McCook et al., 2001:

"There is increasing concern globally that enhanced runoff from human land uses is leading to the degradation of coral reefs. Land-clearing, deforestation, excess fertilisation of agriculture, and sewage runoff have all been implicated in contributing to nutrient and sediment overload of coral reef waters, leading to so-called "phase shifts" in which areas formerly dominated by corals become overgrown by algae (e.g. Smith et al., 1981; Hatcher et al., 1989; Done, 1992; Edinger et al., 1998). These changes have serious ecological, environmental, and economic consequences. On the Great Barrier Reef (GBR) in particular (Figure 1), there is concern that abundant macroalgae on inshore fringing reefs indicate degradation due to anthropogenic increases in terrestrial inputs of sediments and nutrients (Bell & Elmetri, 1995; reviewed in McCook & Proce, 1997a; McCook & Price, 1997b; Wachenfeld et al., 1998; Atkinson, 1999; Prideaux, 1999)." 

AND

"The impacts of chronic long-term stresses such as overfishing or eutrophication on established communities may be relatively small, but may be much more severe where those communities are also subjected to acute, short-term disturbances, whether natural or human in origin. Coral reef communities are naturally subject to frequent major disturbances, such as cyclones, crown-of-thorns starfish outbreaks, or bleaching, and may be able to recover rapidly from such events. However, the recovery process may be hampered by chronic human impacts (Kinsey, 1988), and, in particular, rapid macroalgal growth subsequent to a disturbance may prevent coral regrowth or recruitment and reef recovery (Connell et al., 1997; Hughes & Tanner, 2000).

This is well illustrated by the model results in Figure 3, which show a matrix of community trajectories for increasingly eutrophic conditions and increasing frequencies of acute coral damage. It can be clearly seen that the coral cover declines more severely when subjected to both eutrophic conditions and frequent disturbances than accounted for by either factor alone.

This observation has important implications in terms of attributing causality of the decline in coral cover. The immediate cause of coral death may be natural, but the failure to recover, and consequent long-term decline in reef condition, may in fact be a direct consequence of the human-derived stresses (discussion in McCook, 1999). However, such causality would be very difficult to demonstrate in a field study, because the changes caused by the human impact are intrinsically confounded by the often much larger changes caused by natural events."

 Figure 3 Combined effects of eutrophication and disturbances on coral and algal trajectories. Matrix of community trajectories for combinations of circumstances from oligotrophic to eutrophic (left to right), and from no disturbances to frequent disturbances (top to bottom). It can be seen that overall coral cover (blue line) is reduced more when frequent disturbances occur in eutrophic conditions (bottom right), compared to either frequent disturbances alone (bottom left) or eutrophic conditions alone (top right).

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Citation and/or URL

McCook, L.J., Wolanski, E. & Spagnol, S. 2001, Modelling and visualizing interactions between natural disturbances and eutrophication as causes of coral reef degradation, In Oceanographic processes of coral reefs: physical and biological links in the Great Barrier Reef, ed. E. Wolanski, CRC Press, Boca Raton, 113-125 

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