Bleaching: a coral health and recovery guide
The Caribbean has been among the first regions to suffer when warm water bleaches coral reefs. With annual bleaching events now looking as though they will be inevitable, it is necessary to discover which species, if any, are resilient against this regular onslaught upon their energy reserves, their tissues and calcification. 3 species were chosen.Porites astreoides, the mustard hill coral and Porites divaricata are SPS (small polyp stony )species reaching only 24cm diameter at best, while Orbicella (Montastraea) faveolata is a different and very large colonial stony coral which is also endangered.
The 3 corals were exposed artificially to raised temperatures for 2.5 weeks on 2 consecutive years. Assessment was carried out straight after the second exposure and then 1.5 and 11 months later. Sadly, Orbicella initially fared badly. Cumulative damage resulted in an inability to recover during the period of observation. The bleaching seems to have reduced its capacity to resist. Protein and carbohydrate concentrations within the tissues never recovered to the levels found before bleaching. Heat sensitivity in some species therefore seems likely to be a decisive factor in bleaching resistance. Verena Schoepf of Ohio State University in Columbus, her colleagues there and colleagues from the University of Delaware in the US and the University of Western Australia, Crawley present the results of their Caribbean collaboration in the Proceedings of the Royal Society B, as
Annual coral bleaching and the long-term recovery capacity of coral.
What they conclude however is likely to be influential for as long as coral lasts, given its very unpredictable future. The mound forming Porites astreoides is common because of recent increases in abundance as temperatures affect it very little long-term. It is also resistant to disturbance, which is an unfortunate situation in many Caribbean sites. However even this persistent coral is severely affected by repeat bleaching. Chlorophyll a is depleted, protein and carbohydrate concentrations lower and calcification rates decrease. There is also little sign of any full recovery, even after 11 months. Though its symbiont algae had recovered well, the polyps themselves struggled. Reproduction was also affected, given the lowered resources available for any extra metabolic activity.
The other 2 species of coral studied did recover fully from repeat bleaching, despite the rarity of the big Orbicella. Their overall physiology was able to recover in both species (though in different ways), especially according to the figures produced for chlorophyll a and lipid levels. While Orbicellas calcification stopped completely after repeat bleaching, it recovered completely within 1.5 months. The chlorophyll a level was the only figure that needed longer time scales for recovery. 11 months were still enough for a complete reversion to normal. As Porites divaricata also recovered well from repeat bleaching, it is obviously a specific capability to recover that will enable some corals to survive a likely annual bleaching in the near future. Other regions with their own endemic species will vary, and even coral residents of these Caribbean locations could be different from the same species at different depths and in different locations.
A Hawaiian problem with 2 other Porites spp. alongside many other corals and their associated ecosystem was explained last year here with
Bad news for Corals and Divers. We can only hope all regions can be studied in time to create survival strategy for those species that are most at risk from climate change.