[Coral-List] Annual staghorn bleaching in American Samoa pools

[Douglas Fenner]

Annual Staghorn Bleaching in American Samoa Fringing Reef Pools

 

    Staghorn corals (Acropora formosa = A. muricata) have begun bleaching in the fringing reef pools of Tutuila, American Samoa.  As of 10/29/05, over 95% of A. formosa in Nuuuli pool, Tutuila, American Samoa are partly bleached.  Mike King reports similar bleaching in Alofau pool, Tutuila.  They recovered from the previous summer's bleaching just 5 months ago.

    Some have suggested that due to rising temperatures from global warming, in the future coral reefs may bleach every summer.  This is already happening in the pools of American Samoa.  Because it is a regular, annual event, and there are several possible controls, this offers an opportunity for scientists to plan studies of bleaching instead of having to take advantage of unpredictable opportunities.

    For the past two Austral summers, I have recorded the bleaching of three species of staghorn Acropora  (A. formosa, A. pulchra, and A. nobilis) in the fringing reef pools of Tutuila, American Samoa.  Acopora formosa and Porites cylindrica dominate the pools of Tutuila, but P. cylindrica was not observed to bleach.  Bleaching of the three staghorns has reached as high as 100 percent of the colonies in some of the pools, with colonies nearly entirely white.  At the same time, very few corals on the outer reef slopes were observed bleaching.  Acropora nobilis is present on the outer slopes (though not common), and did not bleach there when the same species strongly bleached in the pools.  The pools reach higher temperatures than on the outer slopes.  Most of the pools had peak temperatures of 32-33 oC in the last summer, but one hit a peak of 34.9 oC (for a half hour).  The highest temperatures occur on sunny summer days when low tide is midday, reducing circulation.  High temperatures are well known in the pools of nearby Ofu island, American Samoa (Craig, Birkeland, and Belliveau, 2001).  Those pools have a higher diversity of corals and less bleaching.

     Mass coral bleaching on outer reef slopes was observed by several people in American Samoa in 2002 and 2003, though the time and intensity were not recorded.  At present, about half of all staghorn colonies in the pools are dead.  Essentially none have died in the past two years, so it is likely they died in 2002 and/or 2003, when bleaching was more intense.  Thus, along with the present bleaching, staghorns in the pools have bleached for five years in a row.

      In 2004, staghorns in the pools began to bleach in March, peaked in April, and ended in August.  Bleaching started again in November (04), peaked in the January to April (05) period, and ended at the end of May (05).  Now bleaching has started again at the end of October (05).  Pool staghorns are spending more of the year bleached than recovered, year after year.

      -Douglas Fenner, Dept. of Marine & Wildlife Resources, American Samoa

 

Craig, P., Birkeland, C., Belliveau, S.  2001.  High temperatures tolerated by a diverse assemblage of shallow-water corals in American Samoa.  Coral Reefs 20 (1): 185-189.