Corals Bleached Whiter than White,
but what went Wrong in the Final Rinse?
 he coral bleaching event in early 1998 might have been expected by some, but it was a surprise to others, me included. Why? Although the El Niño signal was strong and bleaching had already been reported from a number of locations in the eastern Pacific prior to our summer, the north Queensland lead-in to the summer, weather wise, was exceptionally mild. We had an early start to our 'wet' in mid-December and if it wasn't raining, most days between mid-December and mid-January (normally our hottest month) were cloudy, thus keeping water temperatures down. In fact, at the time of the deluge in Townsville on 10 January (550 mm in 24 hours), sea temperatures at Magnetic Island were around 27°C, about 2°C below normal for this time of year.
According to the Walker circulation of global seasonal climate variation (see Exploring CRC Research, April '98), northern Australia is meant to experience cooler oceanic water temperatures during El Niño years, while a warm pool of oceanic water is situated off the South American coast at this time. In theory at least, El Niño years should make western Pacific reefs less vulnerable to bleaching and eastern Pacific reefs more vulnerable to bleaching. In practice too, we find that there is no clear relationship between the Southern Oscillation Index (SOI) and coral bleaching at Magnetic Island off Townsville. In fact, during three of the six recorded bleaching episodes (1980, 1982 and 1994) the SOI was either positive or nil at the onset of bleaching, while during the other three episodes the SOI was negative. Moreover, the most severe bleaching event on record (until this year, that is) was
Our first trip to look for coral bleaching was on 13 January 1998 at Magnetic Island, three days after the floods in Townsville. Although visibility was limited to around 50 centimetres, corals appeared to be in good health, despite the salinity being down to 26 parts per thousand (ppt) in Cleveland Bay (and probably lower in surface waters at the time of the rain). Two weeks later on 23 January 1998, there were still virtually no signs of bleaching at Nelly Bay and only a few pale or white corals at Geoffrey Bay. The salinity at this time had risen only marginally to 29 ppt (0-6 m depth). In Ross Creek however, a site we have been monitoring for five years, all coral colonies were dead except for some hardy Goniastrea colonies. It was not until 30 January 1998 that widespread coral bleaching was evident in both Geoffrey Bay and Nelly Bay. By this time, average daily sea temperatures had climbed to 31°C (6 m depth) and in the following week rose to slightly over 32°C. For nearly four weeks, the average daily temperature on the reef slope at Nelly Bay exceeded 31°C, while seawater salinity at this time ranged between 31 and 34 ppt. Bleaching intensified at Magnetic Island and by mid-February, reports of coral bleaching were also coming in from reefs near Bundaberg and from Orpheus Island. Coral bleaching forms (the crown-of-thorns starfish (COTS) forms you use when you are serious about white coral) were printed in haste and distributed to all and sundry. By mid-March 1998, coral bleaching at inshore reefs was so intense that the extent and intensity of bleaching could be assessed by aerial survey methods. Around 660 reefs (still only 23% of reefs on the Great Barrier Reef) were surveyed for coral bleaching at an altitude of 500 feet. The results confirmed that coral bleaching was indeed happening from the top end of the Great Barrier Reef Marine Park near Horn Island to the bottom of the Marine Park near Bundaberg. The inshore reefs were clearly more affected than offshore reefs with notable exceptions to bleaching occurring on all reefs in the Swains, the Hardline, T-Line and offshore reefs around Proserpine. Offshore reefs north of Lizard Island also appeared to have escaped coral bleaching. Ground truth surveys were conducted at around 30 reefs and we have roughly 900 video transects awaiting analysis at the time of writing (watch out for marine scientists with square eyes, they could be dangerous!). By the end of April 1998, extensive mortality (> 80% of living coral cover) was being reported from the worst affected reefs in the Palm Island group, while reefs which experienced less intense bleaching were showing signs of slow recovery. By this time, it was also clear that coral bleaching was occurring in many other parts of the world, including Lord Howe Island, Western Samoa, Christmas Island, Maldives, Galapagos, Reunion Island, Netherland Antilles, Florida Keys, Yucatan Coast, Cayman Islands, Brazil, Seychelles, Comoro Archipelago, Borneo, California and Panama. The bleaching event on the Great Barrier Reef was clearly part of a major global bleaching episode and may well prove to be the most extensive on record. Assuming this coral bleaching event on the Great Barrier Reef follows the chronology of the 1982 event, we could expect the last vestiges of white coral to disappear from our reefs by the end of September 1998. Follow-up surveys are planned at this time to document the extent of mortality and recovery at most of the ground-truthed sites. We will need to wait until then before we can be certain what the overall effect of the 1998 bleaching event on the Great Barrier Reef has been.
Finally, to the 45+ people who reported coral bleaching to the Great Barrier Reef Marine Park Authority from over 60 locations on the Great Barrier Reef, a big thank you. These reports have been a great help in helping us keep track of the extent and intensity of bleaching and, importantly, in a timely manner. Please keep the reports coming over the next |
Great Barrier Reef Marine Park Authority
PO Box 1379 TOWNSVILLE QLD 4810. Phone: (07) 4750 0700, Fax: (07) 4772 6093
E-mail: registry@gbrmpa.gov.au
few months, especially as corals start to recover or die. The Great Barrier Reef is a big place and clearly we can't be everywhere to find out what is going on. You are our eyes and ears, so please keep up the good work!