| January
2001 Volume 3, No. 1
|
Page
2
|
Apo Island Marine Reserve Two Years After the
1997-98 El Niño Bleaching Event
Laurie J. Raymundo, Ph.D.
Marine Ecology
| |
Dr.
Laurie Raymundo (above) studying the effects of El Niño to a star
coral (Galaxea fasicularis) in Apo Marine Sanctuary. A well-recovered
coral head (inset, top left) and a recovering coral head (inset, bottom
right).
|
The Apo Is.
Marine Reserve, known globally as one of the few success stories in coastal
resources management, was badly hit by the 1997-98 El Niño coral bleaching
event. The reef crest in this reserve is dominated by large (some over 3m
in height), dome-shape colonies of the star coral Galaxea fasicularis. This
feature is, in itself, unique to the Apo Is. Reef; normally colonies of this
species remain quite small and do not dominate reef structure.
Unfortunately, G. fasicularis was one of the worst affected
by the warm water delivered by the El Niño event. A Reef Check survey done
during the height of the bleaching estimated that 90% of G. fasicularis colonies
were bleached. One year later, a resurvey showed that many of these colonies
were dead or recovered only partially. Much of the reef crest was, therefore,
dominated by large amounts of bare substrate, devoid of living coral.
The implications of the response of Apo Is. Reef to the
El Niño are important to understand, as it is predicted that these events
will increase in frequency, and possibly severity, in the future. Coral recovery
from such events can occur either by overgrowth of dead skeleton by patches
of living tissue or by recruitment of new coral from larvae which settle on
the dead skeleton. Reef recovery may depend on the availability of larvae
and the ability of the remaining coral to withstand and survive future bleaching
events. It is important to study and quantify responses of corals to such
events, in order to predict future responses and to determine possible mitigating
strategies. The questions of primary concern are: What is the impact on the
reef community of such high mortality of a dominant, structurally important
species? How fast can we expect the species to recover if major mortality
has occurred? How much new coral recruitment can be expected onto the bare
patches of exposed skeleton?
In a preliminary attempt to address these questions,
L. Raymundo and A. Maypa surveyed these partially dead colonies in November
2000, approximately 2 years after the bleaching event. We wanted to quantify
the extent of damage to G. fasicularis within the reserve, determine if there
was evidence of recovery by the species, and characterize the community recruiting
onto the dead skeletons. The results of our survey revealed some interesting
trends. The star coral clearly dominated a large segment of the reef; 92%
of the total coral cover was taken up by this species. Of the G. fasicularis
colonies surveyed, 65% contained less than 25% remaining living tissue, isolated
in small patches. Only 25% of the surveyed colonies contained greater than
75% living tissue. The average size of the colonies surveyed was 9 m2,
yet the amount of living tissue remaining on those with less than 25% living
tissue averaged 0.93 m2; an order of magnitude smaller than the
surface area of the colony as a whole. This clearly suggested that much of
the bleaching was fatal and recovery of colonies after bleaching by overgrowth
of remaining tissue was a slow process.
The communities recruiting onto these dead and partially-dead
colonies were diverse, consisting of turf algae, encrusting sponges, soft
corals, and hard corals belonging to 15 genera. The average size of these
young recruits was 18.5 cm2; most were between 1½ to 2 years of
age, and had probably recruited, therefore, within the first six months after
the death of the original coral tissue. However, though the recruiting coral
community was diverse, and living patches of the original star coral appeared
healthy and growing, the majority of the surface area on these colonies was
still covered by non-hard coral species; algae and sponges.
The most obvious conclusion which can be drawn from this
preliminary examination of a two-year response to bleaching is that recovery
of hard coral cover, even on a relatively pristine reef with abundant recruits,
is slow. Most colonies which lost at least 75% of their living tissue were
still predominantly devoid of coral two years after the event. On the other
hand, the community which appears to be developing on these bare substrates
shows high diversity; higher than the previously G. fasicularis-dominated
one. Provided this developing community is allowed to proceed undisturbed,
the Apo Is. Marine Reserve should recover its structural and species diversity,
with sufficient time. However, it is difficult to determine what impacts future
predicted bleaching events will have on its recovery. 
