Catastrophic
impact of hurricanes on atoll outer reef slopes in the Tuamotu
(French Polynesia)
Marie
José Langlade , IRD
based
on the article listed below
Introduction
Diving
surveys were made on outer reef slopes around several
islands of French Polynesia (Tikehau, Takapoto, Mataiva,
Moorea), during September and October 1983, subsequent to
the hurricanes which devastated the
South Pacific region between December 1982 and
April 1983. They revealed that coral destruction could be
an extensive and catastrophic phenomenon on atoll outer
slopes. Special
attention was paid to Tikehau atoll reef formations
(Tuamotu archipelago) where quantitative studies on
scleractinians, cryptofauna and fishes were conducted in
1982 immediatly prior to the hurricanes.
Most
of the studies carried out on the effects of cyclones,
typhoons or hurricanes on coral reefs referred to shallow
water areas, reef flats and littoral zones. Except
for recent studies on the effects of the typhoon Pamela on
the coral reefs
of Guam (Randall and Eldredge 1977) and on the
hurricane Allen's Impact on Jamaican coral reefs (Woodley
et al. 1981; Kjerfve et al. 1986), little attention has been
paid to hurricane impact on outer reef slope formations.
However,
our studies have shown that deep outer reef zones were
more severely affected than the shallower ones with coral
destruction reaching 100% beyond 35m.
Hurricanes
between december 1982 and april 1983
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Lisa
(11-13 december 1982); Nano (20-27 january 1983); Orama
(22-27 february 1983); Reva (6-14 march 1983); Veena
(7-13 april 1983); William (15-21 april 1983).
Their
mean characteristics were the following ones (maximum values in
brackets): |
central storm pressure= min. 950
hPa; max sustained
wind speed=45 m/s (62 m/s); radius of maximum winds > 28 m/s
=100 km (150 km); mean sea level change (storm surge) =2-3 m (4
m); local wave height= 8-10m (12m) (Services Météorologiques
Pacifiques 1983). |
Three
of them (Orama, Reva and Veena) seriously affected the atoll of
Tikehau, Tuamotu archipelago (15° S, 148° 10 E) (Services Météorologiques
Pacifique 1983; Auzeneau and Darchen 1983). Veena's eye passed directly
over this atoll moving in a south-west direction Winds reaching
200 km/h devastated the village Tuberahera and wide areas of
coconut groves
Damage
to coral formations on Tikehau
|
General
map of Tikehau atoll (Tuamotu) with location of study site
(arrow).
Diving
surveys were made in Tikehau along the transect A ↔
B (0-1350m), on the west island slope.
|
Description
of the west outer slope of Tikehau.
It
can be divided into three morphological zones (Faure et Laboute, 1984),
on the basis of depth range slope, percentage of coral
coverage and physical environment factors:
-
The
shallow fore-reef area (2-10m deep);
-
a
sloping terrace (10-25m deep);
-
the
slope itself (beyond 25m).
The
western and leeward side of the atoll presented a very
narrow fore-reef terrace (50m) and a steep profile.
Prior
to the hurricanes, pre-storm percentages of living coral
coverage of the shallow fore-reef area ranged from 5 to
25% in the spur and grove zone (0-4m), from 40 to 60%
between 4 and 10 m, from 60 to 75% beyond 10 m.
|
Post-hurricane
surveys on this Tikehau outer slope showed that coral destruction
ranging from 50 to 100% increased with increasing depth contrary
to observations recorded elsewhere on outer reefs (Randall and
Eldredge 1977; Woodley et al. 1981).
|
Cross
section of the west coast of Tikehau
An
hypothesis to explain the deep coral destruction
Direct
coral destruction by storm-induced waves occured between
the surface and 20-22 m. Most of broken corals rolled down
the slope breaking deeper colonies. The avalanche process
resulting in the total destruction of deep coral
communities is generated by a steep slope combined with a
narrow fore-reef terrace. |
|
The
shallow fore-reef coral community composed of small
colonies well adapted to high energy level environments
suffered less than well-developed deep communities. The
cause of coral destruction differs with depth :
-
Between
the surface and 6 m, coral destruction, estimated at
50%, resulted essentially from abrasion by dislodged
material, rolling fragments and scouring sand.
-
A
narrow erosional trench, 0.2 to 0.8 m deep, appeared
at 6 m on the fore-reef resulting probably from the
pressures and shock strength inflicted on the whole
reef by high breaking waves.
-
On
the sloping terrace, more than half of coral colonies
were broken and dead. Accumulations of dead blocks and
fragments occured locally between 10 and 15 m.
-
Estimated
coral destruction ranged from 60 to 80% between 15 and
30 m; most of the colonies were levelled in this
region. Surviving coral patches more than 1.5 m high
formed narrow buttresses perpendicular to the reef
front separated by wide areas of devastated rubble.
-
Beyond
35 m to at least 90 m, the lower limit of our
observations, where earlier living coral coverage
reached 75%
|
Discussion
-
Delayed
mortality in hurricane-damaged corals could be as severe as
the immediate effects of storms and may explain the widely variable
rates of reef recovery.
-
Hurricane-driven
seas frequently strike the lee side of reefs where there is
usually marked development of branching or fragile coral colonies,
like it happened in Tikehau.
-
Deep
outer reef slopes were more severely affected then the
shallower ones. On these slopes an avalanche phenomenon
occured: colonies broken on upper slope areas rolled down the
slope, proceeding to break other colonies and creating a chain
reaction resulting in massive coral destruction.
-
Deep-damaged
reefs all had a narrow fore-reef terrace and a steep profile
(45 to 70°). Both conditions seemed to be required for the
formation of deep reef-avalanches.
-
Massive
coral destruction combined with the downward movement of
broken colonies may act as an important formation agent of
the detrital tore surrounding the atolls. In
the Tuamotu archipelago an
atoll may be ravaged by an hurricane between 4 to 8 times in a
century. Although sporadic, such catastrophic phenomena may be
of importance on a geological time scale.
Conclusion
|
Generaly
described as local phenomena, effects of hurricanes on
outer reef formations were impressive from the point of
view of magnitude in the Tuamotu. In a few hours, million
of cubic meters of coral rolled down onto the outer atoll
slope and accumulated probably at depths of between 200m
and 500m.
Hurricanes
of 1906 exhibited similar characteristics as those of
1983: it could be surmised that similar destructions had
occured subsequent to the 1906 events.
This
hypothesis would suggest that flourishing coral colonies
observed in 1982 could not be more than 77 years old. A
recovery period of 50 years is in accordance with the
estimations for totaly destroyed reefs.
It
should be interesting to follow the aspects of recovery
with successional processes of coral recolonisation on
Polynesian atoll outer slopes. Will previous communities
be restored and, if so, in how many years, or will a new
equilibrium ne organized increasing the heterogeneity of
coral reefs? |
Reference
Harmelin-Vivien
ML, Laboute P 1986. Catastrophic impact of hurricanes on atoll
outer reef slopes in the Tuamotu (French Polynesia). Coral Reefs 5
: 55-62
Bibliography
Faure
G, Laboute P 1984. Formations récifales de l'atoll de Tikehau
(Tuamotu, Polynésie Française, Océan Pacifique) 1. Définition
des unité récifales et distribution des principaux peuplements
de Scléractiniaires. Notes Doc ORSTOM Tahiti 22: 108-136.
Randall
RH, Elredge LG 1977. Effects of typhoon Pamela on the coral reefs
of Guam. Proc. 3rd Int Coral Reef Symp. 2: 526-531.
Woodley
JD, Chornesky EA, Clifford PA, Jackson JBC, Kaufman LS, Knowlton
N, Lang JC, Pearson MP, Porter JW, Rooney MC, Rylaarsdam KW,
Tunnicliffe VJ, Wahle CM, Wulff JL, Curtis JSG, Dallmeyer MD, Jupp
BP, Koehl MAR, Neigel J, Sides EM 1981. Hurricane Allen's impact
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