Bleaching Biodiversity

Life's Essentials

Carbon dioxide (CO2) and water are essential for life's processes. Without these basic components, the formation of organic matter through photosynthesis would not be achievable. However, too much of life's essentials, namely CO2, can cause major concerns for our planet; this is specifically the greenhouse effect, highlighted by CO2 concentrations observed at the Mauna Loa observatory (Figure 1). 

Figure 1. CO2 concentrations up to 2005 (source: IPCC, 2007).

The IPCC 2007 report indicated that whilst the earth's atmosphere has warmed by 1°C since 1850, the majority of this warming has taken place over the last decade (Figure 2). 

Figure 2. Global average temperatures from 1850 to 2000 (source: IPCC, 2007).

Numerous IPCC models have been developed to estimate future global atmospheric warming that may result from possible CO2 concentrations by 2100. Figure 3 highlights the projected changes in atmospheric global temperature ranging from 2 to 4°C greater than the  Year 2000 constant projection. 

Figure 3. Predicted surface warming up to 2100 (source: IPCC, 2007).

These observed and projected increases in CO2 concentrations are very important in relation to earth's ocean system. For example, the global atmospheric warming since 1850 has translated into 0.5°C rise in ocean temperature with pH reducing by 0.1 to present average of approximately 8.1 (Coles, 2008). These values may seem small and therefore unimportant, but they can have a significant impact on areas dominated by coral reef systems, especially in the tropics (Coles, 2008).

Coral Bleaching

The primary concern for coral reef systems in terms of increasing temperature is the link to coral bleaching and the prediction of increased frequency and severity of bleaching events. I am sure that those of you reading this are familiar with coral bleaching after viewing Kate's blog entry entitled 'Coral Bleaching - Expelling Biodiversity', however I hope I can add on from this and convey its importance.

Over the last 20 years the awareness of coral bleaching has increased significantly due to the increasing prevalence of bleaching events. According to Coles and Brown (2003), bleaching has naturally taken place on coral reefs for an inordinate amount of time and was initially described in the 1920s during an expedition to the Australian Great Barrier Reef. From 1970 onwards, the underlying understanding of the processes and thresholds behind coral bleaching have improved markedly. From this improvement, two fundamental concepts were derived:

• The first being that bleaching results from the 'combined and synergistic effects of elevated light and temperature impacting the coral-algal symbiotic association' and;
• The second being that 'threshold temperatures leading to coral bleaching are not fixed limits, but rather closely tied to the ambient annual maximum temperature normally occurring in the local environment of the coral'. 

The process behind the first concept is relatively straightforward. The majority of coral species have a symbiotic relationship with zooxanthellae that live within corals' tissue. The zooxanthellae provide corals with their rainbow colours as well as up to 90% of the energy they require to survive, grow and reproduce. However, coral bleaching occurs predominately due to elevated sea temperatures and also high solar irradiance (Brown, 1997). As a result, this causes the dissociation of the symbionts resulting in the coral turning white in colour (Figure 4). The Great Barrier Reef Marine Park Authority (GBRMPA) states that if elevated sea temperatures persist longer than eight weeks, coral start to decay and die. 

Figure 4. Health distinctions for coral (source: GBRMPA)

Coles (2008) highlights that the second concept was based on the comparison of results from corals in Hawaii where the ambient temperature is 27°C and those from a mid-Pacific atoll where the annual maximum is 29°C. The coral populations of both areas are subject to exposure of temperatures close (1°C to 2°C) to their upper limit during the summer months. This concept has been repeatedly reaffirmed by bleaching episodes such as those in The Arabian Gulf in 1998 (Coles, 1983). Importantly, the range of this threshold highlights the future of coral with relation to thermal stress. With the range being relatively large, it highlights the ability of coral to acclimatise and adapt to ensure the maintenance of the symbiotic relationship with zooxanthallae (Coles, 2008). However, change may be too great, too fast and adaptation may fail (Hey et al., 2002). 

 A Brief Case Study: The Great Barrier Reef from The GBRMPA

The Great Barrier Reef has experienced vast bleaching episodes in the past. There have been two instances relatively recently in 1998 and 2002 that have had a severe impact. The 1998 episode was part of a global mass bleaching where approximately 50% of the reefs on Great Barrier Reef suffered bleaching. This was due to increased sea level temperatures. In fact they were the highest ever recorded according to the GBRMPA. As a consequence of the bleaching, 5% of the coral reefs were classed as severely damaged. Moreover, the bleaching event in 2002 saw 60% of reefs affected giving it the title of largest coral bleaching episode in history. The cause behind this was two periods of extended hot weather resulting in elevated SSTs that were greater than the long term summer maxima thereby exceeding the temperature threshold (concept 2). Here too, 5% of coral reefs were classed as severely damaged. 

I appreciate this is a lot of information but I hope you now understand a little more about coral bleaching and its impact.

Stay tuned,

Seb


References

Brown, B. (1997), ‘Coral Bleaching: Causes and Consequences’, Coral Reefs, 16, 5, 129-38

For all Coles references please view the Coles (2008) hyperlink.

Hay, J. E. (2002), Climate variability and change and sea level rise in the Pacific Islands region: A resource book for policy and decision makers, educators and other stakeholders, Tokyo, Japan: South Pacific Regional Environment Programme and Japan Ministry of the Environment.