Bleaching at Buona-vista reef, Sri Lanka

[Prasanna Weerakkody]

"Coral Bleaching at the Buona-vista reef, Sri Lanka 1998"

The year 1998 observed the most severe coral bleaching observed in Sri Lanka
in documented times.  The following is an account of the observations from
the Buona-vista coral reef in Southern Sri Lanka by the Nature Conservation
Group field teams.

During the month of March 1998 a few instances of coral bleaching was
observed on Poritid coral boulders, these consisted of bleached spots
5-15cm. across on otherwise normal coral heads.  A large and spreading patch
of bleached corals were observed on the reef crest as early as the 26 of
March. The bleaching was restricted to reef crest areas less than 0.5m.deep;
with corals down to 1m. beginning to lose pigmentation.  The affected
species included Acropora formosa, Pocillopora damicornis, some favids and
few colonies of Montipora aequetuberculata. 

By the middle (10th -12th) of April extensive bleaching was reported. A
survey from the 5-7th May observed the corals were bleaching down to a depth
of over 6m. An estimated 90-95% of all coral in Buona-vista reef was
affected. The documented water temperature had reached 36 degrees
centigrade, Only the corals belonging to the Genus Montipora was observed to
be tolerant of the warm water conditions.  Colonies of M. aequituberculata
was affected in the shallow water but retained most of its color in waters
over 1.5m deep. Other species of Montipora were affected to different degrees. 

An unusual phenomenon was observed where some bleached corals developed
large irregular patches of Bright blue pigmentation. This was observed
primarily in genera Acropora, Echinopora and Montipora. The colour remained
for a few weeks before fading out.

In addition to Scleractinian corals, bleaching was observed in Soft corals
(Sarcophyton, Lobophyton etc.) and Zooantharians (Discosoma sp.) The algal
pigmentation of photosynthesizing Ascidians was not observably affected at
this stage.

The Monsoon conditions were beginning to take effect over the reef around
the 15-20 May resulting in heavy wave action, reduced visibility and a
considerable cooling of the water.  A short dive was undertaken on the 23rd.
May.  The Water was observably 'Cold' (no measurements taken) compared to
the previous weeks and signs of re-pigmentation was observed in some corals.
Most colonies of Porites rus had regained normal coloration, followed by
most Montipora colonies, Leptoria, Favids, and some Pociloporids.  Different
species of Acropora showed faint signs of pigmentation.  Acropora formosa
remained bleached.

Continued surveying between 19-21 June reported temperatures fluctuating
between 27 C and 29 C The recovery was slow with most corals still bleached
or only showing signs of partial recovery.  Over 90% of colonies of Porites
rus, Montipora aequituberculata, M. turgescens, Leptoria and Symphyllia
indicated full re-pigmentation. Echinopora lamellosa, Most Favids, Galaxia,
and Leptoria showed irregular patterns of recovery with some colonies fully
recovered and others remaining bleached. Though some species had regained
partial pigmentation most Acropora spp., Pocillopora spp. and Porites
boulder corals showed slowest recovery .  Large tracts of coral close to the
reef crest had died and was overgrown with filamentous brown algae, Didemnid
ascidians and a Black encrusting sponge. The Porites boulders are undergoing
an epidemic of the" Pink-line disease" (Pers.com. Dr. Thomas Goreau) which
attacks the coral as an advancing pink ring or line 1-2mm thick.

The Soft corals belonging to genera Sarcophyton and Lobophyton were
beginning to show significant levels of mortality in areas 4m and deeper.
Some large colonies of Zooanthids belonging to genus Discosoma was also
beginning to regress and show signs of weakness.

Observations were paused due to the onset of the Monsoon with loss of
underwater visibility. Preliminary observations carried out within the past
week indicate that the reef lost almost all its Acropora and Pocillopora
colonies, most of the Echinopora also succumbed. Even the colonies that
indicated some re-pigmentation had later been killed off. The structural
change to the reef is staggering as these were the dominant genera in the
reef previously. The reef possibly lost over 70% of its coral cover. The
Porites rus and Montipora aequituberculata showed the best survival rates.
The Porites domes which remained bleached for over 5 months is only
re-gaining its pigmentation now. The pink line disease has spread on to
cover and has killed considerable areas of surface on the Porites boulders.
Almost all the colonies seem to be affected now. The changes to the reef
both in terms of structure and bio-diversity is heavy. The Zooantharians
(Discosoma spp.) had recovered and was prolifereating over the dead coral
areas with sponges, ascidians and algae.  The larger soft corals killed off
show no signs of return yet.

Contrasting observations were made in the  Eastern coast close to
Trincomalee in early September.  Two reef sites Green bay and Rocky were
surveyed and were found to be in excellent condition with 50-60% live coral
cover, Strangely the reefs did not show any signs of bleaching. Either the
reefs recovered 100% very fast or they were not affected by the warm water.
Possibly due to the morphology of the coast shielding the area from the hot
water current advancing from the South-West. 

....................................................
Hope this information would be of use to Listers interested in bleaching in
the region. 

Regards.

Prasanna Weerakkody
Nature Conservation Group. 
No.9, Balapokuna place,
Colombo 6.
Sri Lanka.

Phone: 941 856041
E-mail: firefish at sltnet.lk


The Nature Conservation Group is a volunteer environmental NGO in Sri Lanka
dedicated to the preservation of Coral reefs and Marine Ecosystems in Sri Lanka.