Nick Graham of the University of Newcastle, UK, reports on the medium-term impacts of the 1998 coral bleaching event on coral reefs and associated marine life.
Fish are still living in the damaged reef of Cousin © N. Graham
Corals are slowly recovering from the bleaching © N. Graham
Coral bleaching in tropical regions is perhaps the most tightly linked and publicised known consequence of climate change to date. The increase in sea surface water temperature is pushing corals over their thermal limits. When this happens, corals reject the algae they live in symbiosis with. This is what makes their white 'skeleton' visible, and kills them if they can't retrieve the algae within a certain time.
These bleaching events are becoming more frequent and severe. The widely publicised 1998 event affected every coral reef region in the world, and resulted in 16% mortality of the world's reefs. The western Indian Ocean was particularly badly affected. The granitic islands of Seychelles, for example, lost over 90% of original coral cover.
Coral loss at this scale is likely to have serious implications. Live corals give a reef its structure. They build it towards the water's surface, and provide refuge holes and caves for a diversity of fish and other animals to live in. When the coral dies, it is much more susceptible to erosion, both by animals that feed on the bleached corals and by wave action. Corals also compete for space with algae, the latter often taking over after bleaching.
Some coral reef fish are dependent on live coral, feeding directly on it, using it as refuge from predators or as a site for settlement of their larvae. These species often decline rapidly in health and abundance following coral mortality. Other species of fish that feed directly on algae may derive short-term benefits from bleaching. However, this positive effect of increased food supply may be offset in time as the structure of the reef erodes. More research needs to be undertaken on a wider scale, covering more species and over a longer time period.
In our recent study, we resurveyed 21 sites previously surveyed in 1994, prior to the bleaching event. The sites are located around Mahe, Praslin and associated islands. Our surveys covered a total area of over 50,000m2 , and included three distinct coral reef habitats:
- fringing reefs with carbonate framework
- coral growth on a granitic substrate
- patch reefs on a sand, rock or rubble base
We measured the cover of different categories of coral, algae and other components of the substrate, and investigated how the structural complexity of the reef had changed during this time. At the same sites we counted the abundance and estimated the size of 134 species of reef fish of 16 different families. Using these unique data, we are now able to assess what impact the bleaching has had on the reef substrate and fish assemblages over this time-scale.
Our preliminary results indicate that the average coral cover in the granitic Seychelles is still below 10%, compared to over 40% prior to the bleaching event. Little of the current cover is comprised of fast-growing, habitat-forming branching and plating corals. Many reefs are now dominated by rubble and algal fields, and structural complexity has declined, particularly on carbonate and patch reefs. The diversity of reef fish has also declined in the face of this degradation. The most sensitive species, such as those that are small and require shelter from predators, or those that feed directly on live coral, have been most affected. However, the impacts are even more wide reaching where most of the degradation of the habitat structure has occurred.
The reefs of Cousin Island Special Reserve are still important in protecting and maintaining stocks of mobile, larger reef fish species, but have suffered equally badly. This may be due in part to the fact that the reefs were made up largely of thermally-sensitive branching and plating coral groups. All of these died in 1998, and most of the structure has now eroded. In time, the high abundance and biomass of large parrotfish around the island may help to control algae and encourage some coral recovery, if external sources of larvae are sufficient. Seychelles, having an isolated reef system, is likely to be largely reliant on self-recruitment of coral larvae. Local management is therefore particularly important in encouraging recovery.
One potential saving grace in this story is the existence of the granitic reefs. This habitat exhibits less algal overgrowth following coral death, and is more resistant to erosive forces. There are encouraging signs of new coral recruitment and stability in fish populations in this habitat. These granitic reefs may provide the brood stocks necessary to repopulate the more degraded carbonate reefs, if physical conditions - such as a reduction of algae and mobile rubble - can be improved. The challenge now is to reduce as many of the other stresses and impacts as possible in order to give the reefs the help they need in this potential recovery.