Green water
michel pichon
pichon at univ-perp.fr
Tue Apr 16 04:25:53 EDT 1996
A " GREEN WATER" EPISODE IN POLYNESIA
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
EPHE
Universite de Perpignan
France
Please email comments or replies to : criobe at tahiti.rio.net or to
bd at univ-perp.fr
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