calcifying versus non-calcifying biota within patches (Hughes 1989; Done 1992a,c). On the one hand, a long period of uninterrupted increase in the cover and biomass of corals and coralline algae can increase metabolic performance in the direction of "continuous coral" (Table 15.1). On the other hand, a period in which mortality and injury of corals and coralline algae exceeds growth and repair can "turn off" the reefs framework building performance altogether, transforming whole reefs or even reef tracts into a state where net physical and biological erosion exceeds net construction (Buddemeier and Hopley 1988). "Turn-on" requires a restoration of coral area, volume, mass and vigor to a level of calcification not less than around 4 kg CaCO* m 2 y which is slightly under half the rate of "continuous coral". Attainment of this level of metabolic performance requires a substantial assemblage of corals and coralline algae, but in the short time scales so far studied, seems to be largely independent of their composition and species diversity.

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