Disturbance and the Dynamics of Coral Cover on the Great Barrier Reef (1995–2009)

Abstract

Coral reef ecosystems worldwide are under pressure from chronic and acute stressors that threaten their continued existence. Most obvious among changes to reefs is loss of hard coral cover, but a precise multi-scale estimate of coral cover dynamics for the Great Barrier Reef (GBR) is currently lacking. Monitoring data collected annually from fixed sites at 47 reefs across 1300 km of the GBR indicate that overall regional coral cover was stable (averaging 29% and ranging from 23% to 33% cover across years) with no net decline between 1995 and 2009. Subregional trends (10–100 km) in hard coral were diverse with some being very dynamic and others changing little. Coral cover increased in six subregions and decreased in seven subregions. Persistent decline of corals occurred in one subregion for hard coral and Acroporidae and in four subregions in non-Acroporidae families. Change in Acroporidae accounted for 68% of change in hard coral. Crown-of-thorns starfish (Acanthaster planci) outbreaks and storm damage were responsible for more coral loss during this period than either bleaching or disease despite two mass bleaching events and an increase in the incidence of coral disease. While the limited data for the GBR prior to the 1980's suggests that coral cover was higher than in our survey, we found no evidence of consistent, system-wide decline in coral cover since 1995. Instead, fluctuations in coral cover at subregional scales (10–100 km), driven mostly by changes in fast-growing Acroporidae, occurred as a result of localized disturbance events and subsequent recovery.

The 27–year decline of coral cover on the Great Barrier Reef and its causes

“…The rate of decline has also increased substantially, and has averaged ∼1.45% y −1 since ∼2006. Both the overall and more recent rates of decline are higher than previous estimates (13,14), which were either based on time series that ended in 2005 (14) or covered a shorter period (1995-2009) and surveyed far fewer reefs using a different survey method (13). The disturbance data for COTS or cyclones show periodic and random fluctuations but no systematic long-term variation over the 27-y observation period, and given that GBR coral cover was likely higher than 28% before 1985 (2), the decline in coral cover may have started long before then.…”

“…The recent frequency and intensity of mass coral bleaching are of major concern, and are directly attributable to rising atmospheric greenhouse gases (3). To date, the GBR has lost fewer corals to bleaching and diseases than many other regions in the world (13,24), but bleaching mortality will almost certainly increase in the GBR, given the upward trend in temperatures (25).…”

“…Fishing, although intense near the coast and urban centers, is banned in 33% of the GBR and is regulated elsewhere (11). Nonetheless, the GBR has been subject to severe disturbances, including COTS outbreaks, mass coral bleaching and declining growth rates of coral due to increasing seawater temperatures, terrestrial runoff, tropical cyclones, and coral diseases (2,3,(12)(13)(14). The runoff of soils, fertilizers, and pesticides from agricultural and coastal development has significantly affected inshore coral reefs (12,(15)(16)(17), and has likely increased COTS outbreak frequencies (5,18).…”

“…The runoff of soils, fertilizers, and pesticides from agricultural and coastal development has significantly affected inshore coral reefs (12,(15)(16)(17), and has likely increased COTS outbreak frequencies (5,18). Conclusions of scientific studies on the condition of the GBR, based on different datasets and various time periods, have ranged from evidence for fluctuations from localized disturbances (13,14) to ecosystem-wide declines (1,2).…”

Coral Reef Resilience through Biodiversity

“…Halford et al [107] found complete recovery of corals and nearly total recovery of �sh assemblages over 80 km of the Great Barrier Reef about a decade aer major storm damage. Monitoring of sites across 1300 km of the Great Barrier Reef (1995 to 2009) indicated increases in coral cover for some reefs but decreases for others [115]. Recently, De'ath et al [9] reported a loss of over 50% coral cover along the Great Barrier Reef from 1985 to 2012 based on surveys of 214 reefs, attributing the decline to cyclones (48%), Crown-of orns star�sh (42%), and bleaching (10%).…”