The El Nino-Southern Oscillation (ENSO) cycle is a quasi-periodic alternation of climate states across the tropical Pacific Ocean on timescale of 2-7 years that underlies Australia's classic cycle of droughts and floods.
For most of 2010-11, the climate remained in a strong La Nina state which is characterised by heavy rainfall and high sea surface temperatures in the western Pacific. As a result, Queensland experienced devastating floods in January that spared little of the state, resulting in huge economic costs and tragic loss of life. In north Queensland, the Burdekin River which drains the largest catchment in Queensland is estimated to have emptied 27 million megalitres (a volume in excess of 50 Sydney Harbours) of silt-laden waters into the Great Barrier Reef lagoon entering just 100 km south of Townsville. The IMOS National Reference Station located in coastal waters to the northeast of the river mouth recorded the passage of low salinity flood plumes with high loads of sediments and nutrients that fuelled algal blooms with impacts on light levels and oxygen. On the few unclouded moments, satellites captured images of discoloured water containing dissolved organic matter extending hundreds of kilometres from the river mouth and 20-50 km from the coast.
At the end of January, Severe Tropical CycloneYasibegan as a tropical low near Fiji but intensified to a Category 5 system as it crossed the Coral Sea towards the Queensland Coast making landfall near Mission Beach on 3 February. According to the Bureau of Meteorology, TC Yasiwas one of the most powerful cyclones to have affected Queensland since records commenced with events of similar magnitude previously recorded only in 1899 and 1918.
Even though Townsville was well south of the belt of very destructive winds, the IMOS equipment deployed across the shelf near AIMS recorded the extreme nature of this disturbance on the local marine environment. Wind gusts of 180 km/h were recorded on the mid-shelf and wave heights of 9-10 metres were recorded in the GBR Lagoon. A mooring in 70m of water was tilted to 45 degrees (they are normally vertical) by the currents forced through the outer reef matrix while instruments recorded a turbidity storm as bottom sediments were mixed throughout the entire water column. A deeper mooring on the continental slope showed surface waters typical of the top 20m at 190m as the cyclone passed over head. AIMS' oceanographers believe this was caused by the cyclone pushing a wave of water onto the continental shelf that could only escape by flowing backwards at high velocity along the bottom.
From the end of the wet season, the Great Barrier Reef Marine Park Authority began to record a marked increase in the numbers of dead dugongs and turtles washing ashore in the region with empty guts and diminished fat reserves. This was attributed to the loss of shallow seagrass beds caused by the extensive flooding. In June, AIMS mounted a joint mission with the IMOS AUV Facility based at Sydney University to survey deep water seagrass beds with an autonomous underwater vehicle equipped for high resolution benthic photography. In 2005-06, the Great Barrier Reef Seabed Biodiversity Project showed extensive seagrass beds in 30m depths in the middle of the GBR Lagoon. In 2010-11, after the summer of extreme weather, extensive searching of these historical sites using the AUV found no seagrasses and everywhere low levels of large animal life.
Future surveys will monitor the rate of recovery of the seabed communities. Meanwhile, turtle experts are worried about the long-term impact of the loss of so many Green Turtles (a 700 per cent increase on average strandings) on the future resilience of their populations.