Australia's coral reefs, including the Great Barrier Reef, are among the region's most sensitive environments to sea-level rise and climate change-through potential inundation, flooding, erosion, saline intrusion, bleaching and death of corals, and possible changes in tropical cyclone occurrence (some of these issues are covered in Section 4.3.4). Climate change impacts will be compounded by the rapid growth in environmental stresses arising from existing population growth and increasing tourism (IPCC 1996, WG II, Sections 9.4.3, 9.4.5). Coral bleaching and algal invasions are sometimes observed on the Great Barrier Reef, but the extent to which they are natural or human-induced is unknown.
Coral bleaching is associated with several factors (including extreme temperatures and solar irradiance, subaerial exposure, sedimentation, freshwater dilution, contaminants, and diseases) acting singly or in combination (Glynn, 1996). During bleaching, corals expel the symbiotic single-celled algae that live within their tissues, and then sometimes die (Brown, 1997). This occurs when sea temperatures rise more than 2°C above normal, which is often associated with ENSO episodes. The frequency of high-temperature episodes will increase as mean temperatures gradually rise, which would result in more frequent and widespread damage to corals-especially those remote from a reliable supply of larvae of reef species or stressed through exposure to local climatic and/or human impacts such as riverine runoff (Larcombe et al., 1996), high solar irradiance (especially ultraviolet wavelengths) (Glynn, 1996), and pollution (Dubinsky and Stambler, 1996). The effects of temperature rise also will depend on latitude, coral height in relation to sea level, and the direction and size of the prevailing wave climate.
There is evidence, however, that the temperature changes by themselves may be slow enough for the coral reef biota to adapt, through changes in their symbiotic partnerships (Brown, 1997) or genetically over a number of generations, as better-adapted genotypes settle and survive. At the scale of whole coral reefs and their communities, migration to higher latitudes may be a possibility. However, there is a host of historical, hydrodynamic, and ecological factors that determine current distributions of coral reef biota (Veron, 1995), and it could be centuries before substantial changes to species composition and increases in diversity are detected at Australia's southernmost coral reefs.
It has been predicted that, by itself, a rise in sea level might benefit many reefs because corals on reef tops would have a renewed and/or extended opportunity for vertical growth-unlike the last several thousand years, when they have been limited by sea level to exclusively horizontal growth (Wilkinson and Buddemeier, 1994; Wilkinson, 1996). Thus, the potential effect of sea-level rise over the next 50 years in tidally limited reef-top coral habitats is an increase in living coral cover and, in many cases, an increase in topographic relief. This scenario is quite likely to be played out in sheltered parts of many reefs, assuming that the majority of these reef-top corals are not killed by increased temperature or other causes (such as burial by sediments if high-intensity rainfall events were to increase). Although the vertical growth of most shallow corals will be able to keep pace with a rate of sea-level rise of up to 5 cm per decade, the mechanisms for reef island growth are not expected to keep pace with sea-level rise, and therefore many low islands may eventually become uninhabitable (Wilkinson, 1996; see also Section 4.3.4).
If reef tops do not keep pace with sea-level rise, there could be dramatic changes in the reef-top zonation of corals, other biota, and abiotic reef substrata because these are strongly controlled by the energy of the waves that pass over the reef (Done, 1983). The tendency would be for the plunging point for waves to gradually move toward the back of the reef, causing changes in the benthic zonation at the new location of the breaker zone and for several meters to tens of meters either side. Moreover, any significant change in the frequency or magnitude of tropical storms (Suppiah et al., 1996; Henderson-Sellers and Zhang, 1997; Holland, 1997) would affect both the structure and growth of coral reefs (Lough, 1994; Wilkinson, 1996), since individual storms can cause considerable damage to reefs, from which it may take a decade or more to recover.
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