Status of Coral Reefs of the World 2000

Status of Coral Reefs of the World: 2000
CASE STUDY 1: CHAGOS ARCHIPELAGO, CENTRAL INDIAN OCEAN
An error was made in the 1998 report with no bleaching reported on Chagos. In fact
these remote reefs were severely damaged like others in the Indian Ocean. The 1998 El
Niño resulted in a major losses in coral cover and abundance across the Archipelago.
The El Niño came at the end of a chain of events that had caused hard coral cover to
decline from 50-75% (plus 10-20% soft coral) to only 12% live cover over the last
20 years. These are very remote reefs with high biodiversity (220 and 770 fish
species) which experience virtually no human impacts. The 1998 ‘Hotspot’ sat over
Chagos for 2 months, so that in early 1999 seaward reefs of the 6 Chagos atolls had
40% cover of recently dead coral and 40% bare rock. The 50% of corals lost in the
lagoon has turned into loose rubble. It is probable that earlier El Niño events (1982-
83 and 1987-88) resulted in conversion of much live coral to rock and rubble, but the
1998 event caused a major sudden loss of corals. Meteorological records show a 1-2 o C
rise in mean air temperature, lower annual atmospheric pressure, reduced cloud cover,
and more variable winds during the last 25 years; strong evidence for global climate change.
In the 1970s, seaward and lagoonal reefs had high coral cover and diversity, with many
large table Acropora spp. in 5–15m depth, and A. palifera in the turbulent shallows. By
February 1999, most of these corals and soft corals were dead with only a few large
Porites heads having more living than dead tissue. There were also losses of calcareous
red algae, Millepora, and blue Heliopora corals from shallow areas. Soft corals were
almost totally eliminated from seaward slopes after 1998, resulting in large areas of
bare substrate. There was much lower coral mortality on lagoon reefs, probably
because these experience large normal fluctuations in temperature and the corals
presumably have adapted to higher temperatures. The massive Porites in lagoons
survived better than those on ocean facing reefs.
The effects on Chagos have been catastrophic, particularly for the dominant genus
Acropora. In early 1999, it was possible to snorkel for 15 minutes in areas of northwestern
Salomon and western Blenheim and not see any live coral amongst the stillstanding
coral skeletons. Whereas before then, there was 50-75% cover. There were
only occasional sites with more than 10% of colonies were alive, with Porites lutea and
other similar large massive Porites species being the best survivors (average cover of
8% on all reefs). The immediate consequences will be a reduction in reef growth to at
least 20m depth, and delayed recovery of corals and the coral community. Erosion of
the coral skeletons will produce unstable rubble and sand, which will prevent the
settling of new larvae from the few living parent colonies. The scale of the losses,
including coral colonies that were 200-300 years old, indicates that partial recovery
will take several decades at least, and may be a few hundred years before a similar
community of corals is re-established. (Charles R.C. Sheppard, 1999. Coral decline
and weather patterns over 20 years in the Chagos Archipelago, Central Indian Ocean.
Ambio, 28, 472-478, E-mail: sh@dna.bio.warwick.ac.uk.)
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