CORDIO Status Report 1999.pdf

much higher in lagoons than on seaward reefs (Figure
2). The lagoons of Diego Garcia, Salomon and Peros
Banhos are included in the comparison. The Egmont
lagoon is not included, since it almost lacks live coral. It
recently became more enclosed due to linking of the
islands, and is presumably suffering a corresponding
restriction in water exchange. It is also very shallow.
Lagoon reefs
1999
Seaward reefs
1999
9
9
17
12 13141516
11
10
12
11
10
8
13 14
15
16
8
17
Figure 2. Pie charts showing reef cover values from lagoons
(left) and seaward reef slopes (right).
Key: 1: Red algae, 2: Porites live, 3: Porites dead, 4: digitate coral live, 5:
digitate coral dead, 6: table coral live, 7: table coral dead, 8: other coral
live, 9: other coral dead , 10: A. palifera live, 11: A. palifera dead, 12:
Heliopora live, 13: Heliopora dead, 14: Soft coral, 15: Millepora live, 16:
Millepora dead, 17: bare substrate.
7
6
1
1
7
2
2
5
3
6
4
3
5
4
The lack of coral in the Egmont lagoon can probably
not be attributed to the 1998 water temperature rise.
DISCUSSION
Several possible consequences of this mortality arise.
1. Direct reef erosion
Direct reef erosion if caused by a reduction of limestone-depositing
species in shallow water. The most
exposed reefs in Chagos are shallow water reefs dominated
by corals which showed high mortality, commonly
total. The reefs also have an extensive cover of
calcareous red algae of the Porolithon group, which was
reduced but not as greatly affected as the corals. This
cover remained high on the wave-breaking spurs,
commonly 10–50%, occasionally more.
2. Indirect reef erosion
Indirect reef erosion is probably going to be serious.
Erosion of dead corals by waves and boring organisms is
proceeding, and coral skeletons are reduced to rubble.
The mobile fragments produced are extremely abrasive
and damaging to marine life, and effectively prevent
new larvae from attaching to the rock. Eventually the
fragments will disappear, but the time required is not
known. Even though most or all species have survived
somewhere in the Chagos Archipelago and can produce
larvae, they may show little reproductive success for
many years.
3. Loss of diversity
Loss of biodiversity is another possible consequence of
bleaching and coral death, but at present we have almost
no idea what the consequences may be. There are
examples from, for example, the Galapagos, where
endemic fish species became extinct, following heavy
bleaching during an earlier ocean warming event. The
warming of the Indian Ocean occurred throughout
most of the coral archipelagos, as well as in the rich
East-African and Arabian regions, so local extinctions
may have occurred in Chagos too, but this is not known
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