Green water

michel pichon pichon at
Tue Apr 16 04:25:53 EDT 1996


     A huge oceanic phytoplanktonic blom caused abnormaly high 

turbidity in French Polynesian waters in March 1996. 

     In early March, fishermen and divers reported high turbidity  
in oceanic waters near Tahiti and Moorea. Samples were  
immediately taken by the permanent staff of the CRIOBE (EPHE 

laboratory in Moorea). Other samples were taken by scientists of  
the French University of the Pacific and by ORSTOM. The bloom  
was sufficiently important to cause Secchi disc measurements of  
less than 7 m, as compared to the usual 25 m plus. Visibility was 

very low below 20 m depth. From various sources, the bloom  
was estimated to cover at least 500 km2, from the island of  
Raiatea to the atoll of Kauhei East to West, and from the  
Tuamotu archipelago to Tahiti, North to South. 

     The phytoplankton bloom was characterized by a pronounced  
green colour and a gummy consistency. It was limited to oceanic 

waters although some green lagoon waters could occasionally be 

observed when oceanic waters were flushed over the reef front. 

Inverted microscope examinations revealed a multispecific 

composition. Picoplanktonic and nannoplanktonic cyanophytes 
dominated together with (but to a lesser extent) many diatom  
species and 4-6 species of coccolithophorids. Conversely, 

dinoflagellates, which usually dominate the oceanic  
phytoplankton were not abundant. Other small phytoflagellates  
(e.g. prasinophytes,euglenophytes, chrysophytes) were  
occasionally encountered. Additional sampling was carried out  
for algal pigments and POC and DOC determinations. 

     The bloom did not cause massive fish mortality but oceanic 

fishes like tuna disappeared from the area. Normal catches were  
made outside the bloom zone or in the places where the waters 

returned quickly to normal colour. 

     The cause of the bloom is uncertain. Above normal seawater 

temperatures were observed to the north and the west of Moorea 

island, but not in the whole of the area where the bloom  
appeared. As oceanic waters in French Polynesia are strongly 

oligotrophic, it is assumed that such a bloom  would need an 

enrichment of nutrients, possibly a localized upwelling of deeper 

waters. However, no low temperature anomaly could be detected  
by the temperature recorders which have been in place for  
several years on the outer reef slopes of Moorea, at 25 and 35 m 

depth. Other hypotheses, like a displacement of the equatorial 

upwelling, or the influence of the eruption of submarine  
volcanoes in the Vanuatu archipelago do not seem realistic. 

Bruno  Delesalle 
Biologie Marine et Malacologie 
Universite de Perpignan 

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