Carbonate budgets and reef production states: a geomorphic perspective on the ecological phase-shift concept

Recent, region-scale estimates suggest that high levels of coral cover loss have occurred in both the Caribbean and Indo-PaciWc reef provinces (Gardner et al. 2003; Bruno and Selig 2007), and that some 60–70% of coral communities globally are directly threatened by anthropogenic activities (Wilkinson 1996; Hoegh-Guldberg 1999; Goreau et al. 2000). The ecological changes that have resulted from these disturbances have been widely discussed in terms of coral reef ‘health’ and resilience (Hughes et al. 2003; Bellwood et al. 2004) and often aligned with the ecological phase-shift concept originally outlined by Done (1992). A phase shift, in the context of a coral reef, has been deWned as a transition in the ecological state of the reef to conditions of low coral cover and persistent high Xeshy macroalgal cover (Done 1992; McManus and Polsenberg 2004). Whilst episodic natural disturbance events can be important drivers of such ecological transitions, a variety of both direct and indirect anthropogenic disturbances are also widely implicated (Done 1999). Such disturbances might modify natural reef processes, either by altering the ecological balance within individual reef systems (Jackson 1997; Jackson et al. 2001) or the environmental conditions under which reef growth occurs (Kleypas et al. 1999). Whilst earlier concerns over passive reef ‘drowning’ in response to greatly accelerated sea-level rise (e.g. Buddemeier and Smith 1988) have not been maintained (Spencer 1995), there remains considerable concern as to how more modest rates of sea-level rise, alongside climate change-related shifts in chronic stress from changing ocean temperatures and ocean chemistry, may lead to damage acute events occurring at ever more frequent intervals (e.g. Hoegh-Guldberg et al. 2007). The implications for reefs: as geomorphic structures, in terms of the extent of reef framework development and in terms of net carbonate accumulation (Edinger et al. 2000); the geomorphic integrity of reef associated sedimentary landforms; and the ecological eVects, in terms of changes in reef community structures (Chadwick-Furman 1996; Harvell et al. 2002; Hughes et al. 2003; Hoegh-Guldberg et al. 2007) remain, however, unclear. In the scientiWc literature, much of the discussion about the eVects of such ecological and environmental change has focused on quantifying the impacts upon coral communities, especially in terms of monitoring changes in coral cover and diversity, and in coral community structure. Central to many such discussions has been the issue of variations in the relative abundance of corals and macroalgae Communicated by Geology Editor Dr Bernhard Riegl