CURRENT DAY THREATS TO CORAL REEFS: NATURE OR NURTURE?

A Review Of Hoegh-Guldberg  (1999) ‘CLIMATE CHANGE, CORAL BLEACHING AND THE FUTURE OF THE WORLD’S CORAL REEFS’. 

Like all good scientists first they give you the back ground information and only at the very last minute do they draw their conclusions and give you their interesting finds, facts and theories.  So to follow suit here is the background;

Despite corals importance (see future blogs) and their persistence over geological time (previous blogs) corals appear to be one of the most vulnerable marine ecosystems of modem day. Dramatic reversals in their diversity and health have been reported worldwide.  It is estimated that between 50%-70% of all coral reefs are under direct threat from humans (through fishing, deforestation, nutrient enrichment, burning of fossil fuels, and use of toxic chemicals), whilst 100% of coral reefs are exposed indirectly through modifications of interactions with their competitors, predators, pathogens and mutualists. Mass coral ‘bleaching’ is also a major contributing factor to a decline in coral reefs. Since 1979, six major episodes of coral bleaching have occurred (Fig 1), each with associated reef mortality affecting reefs in every part of the world. 

Fig.1. Number of reef provinces bleching since 1979 (from Hoegh-Guldberg 1999)

Whilst reef mortality had started to receive some attention at top levels of world governments there are still many unanswered questions. Houegh-Guldberg (1999) looks at the scientific evidence that suggests coral bleaching is a sign of climate change and builds a case for the prediction that thermally triggered coral bleaching events will increase in frequency and severity in the next few decades. He sets out to answer the following;

1) Is coral bleaching a natural signal that has been misinterpreted as a sign of climate change.   

      2)  Has the incidence of coral bleaching increased since 1979? Or has it simply been over looked prior to 1979?

      3) Are bleaching events likely to increase or decrease in intensity in the next 100 years?

      

Symbiosis in coral reefs: zooxanthellae are intracellular organisms that live within the membrane bound values in the cells of the coral. Studies by Trench and Rowen have revealed that zooxanthellae are a highly diverse group of organism which include 100s of taxa with two or three species per host. Corals benefit from this mutualism by receiving photosynthetic sugars and amino acids, whist in return provide the zooxanthellae  are provide with crucial plant nutrients e.g. ammonia and phosphate from their waste material, with this island of productivity amid this arid desert.

The role of temperature: reefs dominate tropical environments between 30 degrees North and 30 degrees South these roughly coincide of water temperature between 18 degrees and 30 degrees C.  Tropical latitudes further suit corals because seasonal and diurnal fluctuations in tropical sea temperatures are generally very small.  However, Thunnell et al. (1994) suggest sea surface temperatures of the tropical oceans have increased by 2 degrees in that last 18,000 years.  There is a strong correlation between warmer than normal conditions (at least 1 degree higher than a summer maximum) and the incidence of mass coral bleaching.  Increasing water temperature evident rapidly cases zooxanthellae to leave the tissue of reef building corals resulting in a reduced number.  Heat stress does not only cause a reduced population density of zooxanthellae but it also acts to reduces their photosynthetic rate. Basically, light that is essential for high productivity of coral reefs under normal conditions becomes a liability under higher than normal temperatures.  

NOTE: A rise in water temperature is not the sole cause of colour loss and reduced salinity, altered light intensity and exposure to chemicals can all cause corals to become pale, although the physiological response and mechanism by which they do is not the same as true ‘bleaching’.

So why has the incidence of bleaching increased? Some commentators have suggested that it is simply a reflection of the number of observers and the ease with which reports can be made and brought to public attention e.g. by internet.

However, palaeo-sediments have shown that tropical seas have under gone a warming in the past 100 years.  In many tropical seas, rates of change are now greater than 2 degrees per century. By comparing simulated sea temperatures to the thermal thresholds of corals estimates of frequency with which sea surface temperatures will exceed the thermal maximum of corals and their zooxanthellae (Fig 2.).

Fig. 2.Weekly sea surface temperature data for Tahiti, arrows indicate bleaching events in the literature. 

Hoegh-Guldberg (1999) ran fours analysis from three climate change models, which thus stimulate changes in sea temperature.  In all four runs he found (Fig 3);

Fig. 3. Number of times per decade that predicted temperarures exceed coral threshold levels (bleaching events). 

-         The frequency of bleaching is set to rise rapidly, with the rate being highest in the Caribbean and slowest in the Central Pacific.

-         Secondly, the intensity of bleaching will increase at a rate proportional to the probability that the thermal maxima of the corals will be exceeded by future Sea Surface Temperatures.

-         Thirdly, most regions will be experiencing bleaching conditions every year within 30-50 years.

-         Lastly, the reason for the relatively low frequency of bleaching events before 1979 becomes clear; tropical sea temperatures have been rising over the past 100 years (Bijlsma et al. 1995) and have brought corals ever closer to their upper thermal limit. The ability for an El Niño event to trigger bleaching was only reached in most oceans in the period from 1970 to 1980. This explains why mass bleaching events are not seen to any great extent before 1979.

It is thought Reef building corals and their zooxanthellae are unable to adapt (genetically) fast enough or acclimatise (phenotypically) to thermal stress. If corals are incapable of changing their physiological response to cope with this stress, bleaching will increase with frequency and intensity with serious consequences.

The acclimatisation of corals and their zooxanthellae refers to the modifications of cellular metabolism which might make them more tolerant to higher temperatures. Whilst adaptation refers to the selection of individual corals within a population that are better able to cope with the new high temperatures and thus survive. Individuals which have a poor affinity to high temperatures either do not survive or do not breed. Acclimatisation can occur over a few hours or days whilst adaptation may require hundreds or thousands of years.

Adaptation: corals and zooxanthellae have variety of thermal optima and maxima across various species. Corals have adapted to local temperatures, this is not surprising since it is universal to all living organisms e.g. the Masi who are adapted to high temperatures and Eskimos adapted to low.  The variation of thermal tolerance suggest that there are genotypes within current coral populations that may be selected for under regimes of increasing temperature. However, this change towards a population dominated by high temperature tolerant genotypes is a slow process and may depend on the stabilization of sea temperatures.

However, the multiple recurrence of bleaching at the same sites over the past 20 years (some coral reefs have bleached during every major bleaching episode) strongly suggests that populations are not rapidly changing their genetic structure to one dominated by more heat-tolerant individuals. A second way that corals might increase their survival is to change their zooxanthellae for more heat-tolerant varieties (Adaptive bleaching hypothesis). Although this idea has attracted much discussion, it is not well supported by evidence.  The observation that corals may have a variety of different types of zooxanthellae in the one colony and experience the selective loss of one type during temperature stress (Rowan et al. 1997) does not necessarily demonstrate that bleaching is adaptive.

Acclimation: Corals and zooxanthellae are able to acclimatise to changes in their environment on a daily or weekly bases e.g. zooxanthellae acclimatize to higher light intensity during the day by modify the concentrations of photoreceptors and associated chemicals.  Despite their ability to acclimatize to changing environmental conditions, reef building corals do not appear to have acclimatized to the rapid increases in sea temperature over the past 20 years. There is no broad pattern suggesting that corals are better at coping when their maximal temperatures are exceeded. Corals seem to be just as close to their thermal limits as they were at the beginning of the 1980s, suggesting that acclimatisation (as well as adaptation) does not seem to have occurred to any great extent. If corals were acclimating (or adapting), then we should see a reduced number of corals affected by bleaching.

Conclusion: Even under moderate greenhouse scenarios (a doubling of current greenhouse gas concentrations by 2100), present and future increases in sea temperature are likely to have severe effects on the worlds coral reefs within 20-30 years. Most coral reef systems are predicted to experiencing near-annual bleaching events that will exceed the extent of the 1998 bleaching event by the year 2040. Some coral reefs (e.g. Caribbean, South-east Asian) will reach this point by 2020. Cooling by anthropogenic aerosols will have little effect on the time that the endpoint is likely to be reached. A better understanding of the capacity for corals and zooxanthellae to adapt to these rapid and on-going changes is required. Present evidence, however, suggests that corals and their zooxanthellae are unable to acclimate or adapt fast enough to keep pace with the present rapid rate of warming of tropical oceans. If the mortality of reef-building corals continues to increase, changes in the distribution of corals will almost certainly occur. Given the central role of corals and zooxanthellae in the structure and function of coral reefs, these changes are likely to have severe and negative effects on the health of coral reefs world-wide by the middle to end of next century. The ecological and economic effects of these changes have not been properly assessed and should be a priority of future research. If, however, the scenario presented in this paper continues to be supported, then a rapid reduction of greenhouse gas emissions over the next decade must be put into effect immediately.

Join me over the weekend for the second instalment of CURRENT DAY THREATS TO CORAL REEFS: NATURE OR NUTURE? Where I will review the atmospheric carbon dioxide.

Reference

Bijlsma, L. Ehler, C. N. Klein, R. J. T. Kulshrestha, S. M. McLean, R. F. Mimura, N., Nicholls, R. J. Nurse, L. A. Perez Nieto, H. Stakhiv, E. Z. Turner, R. K. & Warrick, R. A. 1995. Coastal zones and small islands. In .Climate Change 1995.Impacts, adaptations and mitigations of climate change: scientific-technical analyses: the second assessment report of the Inter-Governmental Panel on Climate Change. (Eds R. T. Watson, M. C. Zinyowera and R. H. Moss.) pp. 6-12. (Cambridge University Press: New York.)

Rowan, R. Knowlton, N. Baker, A. & Jara, J. 1997. Landscape ecology of algal symbionts creates variation in episodes of bleaching. Nature. 388: 265-9.

Thunnell, R. Anderson, D. Gellar, D. & Miao, Q. 1994. Sea-surface temperature estimates for the tropical western Pacific during the last glaciation and their implications for the Pacific warm pool. Quaternery Research. 41: 255-64.