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Coral Reefs, Sea Level Rise and Ocean Warming
Coral reefs are dynamic ecosystems that are both fragile and resilient. Reef communities boast hundreds of thousands of species, many of which are undescribed by science (Hoegh-Guldberg 1999). In order to better understand the reasons why coral reef communities respond the way they do to various climatic occurrences, this section will first provide a brief biological background section. The stresses brought by sea level rise and ocean warming will then be discussed in great detail. Lastly, speculations regarding the future of coral reefs will be discussed with options for better management in hopes of preserving coral reefs for future generations to enjoy. Throughout the section current scientific literature and research will be incorporated.
It is imperative that issues regarding coral reef conservation be addressed immediately in the discussions regarding global warming and greenhouse gas emissions. Coral reefs are critical to the survival of tropical marine ecosystems and hence to local people. More than a quarter of the world's reefs have been damaged by pollution and climate change, and many researchers believe most of the rest could suffer the same fate by 2020 (Kirby 2001). This progression toward destruction must be reversed before more damage is done. The elimination of coral reefs would result in dire consequences for marine resources as well as for the people dependent upon these resources. Any rise in sea level will have significant and profound effects on the economies and on the living conditions of the populations of coastal and island countries (Small Islands Information Network). Coral reef communities provide nearby human populations with critical sources of income and resources through tourism, fishing, building materials, coastal protection and the discovery of new drugs and biochemicals (Hoegh-Guldberg 1999). At this time between 50 and 70% of all coral reefs are under direct threat from human activities including increased eutrophication and sedimentation flowing from disturbed terrestrial environments, over-exploitation of marine species, mining and physical destruction by reef users (Hoegh-Guldberg 1999).
Biological background
The first requirement for active coral reef development and growth is light (Nybakken 1993). If corals are unable to get enough light (whether due to increased water turbidity and the increased suspended sediment clouding the water column, or due to a dramatic and rapid increase in water depth), they stop growing and eventually die (Nybakken 1993). Light is necessary to promote photosynthesis within the corals symbiotic zooxanthallae. Light also enhances oxygen production, which stimulates the coral metabolism and leads to increased calcium carbonate deposition and therefore coral reef growth. Corals require water depth where light intensity is at least 1-2% of surface intensity (Nybakken 1993). This dependence on light limits the depth of waters where corals are found. No species of coral has been found to develop in waters deeper than 70 meters, with most corals growing in waters less than 25 meters (Nybakken 1993). Another factor limiting coral growth and distribution is temperature. Coral reefs dominate between the latitudes of 25º South and 25º North where water temperatures tend to remain consistently warm year round (Hoegh-Guldberg 1999). Nybakken (1993) finds corals prefer a mean annual water temperature between 23-25ºC, although various coral species are known to exist in the temperature range of 18ºC - 30ºC (Hoegh-Guldberg 1999).
Coral dependence on light and ocean temperature are just two variables on a list of potential stresses to reef ecosystems. Corals possess numerous mechanisms for acclimatization and adaptation to various stresses including diverse reproductive strategies, flexible symbiotic relationships, physiological acclimatization, habitat tolerance, and a range of community interactions (NOAA). However, once weakened, their systems become more vulnerable as additional stresses are added. It is when multiple stresses occur at the same time - as is almost always the case in natural systems - that coral reefs are in danger of severe, long-standing damage and perhaps fatality. Corals are slow growing organisms, and when destruction occurs, it takes upward of 25 years to repair and rebuild even the smallest coral colonies (Nybakken 1993). For example, the recovery from an El Nino Southern Oscillation event is estimated to take longer than 100 years (Nybakken 1993). As El Nino events occur more frequently, as is predicted in future climate change scenarios, there is good reason for concern. Corals will not have time to recover between El Nino's, and populations will drop dramatically as a result. The recovery of corals from damaging events depends on the extent of destruction, the nearness of a source for recolonization and favorable habitat conditions including temperature, light, ocean currents and salinity (Nybakken 1993).
Sea Level Rise and the Consequences for Coral Reefs
As has been previously mentioned, corals are dependent on light to maintain their biological functions. A rise in sea level will cause reef ecosystems at the depth limit of coral growth to experience diminished light conditions that will no longer sustain this growth and will most likely result in death (Hoegh-Guldberg 1999). Corals that currently exist at these depths are expected to go extinct with rising sea levels. In addition to corals living at their physiological depth limit, slow growing coral species are also susceptible to the negative consequences brought by sea level rise as they will be unable to grow fast enough to keep pace with rising ocean levels (Hoegh-Guldberg 1999). When coupled with other stresses coral reef systems will be under in association with rising waters (increased water temperatures, and possibly reduced salinity), the ability of reefs to keep up with rising sea levels will be greatly diminished (NOAA). If coral growth rates are reduced by general environmental stresses, then the rise in sea level that is expected under even moderate global climate change projections will present additional and perhaps insurmountable challenges for coral reef communities in the future (Hoegh-Guldberg 1999). For example, computer model simulations have demonstrated that coral reefs in the Caribbean will be unable to keep up with the predicted rates of sea level rise (Graus 1998). As these reefs become deeply submerged, dependent human populations will feel multiple severe consequences.
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