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Coral Recovery
From Bleaching A study carried out on the RV Sea Surveyor Thomas J. Goreau, Ph.D. SUMMARY Mortality of corals in the Alphonse group following the 1998 bleaching event was somewhat less than in the high islands of the Mahegroup. Overall mortality of corals on fore reef slopes was around 90%in shallow water and around 70-80% on vertical slopes, in comparison to values around 95 and 99% in the inner islands. Although mortality of Acroporas was nearly total, several small surviving colonies were found at a range of depths including very shallow seagrasses. In contrast with the high islands, many partially surviving Pocilloporaverrucosa colonies were found. The major survivors were Helioporacoerula (rare in the inner islands), Porites head species (probablylutea), Porites cylindrica,Porites nigrescens, Astreoporamyriophthalma, Hydnophora microconos, and Favid corals. Coral diversity was lower than in the high islands and no Diploastreas or Gonioporas were seen in the Alphonse group, although these were found at every site in the high islands. Survival of corals was very high, roughly 80%, inside the Alphonse lagoon, also higher than equivalent habitats in the Inner Islands. Although the tiny human population is insufficient to cause eutrophication, evidence of high natural nutrient sources was seen. Strong upwelling is the cause of significant algae abundance in deep waters, while the lagoons at Alphonse and Saint Francois (but not Bijoutier, which lacks a lagoon) have green water which affects surrounding reefs and promotes algae growth, apparently as the result of recycling of nutrients from sea grass decomposition. Areas not affected by such sources, such as Bijoutier, are predominantly overgrown by encrusting pink calcareous algae, creating the surfaces needed for new coral settlement. 1) SOUTH EAST BIJOUTIER Bijoutier, a small uninhabited island between Alphonse and Saint-Francois, is encircled on its north, west, and south by a reef crest at some distance from the island. Although Alphonse and Saint Franco’s lagoons contain green water, the waters around much smaller Bijoutier flush rapidly and the water is clearer. A dive was conducted at the southeast end of the ring reef encircling Bijoutier. The bottom dropped off at a slope of about 60 degrees but corals extended down only to around 20 metres, with rubble and sand slopes below. Shallow portions of the reef had been dominated by live corals, predominantly Acropora, with smaller amounts of Heliopora coerula, Pocilloporaverrucosa, and other species. Deeper water showed declining coral cover and a larger proportion of non Acropora species, with about half the bottom covered by Porolithon encrusted limestone reef rock. Dead but intact coral greatly outnumbered living corals. Overall coral mortality appeared to be around 90% in shallow water, and around 70%in deeper water. Corals with high survival rates included Porites heads, but none of these were very large, with most being around 0.5to 1 metre across. Most Heliopora survived, making it now the predominant living coral with roughly half of all live colonies, but nevertheless the majority died in shallow water. Other corals with fairly high survival included Astreopora, Favia,Favites, Montastrea,Montipora, Echinopora, Hydnophora, Fungia, Herpolitha,Diaseris,Pavona, Pachyseris, and Platygyra. Very low survival was seen in Acropora, of which only two small live branching colonies were seen, and a half alive Acropora palifera about50 cm across. Pocillopora didonly slightly better: all Pocilloporaeydouxi were dead but fivePorites verrucosa were found which were between 10 and 50 percent alive. The clean low nutrient conditions were shown by rapidly growing deep pink Porolithon crusts which covered most of the limestone bedrock between corals and which was rapidly overgrowing all the corals which had died from bleaching earlier this year. Other than occasional Halimeda tuna clumps, there were no macrophytic algae, and there were no algal turfs on dead coral. Sediment and nutrient stresses appeared minimal, and the dead corals were being converted to good coral settlement substrate. Coralline Algae Lethal Disease was present but only in small amounts, affecting only a few percent. Coralline Lethal Orange Disease was not seen. Porites line disease was present, but only affected around 5 to 10 % of the Porites heads. 2) EAST BIJOUTIER The site examined was the to the east of Bijoutier, where there is no surrounding reef crest. A deep channel rises steeply at slope of more than 75 degrees to a wide shallow seagrass flat which adjoins the island. The flat is at low tide level, and is covered to the very edge with Thallasodendron lawns. The hard reef rock slope is around half Porolithon and half covered with corals, mostly Acropora, Heliopora, and Pocillopora. Around 70% of all corals were dead, with highest mortality in shallow water, especially for Heliopora, which was the predominant survivor. Some largely alive and healthy Pocillopora verrucosa colonies were seen, as were over a dozen small alive and healthy Acropora of at least 3or four species, up to 40 cm across. Surprisingly around half a dozen small live Acropora were found in small rubble channels on top of the Thallasodendron flat, in waters only 20 cm deep. Strong thermal contrasts were noted between very warm water draining the seagrass flat, in which these corals were found, and in much cooler water entering the deep channel from the Canal a Mort to the north. Other than encrusting calcareous red algae and a few Halimeda clumps, the only other algae seen were fan shape rosettes of Microdictyon, a species which might proliferate during periodic upwelling of cooler nutrient rich waters, or might result from nutrients recycled from sea grass decomposition. 3) NORTHEAST ALPHONSE The reef crest is composed of sand and seagrass at low tide height. The seaward side slopes at a shallow angle, about 15 degrees, to a depth of around 10 metres, where there is a drop off extending to about 25 metres depth, then a rubble and sand slope at about 45degrees with few corals. The shallow reef slope was heavily dominated by Acropora colonies, but these were almost entirely dead, and only two very small live colonies were seen. Pocillopora verrucosa colonies up to a metre across had also been common, and while almost all were dead, around 10colonies were seen which were partially alive. Because of the dominance of these two genera in shallow water, overall mortality was around 90% on the upper slope. Along the lower part of the slope and the drop off the proportion of these two genera was markedly less and overall coral mortality appeared to be around80% on the deeper part of the upper slope from around 7-10 metresdepth, and around70% on the drop off. Along the slope down to 7metres depth the predominant surviving corals were the branching species Porites cylindrica, Porites nigrescens, and Heliopora coerula. Patches of Porites cylindrica and nigrescens were mostly about 0.5 to 1metre across, but in shallow water around 2metres deep larger patches, up to 3 metres across were seen. In waters deeper than 7 metres the dominant live coral were small heads of Poriteslutea, up to I metre across, but typically half that size. Around a dozen other genera were also seen alive, including Astreopora, Hydnophora, Favia, Favites, Fungia, Pachyseris, Montipora,Echinopora, Goniastrea, small Galaxea, and other genera. At the lower part of the drop off large gorgonian fans were seen. A very sharp thermocline was encountered at 20 metres depth, causing strong schlieren effects. Upwelling was indicated by occasional patches of cold water at depths as shallow as 7metres. The impact of upwelling was clearly seen in the algae. Deep water had conspicuous green tufts of Microdictyon widely distributed. This alga appears to be an indicator of upwelling. The deeper portions of the slope were covered with thick low lying mats of Cladophoropsis, alsoan indicator of high nutrient input. The abundance of both of these algae was markedly less in waters above 5 metres depth, where Caulerpa serrulata and another Caulerpaspecies became the most common alga, with tufts of a brown filamentous alga, probably acyanobacteria. Halimeda tuna clumps were found at all depths. The decrease in algae towards shallow water indicates that upwelling of deep water is the major source of nutrients. At all depth most of the corals which had recently died from bleaching were overgrown by Porolithon, but below 7 metres depth Microdictyon and Cladophoropsis were also found on dead coral, although these two largely occurred on reef limestone rock. In water shallower than around 5 metres Porolithon largely covered the limestone between the dead Acropora colonies as well, but often overgrown by Caulerpa clumps. Porolithon was affected by Coralline Algae Lethal Disease (probably only around5%) but no Coralline Lethal Orange Disease was seen. Porites line disease was present affecting perhaps 10% of the head Porites, and ranging in color from brown, pink, and blue. 4) SOUTH ALPHONSE The outer slope reef on the south side of the atoll, to the east of the boat channel, facing the Canal a Mort, was investigated. This reef was in most regards quite similar to the reef on the North East of Alphonse. However coral cover was only around half as much at all depths, even though mortality was very similar at all depths to the previous site. Coral colonies were distinctly smaller, and few colonies larger than a metre were seen except for dead Acropora table colonies. Almost all Acropora were dead, and only two very small live colonies were encountered, but a significant minority of the Pocillopora, perhaps 10%were partially alive. The lower coral cover and coral size indicate that this area has been subjected to physical stress to a greater extent or more recently. Another difference was that Porites cylindrica and nigrescens were quite uncommon in comparison with the previous site. While Porolithon was overgrowing most recently dead coral skeletons, there was little Porolithon on the bare rock substrate between dead and living corals. This was largely covered in Cladophoropsis turfs, fine filamentous algae turfs, Microdictyon, Caulerpa serrulata, and Halimeda tuna. However, unlike the previous site, the Cladophoropsis turfs were not mainly found on the deeper slope, instead they became even more abundant toward the reef crest. The much lower amount of Porolithon seems to indicate strong influence of two different nutrient sources at this site. A strong thermocline was encountered at around 15 metres depth, and patches of cold water were found above this. However there also appeared to be a strong nutrient source from the lagoon. The uppermost 5 metres of water was turbid, very warm, and green, and appeared to be water from the lagoon flowing out onto the reef through the boat pass. The nutrient sources from both shallow and deepwater are preventing the establishment of good Porolithon crusts on which new corals will preferentially settle. As at the previous site, reef fish populations were primarily dominated by schools of small plankton feeders, mixed in with groupers and surgeon fishes. However this site was distinctive in the large numbers of very large fish. These included Napoleon wrasse, sharks, a huge tuna, very large jacks, big groupers, a giant sting ray, and many other large fish, more than seen anywhere else. This exceptional aggregation of large fish is probably due to the currents which flow through the Canal a Mort between Alphonse Atoll to the north and Saint Francois and Bijoutier to the south. 5) NORTH EAST ALFPHONSE LAGOON The inner reef rises almost vertically from around 7 metres depth almost to the surface, and is composed of a very different coral assemblage than the outer slope reefs, including corals much more resistant to high turbidity stress. This was the only site where very large Porites heads were found, up to 5or more metres across. A very diverse coral assemblage was found, but which contained only very few Acroporas, a few of which were still alive. Other species found included the branching corals Porites cylindirica, Poritesnigrescens, Seriatopora hystrix,Pocillopora damicornis,the, the, encrusting corals including Monitipora, Leptastrea,Astreopora, Echinopora, head corals including Physogyra, large Faviaspeciosa up to two or three metres across, Favites, and plate corals including Pavona decussate and Pavonacactus. Algae included Halimeda tuna, and Caulerpas growing in the Thalassodendron sea grasses in the sediments at the base of the reef, but little Porolithon as the result of high resuspended sediment settlement. Only minor Porites band disease was seen. Overall survival of corals here was the highest seen anywhere in the Seychelles, probably around 80%. Similarly high survival of corals was also reported for lagoon reefs in Saint Francois atoll by Martin ?,the captain of the Tam Tam fishing vessel, but there was unfortunately no time to dive there. The remarkably high survival of these reefs, including species which were not found alive in Mahe, make propagating corals from them especially important. A more detailed search needs to be made for living corals in the lagoons of Alphonse, Saint Francois, Aldabra, Cosmoledo, and other of the atoll islands. ACKNOWLEDGEMENTS This work was possible thanks to an invitation from Prof. Jonathan Erez, Director of the Eilat Marine Laboratory, to join the cruise of the Sea Surveyor to Alphonse. Special thanks also goes to David Zakkai for assistance in the field.
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