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Coral Bleaching: El Nino and Global Climate Change or Adaptive Mechanism?
Much research has been done to identify the causes of mass coral bleaching events in the last several decades. Most of the literature has focused on global climatic change and the relation of El Nino Southern Oscillation (ENSO) events and coral bleaching patterns, while some have hypothesized other causal justifications. The complexity and intensity of such research isnŐt precise and the data for statistical analysis is insufficient. This paper attempts to compile knowledge from multiple studies and work and compare a model for predicting the justification of a bleaching cycle and whether or not bleaching events are an adaptive mechanism.
Most of the reef-building corals present today had evolved by the Miocene, while most are Mesozoic in age (Buddemeier and Fautin, 1993). This makes reef-building coral the oldest and most complex systems in the ocean, and furthermore means that they have survived many rapid climate changed-induced environments, especially through the most recent Quaternary era. This has given some paleoclimatologists reason to study more in depth the adaptive mechanisms that coral reefs have developed over time. The types of environmental disturbances that corals face today may not be much different than those that they have experienced in the past. The real question might be then, how intense and quickly have they ever experienced disturbance like has been experienced in the past three decades, and, moreover, why has the phenomena never been reported before the past century?
Reef-building corals are minute polyps belonging to the phylum coelenterata. These reef animals coexist with other animals and algae that eventually die and produce a mat or skeletal structure of detritus material on which corals build themselves. Coral reef communities also have a very important symbiotic relationship with unicellular microscopic plants called zooanthellae that feed on the nitrogenous waste and respire oxygen through photosynthesis to the surrounding water (Reaser et al., 2000). The phenomenon of coral bleaching occurs when the endodermal cells that zooanthellae attach themselves to in the coral polyps become sloughed off due to different environmental stressors (Buddemeier and Fautin, 1993). Once the endodermal cells are sloughed off and the symbiotic relationship no longer exists, the reef loses its color and becomes a pale white. The coral reef and polyps are still living, but depending on how long the stress persists entire communities can die off. These types of regionally to local mass bleaching events have been termed bleaching Ňhot spotsÓ and are defined as anomalies in sea surface temperatures (SST) in coral regions that approximate or exceed by 1.0 degrees C the SST expected climatologically in the warmest months of the year (Buddemeier and Fautin, 1993). Coral bleaching was first reported in the seventies, and have since been recorded using satellite remote-sensing data.
The model that I looked at was trying to give values to all of the influencers of climate change around the globe (stochastically and chaos related) to make predictions as to when mass bleaching events might occur. Huppert and Stone (1998) said that the global coral reef bleaching cycle occurs every three to four years. The two big components that they use are strong El Nino years and background environmental noise (such as random weather disturbances). They found that increases of .5-1.5 degrees C over weeks or 3-4 degrees over days (sometimes hours) causes coral death or dysfunction. They also have found that many reef-building corals live near their upper thermal tolerance levels which make them sensitive to temperature change.
Huppert and Stone (1998) concluded with their model that certain variables in their model could culminate to create a bleaching not spot based on non linear ENSO events and seasonal cycles. The four findings include (1) changes in background environmental noise, such as short-term weather fluctuations, increases the propensity for threshold crossing (bleaching hot spot specifications) for both frequency and intensity. (2) A reduced temperature threshold, due to degradation and decline of coral. (3) Increased temperature due to global warming could push the SST above the bleaching threshold level, and (4) UV radiation during ENSO years in the central and western Pacific which brings clear skies and doldrum-like conditions. With all of these factors one can conclude that not just one certain climate event can cause the massive global coral bleaching events, but that their must be some kind of resonance among these and some kind of environmental background noise that correlate to create these extreme conditions. Although their has been some discussion on coral reefs as bioindicators of global climate change, bleaching is so complicated, its to hard to minimize that much environmental disturbance without a better understanding of coral reef ecology.
Another somewhat opposing hypothesis of why coral bleaching is occurring in more frequency in the last few decades has to do with it being an adaptive mechanism. Buddemeier and Fautin (1993) hypothesize that where there are multiple types of both zooanthellae and host coral species among reef communities, the polyps will only accept the right type of zooanthellae that are most productive for the given environment that they are found in. The potential exchange of symbionts has been studied, and there are restrictions based on reproductive success, and after conditions fall back to pre-stress levels the original zooanthellae will replace the algae that were used by the polyp when it was stressed. Repopulation after bleaching was found to have four different outcomes: (1) an unstable relationship that breaks down rapidly, (2) a stable relationship, but algae gets out competed by another, (3) advantageous combination resistant to invasion by another algae under non-stress conditions, (4) coexistence of more than one type (sea anemones) that harbor zoochorellae and zooanthellae. Therefore algae in a stress-resistant symbiotic unit may be displaced by a less stress-resistant zooanthellae, some of these same characteristics have been found in sea anemones, crabs and oysters. So the adaptive mechanism that corals have developed is a microevolutionary adaptation to cope with the stress of the immediate environmental conditions that have been created on a global scale.
The worst El Nino years on record since record are 1983 and 1998, the latter being the most geographically extensive. Bleaching hot spots that have been recorded using satellite remote-sensing data suggests that there is some kind of correlation to El Nino events and poor ocean circulation around coral reef communities. Models have been shown to show a cyclic pattern to when massive bleaching events might occur, while others believe that bleaching may be an adaptive mechanism that corals have adopted over their 2 billion year old lives (Mesozoic). One thing is for sure, and that is that when trying to understand what coral bleaching is and why it occurs randomly and in random places, one needs further research to understand the different types of zooanthellae and El Nino conditions coupled with local or regional weather anomalies. No direct conclusion can be made until more research is done, but for now we do need to try to understand why these disturbances are occurring in a higher frequency and intensity instead of trying to further complicate what we do understand.
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