IGCAR : Annual Report - Indira Gandhi Centre for Atomic Research

IGC
Annual Report 2007
Fig. 1 Yellowish-green streaks of trichodesmium observed
in the coastal waters of Kalpakkam
western Bay of Bengal current
(WBC) a phenomenon called
upwelling occurs bringing
highly nutrient enriched bottom
water to the surface and thus
found to enhance
phytoplankton growth. During a
regular coastal water
monitoring program, a
prominent discolouration of the
surface water was noticed in
the coastal waters of
Kalpakkam on 16 th March
2007. The bloom was very
dense and created yellowishgreen
coloured streaks (Fig.1)
of about 5 m width and
extended to a few hundred
metres. The phytoplankton
responsible for discolouration
was identified as
Trichodesmium erythraeum
(Fig.2). Although, bloom of
Noctiluca scintillans and
Asterionella glacialis has been
reported in the coastal waters
of the Kalpakkam, there has
been no report of 'green tide'
formation by Trichodesmium
erythraeum. One of the
interesting features of this
bloom was that it was sighted
only for one day and drastically
declined on the next day with
negligible Trichodesmium cell
density. Such a sudden
appearance and decline in
bloom forming organisms in
general and Trichodesmium in
particular is very peculiar and
interesting. The three different
phases of a typical
phytoplankton bloom such as,
the exponential phase (log
phase), peak & senescent
phase (lag phase) could not be
seen during this observation as
usually observed for any
bloom. Its appearance if
continued for a relatively longer
period, could have significantly
affected not only the
commercially important marine
species but also the lives of
people who consume fishes. As
this species has been identified
as toxic and reported to
contain neurotoxin commonly
associated with paralytic
shellfish
poisoning.
Interestingly, the bloom was
sighted just after the southwest
to northeast monsoon transition
during which the current
direction changes from
equator-ward to pole-ward in
Bay of Bengal. Close
monitoring of the coastal
waters during this period with
respect to chemical and
biological characteristics
revealed significant changes in
its quality. An abrupt increase
in ammonia and phosphate
content was noticed on the day
of bloom. Phosphate values
reached a maximum of 2.56
µmol l -1 with the peak
coinciding with the day of
highest cell density. This
appears to be about 10 times
higher than the normal
phosphate values encountered
Fig.2 A bunch of trichodesmium erythraeum diatoms
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