Coral Reefs and Climate Change
Threats to Reefs

Climate change presents a number of threats to coral reef ecosystems. Each of these threats may have different impacts on the functioning of reefs. However, it is important to remember that the synergistic interaction of these threats is really what reefs will be dealing with. Here I will review some of the specific climate change threats that reefs will encounter.

TEMPERATURE
Rising sea surface temperatures lead to coral bleaching. Bleaching refers to the expulsion of the symbiotic zooxanthellae that live inside coral tissue and provide the coral animals with photosynthetic products (Hopley & Suharsono 2000). Once a temperature threshold is surpassed, corals lose their zooxanthellae, thereby losing all the important functions they provide. Besides providing photosynthetic products, zooxanthellae are also responsible for calcification (Hopley & Suharsono 2000). Prolonged bleaching (i.e. prolonged warm temperatures) can cause mass coral mortality (Royal Society 2005). El Nino events cause prolonged increases in sea surface temperature, and have been associated with severe coral bleaching and mortality--especially the 1998 El Nino (see map). These events are predicted to become stronger and more frequent in the future, which could destroy many of the world's remaning reefs.

UV RADIATION
Climate change may produce a 1-10% increase in UV-B radiation in many areas where reefs are found (Hopley & Suharsono 2000). UV-B is physically damaging to DNA, and may effect organisms up to 20 meters below the sea surface. Many reef organisms produce substances that protect them from UV radiation, but as radiation increases, this protection may not be as effective. Sublethal physiological effects of UV may decrease performance of reef organisms without killing them. Larval stages of many animals are particularly susceptible to UV radiation as they spend most time very close to the sea surface (Hopley & Suharsono 2000).

OCEAN ACIDIFICATION
As atmospheric CO2 concentrations increase, the amount of CO2 dissolved in the ocean also increases. This results in more production of carbonic acid, and decreasing pH of the ocean water. As pH decreases, the saturation point of carbonate goes down, making fewer carbonate ions available for reef-building organisms to use for calcification (Royal Society 2005). By 2100, under IPCC scenarios of atmospheric CO2 increase, calcification rates on coral reefs could decline by 17 to 35% (Kleypas et al. 1999). This would result in corals having weaker skeletons, lower growth rates, and greater susceptibility to erosion. If the rate of erosion surpasses that of calcification, then reef structures may begin to disappear altogether (Royal Society 2005).

OVERALL IMPACTS
The overall effects of climate change may produce reefs on which corals are rare, which will significantly reduce structural complexity and biodiversity. Over long periods of time, reductions in reef-building corals may cause the disappearance of reefs altogether (Royal Society 2005). Coral mortality will cause a collapse of the whole reef ecosystem, as corals are the main primary producers on reefs. Graham et al. (2006) found that climate change-driven loss of live coral cover results in significantly decreased structural complexity on reefs. The high structural complexity on reefs is what allows so many microhabitats and niches, thus as complexity decreases, so does biodiversity. In the same study, Graham et al. found that loss of live coral cover reduced fish assemblages and caused a loss of key functional groups of reef fish. Small fish were lost most, along with corallivorous and planktivorous species. These shifts in community composition will result in the loss of coral reef ecosystems as we know them.

 

Photo credits: Australian Institute of Marine Science