Coral Bleaching Events 1998 and 2002, Great Barrier Reef

Brief description

The extent and intensity of two large-scale (>2,000 km) bleaching events on the Great Barrier Reef in 1998 and 2002 were visually assessed by aerial survey.Out of approximately 2900 reefs in the GBR, the aerial survey covered 641 reefs in 2002 (3-20 March) and 654 reefs in 1998 (9 March to 10 April). Note that the same reefs were not necessarily sampled in both years.There was a difference in the way bleaching category labels were recorded between the two years though the amount of bleaching remained the same.Bleaching categories used in 2002 were:0 (60% bleached).Bleaching categories originally used in 1998 (as referred to in the 2001 paper) were:1 (Extreme >60%), 2 (Very High 30-60%) 3 (High 10-30%), 4 (moderate 1-10%), 5 (Low In the dataset the 1998 values have been modified to match the 2002 values. Therefore data in the e-Atlas are now consistent with one another.The 1998 and 2002 satellite SST data which were used to correlate with bleaching observations were aligned by applying an offset based on weather station data. This alignment was because different AVHRR satellites were used for 1998 and 2002 which had different calibrations.Satellite derived SST variables (maximum temperature, days above threshold, degree-days above threshold, spatial adjustment of temperatures by long-term average) were investigated to see which variable correlated best with bleaching intensity. Max3d (the maximum SST occurring over any 3-day period) was the best predictor of the presence or absence of bleaching. SST data period covers 20 December to 7 March in both 1998 and 2002.Geographical range cover the whole Great Barrier Reef: inshore (fringing reefs around islands and isolated reefs close to the mainland) and offshore (located on the mid- and outer-shelf); and regions (northern, central and southern). To determine scale and intensity of coral bleaching on the Great Barrier Reef and its relationship to SST variables. Modelling using nearest-neighbour-analyses was then used to predict bleaching status for all GBR reefs.In 1998, ground-truth surveys carried out on 23 reefs (11°S-18°S) indicated that aerial survey data are likely to be underestimates.In 2002, temperature data was collected using an in situ data logger (Magnetic Island) and weather station (Davies Reef). Field observations of bleaching were also recorded at both these sites.The category labels from 1998 were changed to correspond to 2002 labels.160 m was the flying height for aerial surveys in both years.Thumbnail images are of coral bleaching from the air and below water at Halfway Island in the Keppel Group.Data have been used for the e-Atlas:http://e-atlas.org.au/content/coral-mass-bleaching-extent-1998http://e-atlas.org.au/content/coral-mass-bleaching-extent-2002

Lineage

Maintenance and Update Frequency: asNeeded

Statement: Statement: SST data derived from advanced high resolution radiometer sensors aboard NOAA14 and NOAA16 satellites at a spatial resolution of 1 km (Skirving WJ and Guinotte J (2001). Data were processed using the non-linear NOAA algorithm designed to give an estimate of SST at 1m depth (Skirving et al 2002).In situ temperature data collected using high-precision (0.02°C) dataloggers (Dataflow systems) at 1m and 6m depths and automatic weather station at 2m depth. Logger and weather station sensors calibrated/corrected to an accuracy of ±0.1°C.Source Description: Skirving WJ, Guinotte J (2001) The sea surface temperature story of the Great Barrier Reef during the coral bleaching event of 1998. Chapter 18. pp. 301-314. In: Wolanski EJ (ed) Oceanographic Processes of Coral Reefs: Physical and Biological Links in the Great Barrier Reef. CRC Press. 356 p.Skirving WJ, Mahoney M, Steinberg CR (2002) Sea surface atlas of the Great Barrier Reef, 1990-2000. CD Rom, Australian Institute of Marine Science and the CRC Reef Research Center, Townsville.

Notes

Credit
Berkelmans, Ray WC, Dr (Principal Investigator)