There is a growing consensus amongst leading scientists that coral reefs throughout the world will undergo massive changes within the next few decades. The causes are increasing levels of anthropogenic atmospheric gases responsible for global warming (causes increased incidences of coral bleaching and related mortality), and a reduction in carbonate saturation in tropical surface (this reduces the ability of reef corals and other organisms to calcify, with possible dire consequences to existing populations). If you add to this other anthropogenic effects such as destructive fishing methods, sewage discharge, eutrophication etc. the outlook for coral reefs is not good.

Threats which have implications for the health of the Aldabra marine community:
Aldabra is a UNESCO World Heritage Site and therefore protected from many anthropogenic disturbances, added to this it is very remote so activities elsewhere in the area do not appear to affect life on the atoll to any great extent. However, this is no reason for complacency and the following threats could become a problem in the future:

Global Threats

• Sea temperature anomalies This causes corals to expel symbiotic algae that live in their tissues, after which they either recover and take on new algae or die (mortality may only be of part of the colony, or may be complete).
• Changes in weather patterns Global temperature changes would change weather patterns which could threaten reefs through storm damage or changes in ocean current routes. Reef distribution would change given time.
• Sea level rise This is more of a threat to humans, if sea level rises corals will probably keep growing upwards and keep up with it as has occurred before in the past.

Local threats

• Pollution
• Oil Spill This would have devastating effects on all terrestrial and marine ecosystems. If the proposed Astove Oil Terminal is to proceed careful consideration needs to be given to a contingency plan as Aldabra is directly downstream and potentially could suffer long term low level chronic stress from this facility. Improper waste disposal
• Damage from tourism - damage to coral reefs when snorkelling and diving. This is minimal at Aldabra but an orientation programme on the terrestrial and marine environment must be carried out to all visitors
• Illegal fishing - over exploitation of reef and pelagic fish species (including rays, sharks), as well as turtles and lobsters. Illegal foreign drift nets and purse seine nets
• Macroalgal invasions may result through a loss of coral cover, decline in herbivorous fish population and/or increased input of nutrients into the marine system. The latter is unlikely to be a factor in Aldabra given the low level of human activity on the atoll. Algal blooms have occurred following the bleaching event but herbivorous fish populations do not appear in decline and will reduce the likelihood of algae dominated reef.

The 1998 Coral Bleaching Event at Aldabra

In early 1998, the waters of the Seychelles experienced unseasonably high sea temperatures ranging from 29 to 34 C and exceptionally reaching 37 C in some lagoons. High sea surface temperatures for the Seychelles were unusually prolonged with temperatures in excess of 30 C persistent for approximately four months in early 1998. This was later confirmed with satellite derived SST data for the region obtained through the Hadley Centre - UK MET Office.

This led to a coral bleaching event that was widely noted in the granitic Seychelles, and was extensive on all the southern Seychelles reefs in 1998. Bleaching was not exclusive to hermatypic corals and incidences of bleaching were widespread in alcyonaceans, non-scleractinian coelenterates (Stichodactyla spp. and Heteractis spp.) and bivalves (Tridacna spp.).

Mortality was particularly high in the branching corals - Acropora spp., Pocillopora spp., Millepora spp. (fire coral) and Heliopora spp. (blue coral). Death in the massive or boulder corals such as Porites spp., Favia spp., Pavona spp. and Diploastrea spp. was in most cases partial and spatially patchy. In Aldabra, bleaching was generally worse in shallower waters (10m or less). Areas which were least impacted were those influenced by cooler currents and those coral colonies which are found within lagoonal channels where water fluxes are high. Furthermore, coral subjected to frequent high temperatures, such as in lagoons, faired well; thus many of the corals survived in the Aldabra Lagoon suggesting an adaptation of corals there to periodic inundation by high sea temperatures.

The strength of the 1997-98 ENSO, whose growth was more rapid and SST anomalies larger than any other El Niño since 1950, was such that warm water anomalies were significantly strengthened in the tropical Atlantic and Indian Oceans. The strong and prolonged nature of this warm water SST anomaly in the Western Indian Ocean led to the extensive and pervasive coral bleaching in this region, including the Southern Seychelles. Previous studies have shown that small temperature and short duration excursions above the mean monthly summer maximum result in partial and complete recovery of bleached coral colonies, yet larger temperature changes maintained for prolonged periods, as was the case in 1997-98, lead to the mass mortality of affected corals.

Study of sea surface temperature archives extracted for the Southern Seychelles suggests that a bleaching event of this magnitude has not been witnessed within the last three decades in this area. This is largely based on the assumption that the coral bleaching events occur following a >+1ºC above the mean maximum monthly SST.

Records for Aldabra also indicate that SSTs for 1998 were the highest of the previous three and a half decades (See figure below). Anomalous temperatures began with a rapid increase in SSTs from November 1997 to a +1°C SST anomaly by January 1998. Peak SSTs (30.65°C) were reached in March, representing a +1.31°C anomaly above the long term mean maximum SST for that month. The +1°C anomaly persisted until April 1998 indicating a duration of almost four months (Figure 5). All temperatures recorded for the period leading up to the bleaching event and those following ranged from +0.5 - 1ºC higher than the long term average of the monthly mean maximum temperatures (1961-1997).

Monthly SST anomalies for Aldabra (5-10ºS 45-50ºE), using 1961-1990 baseline. Note the 1997 anomaly.

Monthly mean maximum sea surface temperatures for Aldabra (9-10ºS, 46-47ºE) from September 1997 to July 1998 in comparison to the average monthly mean maximum sea surface temperatures for the period September to July 1961-1996.

In April 1998, at the peak of the coral bleaching event, a Cambridge Coastal Research Unit (CCRU) based research team (SSARP) found that widespread bleaching and mortality was common on the outer reef slopes surveyed (3m-25m) from the western to northeastern sides of Aldabra. Coral coverage was found to be 37% with 41% of those having been bleached or displaying recent mortality. Bleaching intensity in Aldabra was not as high as other areas in the Southern Seychelles as peak warming was 0.5ºC lower than other areas in the region. Bleaching and related mortality was primarily seen in the branching and tabular species of coral (i.e., Pocillopora and Acropora), and partial to patchy in most massive species (i.e., Porites, Pavona). Bleaching was in some areas confined to a single side of the coral colony. A high proportion however of the massive species of corals displayed signs of previous mortality indicative of a thick of algal overgrowth and the presence of encrusting and boring invertebrates. As with other areas soft corals had high levels of bleaching and mortality. Although no quantitative data were gathered for the reef communities in the lagoon, extensive observations were made in all of the channels and in the western half of the lagoon. Most of the coral species found in the channels, with the exception of isolated incidences of branching corals, were observed to be alive and displaying no obvious signs of perturbation. Notably in the channels was the presence of normally deepwater corals (Tubastrea micrantha) which have been displaced to shallow waters as a result of the channel hydrodynamic activity mimicking conditions of the deep water. This may explain the high survivorship of massive and branching coral colonies in the channels. Lagoonal patch reefs and individual heads of massive coral species displayed very limited bleaching. Distinctive species such as Galaxea, Serioptera, Acropora and Pocillopora were completely bleached and was most often the case with increased distance from the flux of water from the channel.

Warm water events, how should we deal with them?

There is little doubt that we shall experience more warm water events in the next 50-100 years, and that some of them will be more severe than the 1998 event. Unfortunately very little can be done to prevent these threats but, from experience, we know that some corals survive such events. Corals may survive disturbances because they live on deeper reefs where water temperatures are less variable; in lagoons, where they may be used to large daily fluctuations in temperature; or protected by specific oceanographic phenomena, such as the upwelling of cool deep waters. These potential sources of larvae need to be identified, managed appropriately and protected from further damage in order to promote recovery and boost the resilience of individual coral colonies and the reef system as a whole. Areas which may be less affected by anomalous SSTs such as channels, lagoon and areas of upwelling should be considered as important

Corals that do survive warm water events will be of key importance for the supply of coral larvae to replenish degraded areas. Some surviving reefs will have the potential to supply larvae (source reefs), while others will receive larvae via ocean currents (sink reefs). Some reefs may be sinks at one time of year and sources at another time, where monsoonal currents reverse in different seasons.

Several factors determine whether a reef is a good source of coral larvae:

• The presence of large coral colonies that may produce large numbers of larvae.
• High coral diversity, which may increase the chance of rapid colonisation by opportunistic, fast growing species and later by slower growing species.
• Minimal presence of human impacts on the reef, such that the chance of coral reproduction and larval survival is maximised.
• Presence of upwelling water, which will assist with the transportation and survival of coral larvae.
• The presence of prevailing wind and oceanic currents that flow past the source reef and towards the degraded (sink) reef.