05 April: Condition of Great Barrier Reef corals before the mass bleaching event in 2016

• In 2012, the Australian Institute of Marine Science reported that average coral cover on the GBR had fallen by half over the preceding 27 years.
• Recent data collected by the AIMS Long Term Monitoring Program from 2012 – 2015 provide a more accurate reference point for understanding pre-bleaching conditions on the GBR.
• An updated analysis of the regional and Great Barrier Reef-wide trends shows that from 2012 to 2015 hard coral cover in the central and southern sections of the reef had increased (see Figure 1 ). In contrast, the northern section shows a decline in coral cover over these three recent years because of an intense cyclone (a second cyclone occurred after the most recent survey) and renewed activity of crown-of-thorns starfish in the region.

Figure 1: Temporal trends in coral cover (mean >+/-95% confidence intervals) for the entire GBR and for the Northern, Central and Southern regions (see Figure 3 for boundaries of regions and sampling sites in 2013-15). N indicates the number of reefs contributing to the analyses. Hard coral cover was estimated using the manta tow technique and trends in coral cover were modelled using logistic GAMMs incorporating cubic spline smoothers¹ for Year. (see Table 1 for a summary of selected data)

• With AIMS’ reef surveys extending over more than 30 years, the AIMS Long Term Monitoring Program provides an invaluable record of change in coral reef communities over a large area of the Great Barrier Reef (GBR). Several survey trips are completed each year and detailed reports on the condition and trends of individual reefs are available here.
• This past week, field observations and aerial surveys reported severe coral bleaching from north of Cairns to Torres Strait, and the Great Barrier Reef Marine Park Authority (GBRMPA) lifted its response to “level 3” – GBRMPA’s highest level of bleaching incident response. Reef-wide surveys are underway right now with results expected over the next weeks. Recent observations of bleaching on the reefs further south indicate widespread bleaching of variable intensity, ranging mostly from minor to moderate.
• While observations made from aerial and diver-based surveys indicate a clear north-south gradient in severity of bleaching, the situation changes almost on a daily basis as corals remain under stress from higher than normal water temperatures. With water temperatures forecast to remain warm for at least the month of April and corals already suffering from accumulated heat stress from the summer conditions, we will have to wait until later autumn to know the total mortality associated with the current bleaching event.
• While the Reef’s coral cover has improved in recent years, the widespread bleaching event will affect its condition. Not all corals that bleach will die, but even partially bleached corals have reduced reproduction and growth for up to two years, which is likely to slow or halt further recovery.
• Especially in the Northern region, which up to 2012 was seen as the healthiest region, the significant loss of coral cover that occurred from 2012-15 is now compounded by the severe bleaching. In the Central and Southern regions, the prospects for ongoing recovery will depend on whether the bleaching will lead to significant coral mortality.
• The decline of coral cover on the mid-shelf and offshore reefs from 1985 to 2012 was caused by the cumulative impacts of severe tropical cyclones, damage by the predatory crown-of-thorns starfish (COTS) and the previous two mass bleaching events in 1998 and 2002. Additional environmental pressures such as reduced water quality and increased water temperatures further reduce reef resilience, i.e. all affecting the ability of coral reefs to recover from acute disturbance events such as such as cyclones and storms.
• There is a concern that the impact of severe disturbance events is increasing, which together with chronic pressures such as rising sea temperatures (Figure 2) will have negative implications for coral health. For example, a recent study predicted that extreme El Nino events, like the one associated with the 2016 bleaching event, may become more frequent in the future.

Figure 2: Annual sea surface temperature anomalies (from the 1961-1990 mean) averaged for the Great Barrier Reef, 10.5-24.5S, 1871 to 2014. Data source: HadISST1 (Rayner et al 2003, J. Geophys. Res. 108, doi:10.1029/2002JD002670.

Figure 3: Boundaries of Northern, Central and Southern regions used in the trend analyses (Fig 1), and locations of the manta tow surveys of hard coral cover in 2013-2015.

¹Temporal trend estimates for 1986-2012 differ slightly from the trends published previously by De'ath et al. (2012) because of the addition of new data (an additional 25 reefs for the whole observation period, 3 new years and slightly different boundaries between regions) and the inevitable dampening of extremes from modelling approaches that utilise spline smoothing functions.