Variation on coral reefs
Environmental Status: Corals
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| Storm waves have significant effects on coral cover and composition. A cyclone can reduce coral cover to zero or leave only the most robust corals standing |
Variation on coral reefs
Variation across space
Although some systematic cross-shelf and north-south trends can be found among the reefs of the Great Barrier Reef, a great deal of natural variation can exist among these reefs. It is not unusual for coral cover to vary greatly between two reefs that are relatively close to each other, as events such as storms or crown-of-thorns starfish outbreaks may devastate one reef while leaving a nearby reef relatively untouched. Furthermore, widely differing levels of coral cover and species assemblages (or community composition) are normally encountered at different depths, or on different sides of a reef.
Variation over time
Coral cover and community composition can vary greatly over time. During the summer, warmer water temperatures prompt the rapid growth of macroalgae on many inshore reefs, altering the reef’s appearance and community composition. However, during the cooler winter months much of this marcoalgae dies and corals become dominant once more. Variability over time is further increased by disturbance events such as crown-of-thorns starfish outbreaks and cyclones. These events can reduce coral cover to zero but subsequent recovery and regrowth should return coral cover to pre-disturbance levels. As such, a ‘normal’ reef could exist in any of the following states:
- low level of coral cover and reduced diversity (e.g. reef severely damaged by a recent cyclone with more fragile species being disproportionately affected);
- intermediate levels of coral cover and fluctuating community composition (e.g. reef recovering from the cyclone, some species recovering, pulse of new recruitment and growth to colonise empty space, coral cover gradually increasing and bare substrate being colonised by new corals);
- high levels of coral cover (e.g. the reef exists in a relatively stable state or state of gradual change with mildly fluctuating coral cover and community composition).
The transition between these states may be as rapid as one or two years and any of these three states could be considered “normal” for a healthy reef. As such, the Great Barrier Reef has been described as a “patchwork mosaic” of reef communities at various stages of growth or recovery at any one time.
A number of studies have demonstrated the level of variability and extent of reef community mosaics. In one 30-year study on Heron Island in the southern Great Barrier Reef, coral cover was found to vary between 0 and 80% depending on the site. In another study, annual surveys of inshore reefs demonstrated large fluctuations in coral cover and community composition from year to year in response to cyclones, freshwater flood events and coral bleaching (see below). Furthermore, the extent of these impacts and recovery rate depended on the community composition present before and after the impact, and environmental conditions.
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A comparison of coral cover at two locations at Snapper Island demonstrates the variability of coral reefs. In March 1996, the Daintree region received heavy rainfall resulting in major freshwater runoff. Flood plumes inundated Snapper Island and coral cover on the south facing sites was reduced from 90% to just 10% with most of the surviving colonies being resistant Porites corals. Meanwhile, the northern facing sites (only 3 kilometres away from the southern sites) were only slightly affected, as they were protected from the flood plume. However, coral cover was dramatically reduced at these northern sites later by the 1998 reef wide bleaching event, and cyclone Rona in 1999. In contrast, the southern sites, while still having low coral cover, did not suffer such dramatic reductions as the remaining Porities corals are very robust and weathered the impacts of both the bleaching event and the cyclone. New growth of fast growing Acroporidcorals is now occurring at the southern Snapper Island sites which should result in a rapid increase in coral cover - unless the reef is affected by some other disturbance event.
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| Photographs courtesy of Dr. Terry Done, Australian Institute of Marine Science | ||
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The variability of coral reefs is vividly demonstrated in another long-term study by AIMS scientists who regularly photograph patches of coral reef. In the example shown below, a patch of reef at Rib Reef in the Townsville area of the Great Barrier Reef Marine Park (GBRMP) had very high coral cover and diversity in 1980. However, an outbreak of crown-of-thorns starfish in 1985 and damage caused by Cyclone Aivu in 1989 reduced coral cover to almost nothing. However, over the next seven years, coral cover rapidly increased and by 1996 had reached pre-disturbance levels.
A different type of long-term study has involved the analysis of coral density bands, which provide estimates of growth similar to that obtained from tree rings. This study has shown that a full analysis of the last 231 years indicates a series of repeated declines and recoveries of similar or even greater magnitude.
Another method of investigating possible cases of reef degradation over long periods is through the comparison of historical photographs of the reef with contemporary scenes from the same location. A comprehensive survey of historical photographs dating back to 1893, undertaken by the Great Barrier Reef Marine Park Authority (GBRMPA), has indicated that out of 14 reefs investigated:
- six showed no obvious changes;
- four showed decreases in hard coral cover; and
- four showed decreases in coral cover only in certain areas.