These processes are interconnected and the overall health of the ecosystem requires all to be in good condition. Many are important attributes recognised as contributing to the outstanding universal value of the Great Barrier Reef World Heritage Area (Appendix 3). The individual processes assessed have remained the same as those in the Outlook Report 2009, except the ecological process of recruitment has been included; and, recognising that it is not a natural process, consideration of pesticide accumulation has been relocated to Section 6.5. An assessment of the condition of terrestrial habitats that support the Great Barrier Reef is also included in this Outlook Report. This recognises the important role of terrestrial habitats in the health of the Great Barrier Reef — from capturing nutrients and sediments to providing feeding and breeding areas for a range of species. As in the Outlook Report 2009, outbreaks of disease and introduced and pest species are examined as their frequency and severity are a gauge of overall ecosystem health.
3.2 Current condition and trends of physical processes
The Great Barrier Reef is part of a larger system of ocean circulation throughout the Pacific Ocean, which delivers nutrients and larvae from other regions as well as deep water into the Great Barrier Reef Region (the Region). Currents and upwellings are recognised as key ecological processes that contribute to the Reef’s outstanding universal value.3 At the largest spatial scale (thousands of kilometres), major oceanic currents of the Coral Sea affect patterns of connectivity and the temperature of the Region’s waters.4 At very small scales (centimetres to metres) turbulence can affect the larval settlement patterns of a range of species such as corals.4 While surface currents are primarily driven by wind, deeper ocean currents are mainly driven by relative densities of seawater, affected by salinity and temperature.4
There is evidence of intensified flow and accelerated warming in the East Australian Current.
Upwelling of cold, nutrient-rich waters to the sea surface creates ‘hotspots’ of marine primary production.4 In the Great Barrier Reef, upwelling intrusions include those on the central Great Barrier Reef which are enhanced during consistently low winds.5 During these conditions, the southward-flowing East Australian Current flows faster, lifting the thermocline closer to the surface, spilling cooler waters onto the shelf.4,5,6 The Outlook Report 2009 reported there was little information about any changes to ocean currents on the Great Barrier Reef.1 Since then there has been increasing evidence of intensified flow and accelerated warming in the East Australian Current adjacent to the Region’s southern coast (see Section 6.3.1).7 This current is transporting greater volumes of warmer water southward, carrying larvae and juveniles with it.8 There remains little information about the Hiri Current which moves north along the coast in northern Great Barrier Reef waters.7,9,10
3.2.2 Cyclones and wind
Cyclones regularly affect tropical marine and terrestrial habitats at regional and local scales. In addition to strong winds and rain, the powerful waves generated during cyclones can seriously damage habitats and landforms, particularly coral reefs and shorelines.4,11,12,13 It is estimated that cyclone damage has been one of several factors in coral cover loss in the Region.14 Between 2005 and 2013, there were six category 3 or above cyclones that affected the Great Barrier Reef (Figure 3.2).15,16
Figure 3.2 Number and severity of cyclones, 1970–2013
A number of severe cyclones have affected the Region over recent years. Source: Bureau of Meteorology17