McCook, 1999:

"Degradation of coral reefs often involves a 'phase shift' from abundant coral to abundant macroalgae. This paper critically reviews the roles of nutrient increases in such phase shifts. I conclude that nutrient overloads can contribute to reef degradation, but that they are unlikely to lead to phase shifts simply by enhancing algal growth rates and hence allowing overgrowth of corals, unless herbivory is unusually or artificially low. Concentrations of dissolved inorganic nutrients are poor indicators of reef status, and the concept of a simple threshold concentration that indicates eutrophication has little validity. I discuss the significance and consequences of these assessments for reef management, focusing on the Great Barrier Reef, and conclude with some specific recommendations, including protection of herbivorous fishes, minimisation of terrestrial runoff, and protection of coastal reefs."

AND

"Reduction in herbivory Given the well documented, high primary productivity of coral reef algae in oligotrophic waters (e.g. Hatcher 1988), a substantial reduction in herbivory can be assumed to lead to increases in algal standing crop, as tissue production outweighs consumption by herbivores, even if there is no change in nutrient availability (also see Bell 1992). Numerous studies (cited previously) have shown increases in algal abundance in response to herbivore reduction with no change in nutrient supply, demonstrating that enhanced nutrients are not necessary for phase shifts at a wide variety of locations in the Caribbean and GBR.

Combined increase in nutrient supply and reduction in herbivory As suggested by Littler and Littler's (1984) model, reduction in herbivory may have a greater effect if there is simultaneous nutrient enhancement and consequent increase in algal production."

AND

"Mechanisms and significance of herbivory tracking algal production

The conclusion that consumption by herbivores can track and absorb even several-fold increases in algal production (reviewed in previous section) is particularly significant because it suggests that abundant herbivore populations can buffer and protect reef communities against changes in algal production. Russ and McCook (1999) suggest that the mechanism of this tracking is more likely to involve changes in individual consumption rates than changes in herbivore population densities, given the rapid time scale of the response (although herbivore populations may show some response in post-recruitment survival at time-scales of years e.g. Carpenter 1990; Robertson 1991). Assuming an upper limit on individual consumption rates, this in turn implies an upper limit on the potential of herbivore populations to absorb extra algal production. Thus the ability of herbivore populations to absorb extra algal production may also serve to suppress indications of shifts in trophic balance, delaying recognition of nutrient overloads until the reef is very stressed and vulnerable. This herbivore 'buffer' effect will be least effective on reefs or zones where populations are naturally or artificially low (such as fringing reef flats on inshore continental islands of the GBR, McCook 1997)."

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Citation and/or URL

McCook, L.J. 1999, Macroalgae, nutrients and phase shifts on coral reefs: scientific issues and management consequences for the Great Barrier Reef, Coral Reefs, 18: 357-367 

Spatial Coverage

Review - Great Barrier Reef and Caribbean

Temporal Coverage

Not applicable

Update Frequency

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Other Information

None