December 1997 Newsletter

LONG-TERM DETERIORATION OF FRINGING REEFS OF THE SOUTHERN GREAT BARRIER REEF?

During surveys of near-shore fringing reefs in the southern Great Barrier Reef, a team of AIMS scientists have discovered that previously healthy, coral-dominated reefs have become overgrown by macro-algae, reports CRC researcher Dr Lyndon Devantier.

International concern that replacement of reef-building corals by non-reef-building organisms (mainly algae) has been linked to increasing human activities in the coastal zone. Perhaps the most striking example is from Jamaica in the Caribbean Sea, where over-fishing algal-feeding fishes, epidemic disease of sea-urchins, and coral death from hurricanes all caused a major shift from coral-dominated to algal-dominated reefs.

Death of corals around the Keppel Isles from Fitzroy River flood plumes following Cyclone Joy in 1991 has been well documented. But further offshore and to the north, around the Percy Isles, cause of coral mortality is less clear-cut. The team found evidence of past reef growth around the Percy Isles in the form of large areas of staghorn coral rubble and dead massive corals, particularly at Pine Islets and Pine Peak Island.

The dead reef-building corals were densely covered by thick layers of macro-algae. The macro-algal overgrowth occurred from the reef flats down to the base of the reef slopes below 10 metres - quite unusual on near-shore reefs. During the brief time available for the surveys it was not possible to determine the precise time or cause of death of the corals. However, growth of other organisms on the dead corals suggest a minimum period since the mortality event(s) of more than a decade. Although many of the algae exhibit marked seasonal fluctuations in biomass, it is apparent their continued growth is inhibiting coral recovery.

The islands lie near an area of high tidal range (~10m) which peaks in Broad Sound. It has long been recognized that the amount of near-shore fringing reef development diminishes with increasing proximity to Broad Sound. The traditional explanation has been that resuspension of sediments by the strong tidal currents causes turbidity reducing light penetration, thereby inhibiting coral growth and reef development. Nonetheless, the presence of large (now dead) massive corals and abundance of staghorn coral rubble at the Percy island group and Pine Peak Island indicate that there have been periods of at least 1 - 2 centuries when coral growth has flourished.

The tidal hypothesis may not be sufficient to explain the apparently recent (decade - century) large-scale death of corals and growth of macro-algae in the area. Indeed, the proliferation of the photosynthetic algae to the base of the reef slopes suggests that light alone is not a limiting factor. Further work is required to determine time and cause of the coral mortality in the area, and the prognosis for recovery based on medium to long-term dynamics of the coral - algal interactions. For example, continuing research on other near-shore GBR reefs indicates that corals can replace algae over periods of a decade following disturbance provided that conditions for coral growth remain favorable.

Nonetheless, the present findings raise cause for concern about the continued `health' of our near-shore coral reefs. Clearly, there is no room for complacency in our approach to research and management for a sustainable future for the Great Barrier Reef. These CRC surveys form part of a large research program focused on the environmental status of near-shore reefs in relation to natural disturbances and human activities in the coastal zone.

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