Bagnis, R.

Jardin, C.

Orempuller, J.

Rougerie, Francis

Horizon / Pleins textes

Bull. Soc. Path. Ex., 
85, 1992, 525 
Key-words: Coral bleaching, Gambierdiscus toxicus. 
Mots-clés : Blanchiment corallien, Gambierdiscus toxicus. 
LE BLANCHIMENT CORALLIEN : CAUSE DE PROLIFÉRATION 
LATENTE DE GAMBIERDISCUS TOXICUS 
CORAL BLEACHING AS A CAUSE 
OF POTENTIAL PROLIFERATION OF GAMBIERDISCUS TOXICUS 
By R. BAGNIS (*), F. ROUGERIE (**), J 
From march-apri1 to july 1991, an important blea- 
ching event occurred in most reefs of Society Islands. 
Severity of the bleaching and mortality of scleracti- 
nian corals varied according to genera. Among the 
dominant (Pocillopora, Acropora, Montipora, 
Porites), Acropora was the most sensitive : in some 
areas of Moorea and Tahiti, many of its colonies 
bleached and a quarter died, then were covered by 
macroalgae (2). The succession of algal populations 
did not show any specific character linked to blea- 
ching. A turf of indistinct flora set up first, followed 
one month later by a turf with dominant filamen- 
tous or calcareous red algae mainly Florids and brown 
algae, that roughly maintained until july (C. PAW, 
personal communication). 
MATERIALS AND METHODS 
The populations of the dinoflagellates Gambier- 
discus toxicus associated to these macrophytes were 
quantitatively monitored in may, july and September 
1991 in a few samples of dead branching Acropora 
and Pocillopora from Tahiti (Pirae pass, Arue frin- 
ging reef) and Moorea (Tiahura outer reef slope), 
using our routine method' of sampling and 
counting (1). 
RESULTS 
The presence of G. toxicus was noted in all the 
samples. The densities of the dinoflagellate per gram 
of algae (GTD) ranged from 250 to 800 without signi- 
ficant variation in the same area from a sample to 
another during the coral bleaching event. In some 
Tahitian Acropora samples from may 1991, GTD was 
higher on bleached dead corals than on the other 
dead Acropora from the neighbourhood. One year 
later, GTD decreased under 50 in surveyed Acropora 
and none difference was pointed out with the neigh- 
bour and cdral colonies. 
(*) Université Française du Pacifique, Centre Universitaire de Polynésie 
(e*) Centre ORSTOM, Tahiti, French Polynesia. 
(***) URA 1453 CNRS, EPHE, Centre de l'Environnement de Moorea, 
Française, BP 4635 Papeete, Tahiti, French Polynesia. 
French Polynesia. 
1 7 NOV. 1993 . .  - 
OREMPULLER (**), C. JARDIN (*) (***) 
From the available data, the highest GTD among 
the macroalgae covering dead Acropora and Pocillo- 
pora were observed about 3 to 4 months after the 
onset of the bleaching event. 
DISCUSSION 
It seems that when a new surface is offered to 
macroalgal colonization the best environmental condi- 
tions for a proliferation of G. toxicus are filled during 
a few months period. The coral reef community is 
probably the most complex in the sea and a very 
delicate balance must exist between organisms in com- 
petition such that any change in the environment can 
result in the proliferation of one at the expense of 
the others. The dinoflagellate G. toxicus appears as 
one of these competing forms. It was one of the first 
microalgae developing massively among- opportunist 
macroalgae that cover dead bleached surfaces in 
normal ecological succession. AS ..far as the ability 
of G .  toxicus to produce ciguatoGn might depend 
upon the strain or the associated microflora, (of 
which mainly bacteria), it would be interesting in the 
future to culture clones of G .  toxicus sampled in 
macroalgae covering dead bleached corals and check 
their toxicity. 
Since 1980 bleaching events have been increasing 
in number, extend and severity throughout the global 
coral reef ecosystem. Links with global warming are 
evoked. Under this scope their role in the increase 
of overall ciguatera incidence for the next decades 
is questionable. This side effect of coral bleaching 
should be taken in account in any long term global 
plan to monitor natural and anthropogenic changes 
on coral reefs. 
REFERENCES 
1. BAGNIS (R.), LEGRAND (A. M.) & INOUE (A.). - Follow 
up of a bloom of the toxic dinoflagellate Gambierdisctfs 
toxicus in a fringing reef of Tahiti. In: Toxic marine 
phytoplankton, Eds E. GRANELI, D. M. ANDERSON, 
L. EDLER, B. G SUNDSTROM. Elsevierd New York Was- 
hington DC, 1989, p. 374-378. 
2. SALVAT (B.). - Blanchiment et mortalité des scléracti- 
niaires sur les récifi de Moorea (Archipel de la Société) 
en 1991. CR Acad. Sci. Paris, 1991, 314, 105-111. 
O. R.S.T.O.M. Fonds Documentaire 
N 0 ! 3 8  34-9 Q Y 4  


Le blanchiment corallien : cause de prolifération latente de Gambierdiscus toxicus = Coral bleaching as a cause of potential proliferation of Gambierdiscus toxicus

Le blanchiment corallien : cause de prolifération latente de Gambierdiscus toxicus = Coral bleaching as a cause of potential proliferation of Gambierdiscus toxicus

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