Disturbances affecting the reef
The health and resilience of the reef is affected by a range of short-term and long-term disturbances including:
- elevated sea surface temperatures
- crown-of-thorns starfish outbreaks
- pollutant loads.
The impact of disturbances on the reef depends on their frequency, duration and severity as well as the state of the ecosystem (Fabricius 2005; Osborne et al., 2011). Multiple disturbances may have a combined negative effect on reef resilience that is greater than the effect of each disturbance in isolation. Between 2009 and 2011, repeated floods and cyclones had a considerable impact on the water quality and ecosystem status of the inshore area of the Great Barrier Reef.
The summer of 2010-2011 was the second wettest on record in Australia. This extreme weather caused flooding in several catchments and much higher than normal discharge from most rivers. A large expanse of the inshore reef south of Mackay was exposed to persistent flood plumes from the Fitzroy, Burnett and Mary Rivers.
La Niña caused significant rainfall events across Queensland during 2010-2011 which led to greater overall freshwater discharge to the reef. This was primarily due to high flows from the Burdekin, Fitzroy, Burnett and Mary Rivers and all rivers in the Mackay Whitsunday region. Discharges from most rivers in the Wet Tropics and Cape York regions were also at least 1.5 times their annual median flow. This is the fifth consecutive year where the overall freshwater discharge from all rivers has been greater than the long-term annual median. For example, in the 2010-2011 wet season, flows from the Fitzroy and Proserpine Rivers were the largest on record and discharge from the Herbert River was comparable to the record flood in 1994.
The influence of flood plumes on the marine environment depends on the volume and duration of river flows, the influence of wind direction and velocity, local currents and tidal regimes. Flood plumes had an impact on inshore areas along the Queensland coast. The southern section of the Great Barrier Reef Marine Park, in particular, was exposed to large volumes of low salinity flood waters for an extended period which is likely to have contributed to localised coral bleaching on shallow, inshore reefs in the area (refer to coral bleaching section).
In addition to large volumes of freshwater, wet season floods deliver the majority of annual loads of nutrients, sediments and herbicides to the reef lagoon. In 2010-2011, concentrations of herbicides in flood plumes sometimes exceeded those known to have a negative effect on coral and seagrass (Haynes et al., 2000b, Jones and Kerswell, 2003, Magnusson et al., 2010). Regions with a high probability of exposure to elevated concentrations of dissolved inorganic nitrogen and suspended solids were the Wet Tropics, Burdekin and Mackay Whitsunday regions, respectively.
Three tropical cyclones had an impact on the reef in 2010-2011. Cyclone Tasha (category 1) crossed the coast near Innisfail and caused large-scale flooding in the Burnett, Fitzroy and Burdekin Rivers. Cyclone Anthony (category 2) passed through the Burdekin region and was closely followed by Cyclone Yasi (category 5) which crossed the coast near Cardwell in early February 2011. About 13 per cent of the reef, from Cairns to Townsville, was exposed to Yasi's destructive or very destructive winds. The affected area represents a 300 km stretch of the 2400 km-long reef; however, the influence of Yasi extended beyond the destructive wind band with damage also occurring south of Townsville. This has led to the poor marine condition in many regions for 2011. Underwater surveys indicate that 15 per cent of the total reef area sustained some coral damage and full recovery will take decades. Cyclones may cause extreme physical damage to reef structure.
Since 2005, many areas of the reef including the inshore area have been affected by category 4 or 5 cyclones. The combined paths of these cyclones have exposed 3889 reefs (80 per cent of the Great Barrier Reef Marine Park) to gale force winds or above. Most of the affected reefs were outside the inshore area, which is a relatively small proportion of the whole Great Barrier Reef Marine Park (7.8 per cent). Recent estimates attribute 34 per cent of total coral mortality recorded between 1995 and 2009 to cyclones and storms (Osborne et al., 2011).
Coral bleaching commonly occurs when accumulated temperature stress, measured as degree heating days over the summer months, exceeds a threshold of about 60 to 100 degree heating days (Maynard, J.A. 2009). An increase in the long-term average temperature of reef waters is narrowing the gap between a regular summer and a coral bleaching season. For example, the frequency of mass bleaching events has increased over the last two decades, corresponding to higher seawater temperatures. Major coral bleaching events caused by unusually warm water temperatures were not recorded in the Great Barrier Reef Marine Park before a major episode in 1998 that was part of a global event. Similar conditions returned in 2002 and to a lesser extent in 2006. Prolonged exposure to elevated seawater temperatures may increase the susceptibility of corals to disease (Bruno, J.F. 2007).
Degree heating days are a measure of only one potential stress. Coral bleaching may also occur in response to other stressors, such as water that is too cold or too fresh as occurs after floods, poor water quality and exposure to certain chemicals.
In 2010-2011, sea surface temperatures around Australia were the highest on record (Bureau of Meteorology 2010). However, summer conditions on the reef were influenced by a series of extreme weather events including monsoonal cloud cover, rainfall and cyclonic activity which collectively minimised the build-up of heat stress. Coral bleaching across the reef was low to moderate. Most of the bleached areas were in the central and southern sections of the Great Barrier Reef Marine Park following Cyclone Yasi and exposure to large volumes of freshwater, respectively.
Most outbreaks occur on midshelf reefs, beginning along the narrow northern shelf between Cairns and Lizard Island and then moving to southern reefs as larvae are transported by the East Australian Current. The Swains Reefs in the Fitzroy region have had low-level chronic infestations throughout most of the past three decades explained by the high density of reefs in this region and the regional oceanography.
In 2010-2011, few outbreaks of crown-of-thorns starfish were detected on the northern reefs despite evidence of feeding scars on some reefs. This is because young starfish hide in the reef interior for the first two years emerging only to feed at night. The situation in 2010-2011 is consistent with a new cycle of crown-of-thorns starfish outbreaks on the Great Barrier Reef caused by severe floods in 2009.
Crown-of-thorns starfish have had a major impact on the reef with a recent analysis of long-term monitoring data showing the starfish has been responsible for more than 40 per cent of the decline in coral cover since 1985. The increasing incidence of crown-of-thorns starfish in recent decades may be linked to enhanced survival of larvae from nutrient-rich flood waters and increased availability of phytoplankton as a food source (Brodie, J. 2005; Fabricius, K.E. 2011). However, a reduction in predator populations has also been suggested as outbreaks are lower in zones closed to fishing (Osborne, K. 2011). The high discharges from most rivers draining into the Great Barrier Reef lagoon in 2010-2011 created conditions likely to trigger additional outbreaks to the ones started by the 2009 floods.
The extent and persistence of damage to the reef will largely depend on the rate and magnitude of change in the world’s climate and on the resilience of the reef ecosystem (GBRMPA 2009a). The future is not easily forecast, but there is strong evidence that halting and reversing the decline of water quality in the Great Barrier Reef lagoon will increase the natural resilience of Great Barrier Reef ecosystems to future challenges.
The reef receives runoff from 35 major catchments which drain 424,000 square kilometres of coastal Queensland. The reef region is relatively sparsely populated; however, there have been extensive changes in land-use since European settlement driven by increased urban, agricultural and industrial development particularly in areas adjacent to the coast (Furnas, M. 2003; Hutchings, P. 2005). Unfortunately, the combination of expanding catchment development and modification of land-use has resulted in a significant decline in the quality of water flowing into the reef lagoon over the past 150 years (Moss, A.J. 1992; Neil, D.T. 2002; Furnas, M. 2003; McCulloch, M. 2003). Flood events in the wet season deliver low salinity waters and loads of nutrients, sediments and pesticides from the adjacent catchments into the reef lagoon that are well above natural levels and many times higher than in non-flood waters (Department of the Premier and Cabinet 2009). Pesticides, which are manufactured chemicals with no natural level, are now widespread in Great Barrier Reef waters.
Numerous studies have shown that nutrient enrichment, turbidity, sedimentation and pesticides all affect the resilience of the reef ecosystem, degrading coral reefs and seagrass meadows at local and regional scales (Department of the Premier and Cabinet 2008; Waycott, M. 2010; Fabricius, K.E. 2011). Pollutants may also interact to have a combined negative effect on reef resilience that is greater than the effect of each pollutant in isolation. For example, the reduced light and excess nutrients found in turbid flood plumes combine to increase the level of stress on seagrasses, and differences in tolerance between species of adult coral to nutrient enrichment and sedimentation can lead to changes in community composition (Collier, C. 2009; van Dam, J.W. 2012; Fabricius, K.E. 2005; Fabricius, K.E. 2011).
Generally, reef ecosystems decline in species richness and diversity along a gradient from outer reefs distant from terrestrial inputs to near-shore coastal reefs more frequently exposed to flood waters (Cooper, T. 2007; Fabricius, K.E. 2011). The area at highest risk from degraded water quality is the inshore area which makes up approximately eight per cent of the Great Barrier Reef Marine Park within 20 kilometres of the shore. The inshore area supports significant ecological communities and is also the area of the reef most utilised by recreational visitors, commercial tourism operators and commercial fishers.
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