[Coral-List] BIOT coral bleaching event

Andrew Bruckner Bruckner at livingoceansfoundation.org
Fri May 1 12:35:20 EDT 2015

Hi All,

The following is a brief report of a recent bleaching event we've been following in the British Indian Ocean Territory.

During recent coral reef surveys in the British Indian Ocean Territory (BIOT) conducted by the Khaled bin Sultan Living Oceans Foundation, we noticed a dramatic increase in bleaching and bleaching-associated mortality. On our first research mission (March 10-31, 2015) bleaching was rare; only a few isolated pale colonies were noted.  Beginning on April 15 we started to see a high number of table acroporids that were pale to yellow in color, and partial bleaching (mottled and pale colonies) on other corals.  Over the next week bleaching increased substantially on table acroporids, with over half showing signs of bleaching.  Completely bleached colonies of Pocillopora, small digitate acroporids, Seriatopora and other smaller corals were also common in shallow water, and several genera, such as Stylophora, displayed bleached branch tips. By April 25, most of the table acroporids (especially A. cytherea) above 15 m were white, light yellow or light blue, while fewer (20%) deeper colonies were bleached. Numerous other corals, especially Lobophyllia, all of the fungiids, most acroporids, Stylophora, Symphyllia and Goniopora were various fluorescent shades of blue, yellow or green; many massive corals, such as Galaxea and Pocillopora,  numerous genera of faviids, hydrozoan corals (Millepora) and certain anemones were stark white.  Bleaching has continued to increase in severity on reefs off Salomon Atoll, with most corals in shallow (<10 m) fore reef locations fully bleached (e.g. Isopora, Stylophora, Pocillopora, digitate and branching acroporids, faviids, Astreopora, Hydnophora, Pavona, and Physogyra) as of April 26. Shallow lagoonal reefs (2-10 m depth) are also showing severe signs of bleaching, and more than half the corals in deeper areas being pale, mottled or fully bleached, including many of the large Porites lutea colonies, plates of Porites rus, foliaceous Echinopora colonies, and plating Leptoseris, Pachyseris, Merulina and Mycedium, with bleaching seen to 35 m depth.  On the fore reef, bleached table acroporids (mostly A. cytherea), Acropora tenuis, digitate acroporids, Isopora and some smaller colonies of Porites are beginning to show signs of tissue loss and algal colonization.  Other species are currently showing less mortality.

There appears to be considerable spatial variability in the extent of bleaching. Some atolls and reef systems, especially lagoonal and fore reef systems off Salomon and Peros Banhos show the highest prevalence of bleaching, while others (Speakers, Blenheim, Great Chagos Bank) had many fewer bleached corals.  Within a reef system, passes between islands and deep reefs (below 30 m) show less bleaching, as do murky lagoonal environments below 15 m depth, while bleaching is often concentrated on either side of a sand channel with affected corals extending down the reef slope.  We've seen a high variability within species, with bleached colonies occurring adjacent to a normal unbleached colony of the same species.  In several instances, the white bleached colony is exposed to full light, while normal, fully pigmented colony usually are shaded.

Weather conditions have been ideal for diving (but not for the corals) with weeks of doldrum-like conditions, gin-clear water, and warm temperatures. Surface water is 31-32° C at the surface, on both fore reef and lagoonal reefs; reef temperatures are approximately 30° C to 35 m depth.  In March, sea water temperatures on the reef were closer to 29° C, while temperatures in April have only been as low as 29.2° C in a few deep locations.

The duration and ultimately the impact of this event is unknown. It appears that we may be near the end though, as the seasonal trade winds appear to be increasing and we've noticed some upwelling of cooler water.  Even if some corals die, it appears that these reefs are likely to bounce back quite rapidly. There is evidence that BIOT sustained high mortality (up to 100%) following the 1998 El Niño, but the reefs quickly rebounded.  A study carried out in 2006 suggested that shallow reefs were similar in structure and cover to that of 25 years earlier. Based on the sizes of the large table acroporids we have documented, these likely recruited after the 1998 event and now cover 60-80% of the reef in many locations, to depths of 20 m or more.   We see unusually high levels of recruitment on most reefs.  On sites that were damaged in the recent past by crown of thorns starfish, bleaching (2010?), and white syndrome there are high numbers of coral skeletons in growth position, but these sites also have high numbers of recruits and populations of healthy, juvenile corals (possibly 3-5 years old, based on their sizes).  Most reefs also have high cover of red crustose coralline algae, near absences of cyanobacteria and macroalgae, and a high biomass of herbivores. Furthermore, previous studies have also reported very high levels of herbivory, low nutrient levels, and a near absence of pollutants, all of which suggest these systems can rebound quickly from a bleaching event.

For photographs and more information on this event please see:

Andrew Bruckner, Ph.D.
Chief Scientist, Khaled bin Sultan Living Oceans Foundation, 8181 Professional Place, Suite 215, Landover, MD 20785, 301 577 1288 ext. 203

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