We have recently finished two days of survey in Nan Wan (South Bay) situated in Kenting National Park, which is a multi-use terrestrial and marine protected area in Taiwan. This survey has had the team and our chief scientist, Professor Ove Hoegh-Guldberg intrigued by the chance to survey the area due to the presence of the Maanshan Nuclear Power Plant (NPP), which was commissioned in 1984. Maashan Nuclear Power Plant is Taiwan’s third NPP and the only NPP adjacent to a coral reef in the world, making Nan Wan a fascinating place to study coral reef science. The hot-water effluent from cooling the nuclear reactor is discharged directly onto the coral reef making the average temperatures 2-3°C higher than other reef sites in the area. The hot water is then trapped in front of the outlet site due to currents and tides in Nan Wan.
When the Maashan NPP was first built, the fate of corals in the area was uncertain, and research by marine scientists in Taiwan including our chaperone for the trip, Professor Allen Chen, and his team has shown the hot water effluent has certainly had an impact on coral cover and distributions in the area. However, the important and fascinating part of the story is some coral species have survived and persisted, making Nan Wan a unique site to study the effects of thermal stress on coral in their natural habitat. With human-induced climate change increasing sea surface temperatures and the frequency of ocean warming events, understanding the ecology, physiology and genetics of corals that can survive in such harsh temperature conditions are important to coral reef scientists for understanding how our reefs might look like and perform in the future.
It must be stressed that survivorship of corals in these increased temperatures does not mean coral reef ecosystems are safe from the effects of increasing carbon emissions and climate change. The presence of this hot water effluent has changed the ecology in the area. Stress-tolerant, yet slow-growing massive coral from the Families Faviidae, Poritidae, and Mussidae, etc. dominate in front of the outflow, and from 12m onwards, the site becomes dominated by soft corals. It is important to realise that domination of a few types of coral due to a disturbance leads to a decrease in habitat heterogeneity (i.e. the amount of different organisms in an area), and this can have negative effects on the ecosystem as a whole. There is often a loss of structural complexity leading to fewer habitats for mobile species such as fish, and the ecosystem is less resilient to further disturbances, as there are fewer species that have the potential to survive and recover.
Professor Allen Chen and his team have been focusing their efforts on understanding the effects of the hot water effluent on the coral symbiont, Symbiodinium. Symbiodinium is a genus of algae hosted in the tissue of coral polyps and is often referred to as zooxanthellae. The symbiont is responsible for up to 80% of the corals nutritional requirements, depending on the corals feeding mechanism and available nutrients in the water.
Professor Chen and his collaborators(Keshavmurthy et al. 2014) have found the Symbiodinium in front of the outflow have been reduced to one genetic clade (i.e. group of biological organisms descended from a common ancestor). There are four main clades of Symbiodinium (A, B, C, D) and the clade found in front of the outflow is the clade group D, which is known to be tolerant to thermal stress. Two genera of coral Stylophora and Seriatopora which were present at the outflow site in 1986 are no longer found there and are seemingly unable to host clade D, which could explain their absence. I find this an interesting result as the very same genus all but disappeared from the Maldives archipelago after the 1998 global mass-bleaching event. The Central Indian Ocean is currently undergoing severe mass bleaching and given Taiwan Sea Surface temperatures have been 2°C above the seasonal averages there is now great concern the reefs of southern Taiwan could be facing mass coral bleaching in the next couple of months.
Keshavmurthy, S., P.-J. Meng, J.-T. Wang, C.-Y. Kuo, S.-Y. Yang, C.-M. Hsu, C.-H. Gan, C.-F. Dai, and C. A. Chen. 2014. Can resistant coral-Symbiodinium associations enable coral communities to survive climate change? A study of a site exposed to long-term hot water input. Peerj 2:e327.