CURRENT DAY THREATS TO CORAL REEFS: NATURE OR NURTURE? (PART 2)

A Review of Hoegh-Guldberg et al. (2007) CORAL REEFS UNDER RAPID CLIMATE CHANGE AND OCEAN ACIDIFICATION.

Introduction: Atmospheric carbon dioxide concentration is expected to exceed 500 parts per million and global temperatures to rise by 2 degrees between 2050 and 2100. This significantly exceed conditions of the last 420,000 years during which most extant marine organisms evolved. Under expected conditions for the 21^st century, global warming and ocean acidification will compromise carbonate accretion, with corals becoming increasingly rare on reef systems.

What is ocean acidification? During the 20th century, increasing atmospheric CO₂ concentrations has driven an increase in the global oceans’ average temperature by 0.74°C, sea level by 17 cm, and has depleted seawater carbonate concentrations and increased acidity by 0.1 pH units. Approximately 25% of the CO₂ emitted from all anthropogenic sources currently enters the ocean where it reacts with water to produce carbonic acid. Carbonic acid dissociates to form bicarbonate ions and protons, which in turn react with carbonate ions to produce more bicarbonate ions, reducing the availability of carbonate to biological systems (Fig. 1A). Decreasing carbonate ion concentrations reduce the rate of calcification of marine organisms such as reef-building corals, ultimately favouring erosion.

Figure 1: Links between the buildup of atmospheric CO2 and the slowing of coral calcification due to ocean acidification. 

Modeled scenarios: Hoegh-Guldberg et al. (2007) projected three scenarios for coral reefs over the coming decays and centuries.

1)      375ppc and 1˚C increase (stabilising at the present atmospheric CO₂ concentration) coral reefs will continue to change but will remain coral dominated and carbonate accreting in most areas of their current distribution (Fig 2A).

2)      However if we move towards higher CO₂ concentrations (450-500ppm and 2˚ increase) coral community compositions will change with some areas becoming dominated by more thermal tolerant corals and others potentially dominated by thermally sensitive but rapidly colonising genera. The density and diversity of corals on reefs are likely to decline leading to vastly reduced complexity and loss of biodiversity including losses of coral associated fish and invertebrates (Fig 2B).

3)      As atmospheric carbon dioxide levels of more than 500ppm are approached levels of carbonate ion concentrations fall well below today’s value and we exhibit an increase of more than 3˚ in sea surface temperature. These changes will reduce coral reef ecosystems to a crumbling frame work with few calcareous corals. Continuously changing climate (which might not stabilise for hundreds of years) is likely to impede migration of corals. Reefs will be rapidly eroded and ultimately become drowned reefs (corals and reef growth fail to keep up with rising sea levels). Under this climate scenario coral reefs become non-functioning (Fig 2C).

Figure 2: Extant reefs from the Great Barrier Reef that are used as analogs for the ecological structures under scenarious 1-3. A). reef slope communities at Heron island. B). mixed algal and coral community  & C). Inshore reef slope.

Impacts: These three scenarios are likely to have serious consequences not only on the corals reefs diversity and density but through wider regional economies e.g. coastal protection, fisheries, and tourism. These consequences become progressively worse as we move through the three potential scenarios.  

 

Conclusion: It is worrying to think that when using the lowest rage of IPCC scenarios there is still devastating effects for coral reefs. At carbon dioxide atmospheric concentration of more than 500 ppm the consequences for coral reefs are extremely risky for both corals and those that depend on them. Climate change also exacerbates local stresses from declining water quality and overexploitation of key species, driving reef increasingly toward the tipping point for functional collapse.

*ppm – parts per million. 

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.

ONE CORAL BANK AND HOW A SINGLE 12YR OLD CAN MAKE A DIFFERENCE.

Hello, 

Apologies for my lack of blogs over the last 2 weeks: dissertation and poster making seemed to have consumed all my time. Anyway, draft of the dissertation is in and the poster for this course is submitted. Here is a preview of my poster which will be presented on Tuesday, (at least it would be if I could work out how to upload a power point file!  Any suggestions would be more than welcome).

Amid the search for poster material, I came across Reef Quest - established by 12 year old Dylan and now joined by over 6000 people from 23 countries who work to monitor reefs and protect them for the future. Among their work they take their knowledge into schools and communities to emphasize the importance of coral reefs.

One of the tools for doing so is this video which highlights the links between atmosphere, ocean and humans: 

http://vimeo.com/8079833

Reef Quest is an excellent example of how one person can help the cause. This is his website.... http://www.reefquest.org/ .... Join the cause!  

On an exciting note, corals have made the news this week - with cryogenic plans getting under way in Australia.

A team of scientists from Taronga Zoo, Australia is working on an ambitious plan to create a 'coral bank' (a bit like a sperm bank).  As the Great Barrier Reef is eroded by global warming, ocean acidification and coral bleaching, embryos will be taken and stored at -296 degrees (Fahrenheit), the temperature of liquid nitrogen. This proposal means the coral reefs could be repopulated from embryos when environmental stresses are reduced. 

'This is really an insurance programme to take the coral out of an uncertain situation and put it in a place that is 100 per cent safe for a very long time' said the zoo's manager of research and conservation, Rebecca Spindler. 

Initial research will concentrate on heat sensitive corals - those most at risk from ocean warming.  If successful, this project could ultimately prevent corals becoming extinct.

http://www.smh.com.au/environment/earth-hour/coral-reef-cryogenic-plan-gets-under-way-20110314-1buky.html

See you next week when I will finally give you a comprehensive look into current day threats.

APOLOGIES: A NEW REVIEW WILL APPEAR SOON.

Sorry about the lack of blogging of late. Dissertation crisis! Come back on Wednesday to see a review of current day threats to the rainforest of the seas.

CORALS IN A CRISIS?

The latest report seems to suggest so ....


http://www.guardian.co.uk/environment/2011/feb/23/coral-reef-report-dying-danger


This articles touches on some of the threats to modern day reefs. Join me over the coming weeks to look at these in more detail and see what can be done to save our reefs from extinction.

THE EVOLUTION OF A TROPICAL PARADISE.

Hawaii is one of the few places in the world where the majority of the stages of reef evolution can be seen.  Recently created lava flow on the big islands become home to new reefs and developing communities. Fringing reef forms  near shore can be seen throughout Hawaii. Barrier reefs are seen off  the coast of Oahu and Kauai and coral atolls are built on top of sunken volcanic islands in parts of the north-western Hawaiian Islands (Figure 1).

Figure 1: A. The Hawaiian Archipelago. B. The Island of Oahu.

After my last entry you will remember that the accretion of coral reefs during the Holocene is sea level dependent, after the peak of the Wisconsin glaciation some 21ka the glacier melted, sea level rose and approached the critical depth (30m) of modern reefs....

.... BUT wave exposure is also a major determinant that controls the development of coral reefs (Grigg 1998). In many parts of the world windward reefs may be favoured over leeward reefs because of the greater exposure to ‘moderate’ wave action which in turn optimises misting, and nutrient availability.

Crigg (1998) showed that of the coast of the island of Oahu (Figure 1) areas exposed to between 0-30% wave exposure have an average long term growth of corals of 2mm/y. Albeit small this is still positive.  However along wave exposed coasts (waves greater then 7.5m) Holocene reef formation is only a thin layer sitting on top of a Pleistocene limestone foundation.  

Modern coral communities in wave exposed environments undergo constant turnover associated with morality, recruitment or re-growth of fragmented colonies and are rarely thicker than a single living colony. High Hawaiian islands are highly susceptible to wave action and it is this that has been associated with its lack of mature barrier reefs. The only reef off  the coast of Oahu that is close to become a barrier reef is off Kaneohe bay. This side of Oahu faces east and is therefore exposed to only moderate trade winds. This lack of destruction from large waves and hurricane swells has allowed the reef to establish itself.   

But barrier reefs are present in wave exposed coastline in other parts of the world (e.g. tropical pacific islands) so another factor must play an important role. One proposed theory is the low species diversity of coral in the high Hawaiian islands.  Acropora the fastest growing species of coral is present in the middle of the Hawaiian archipelago but absent from the high islands. On the island of Oahu in protected and deep water environments the dominant coral species is P.compressa. Whilst in more exposed habitats  P.lobata is the dominant reef builder. Another theory is that Hawaii’s waters are on the cool side for substantial coral growth. 

In short it isn’t only the change in Holocene sea levels which influence the Hawaiian corals and their formation, community structure appears to be controlled by wave energy and depth, alongside species diversity. 

Crigg, R.W. 1998. DOI: 10.1007/s003380050127

CORALS EVOLUTIONARY HISTORY

This week I’ve been reading some papers on coral evolutionary history from the first fossil of the Permian through to their current day biodiversity. I’ve looked at the dips and troughs along the way and the environmental causes. This is a brief over through the 500mya or so....

The Permian

Corals first appeared in the Cambrian period (Pratt et al. 2001) some 542 mya. However fossils were rare until the Ordovician some 100 mya later where rugose and tabulate corals began their period of great success.  Like there modern day counter parts, Rugosa and Tabulata formed reefs which are now held in sedimentary rocks.  They are characteristic of the shallow waters of the Silurian and Devonian (figure 1).

http://www.stratigraphy.org/upload/ISChart2009.pdf
(just in case the figure is poor quality here is a link)

Wang & Sugiyama (2000) showed two decreases in coral diversity during the Permian from China. The first shows a loss of 80% of coral species and is associated with the eruption of Omeihan basalt. The second caused the total demise of rugose and tabulate corals along with many marine and terrestrial species. This occurred at the end of the Changhsigian (figure 1) and is related to the Permian global regression (the Permian mass extinction).  

Evolution of modern day corals.

Following this extinction scleractinian corals evolved to fill this niche, with a complete fossil record that dates back to 240my (mid Triassic). These consist of the corals we all know and associate with the modern reefs. They are a group of calcified anthozoan corals that populate shallow water in tropical to sub tropical reefs.  They stand out as being one of the few calcified metazoan to arise in the Triassic as the majority of orders originated in the Palaeozoic.  The origin of this coral group has remained an unsolved problem in palaeontology and has several proposed hypotheses. These include: the idea that Scleractinia evolved from Palaeozoic rugose corals that somehow survived the Permian mass extinction (Stanley 2000), However Scrutton (1997) does not consider Rugosa to be ancestral to Scleractinia. Alternatively the idea that Scleractinia were derived from soft-bodied, ‘‘anemone-like’’ ancestors that survived the Permian mass extinction, has become a widely considered hypothesis (Stanley 2002). Paleozoic scleractiniamorphs also have been presented as possible ancestors (Stanley 2000).

After their resounding successes in the Silurian and Devonian, corals experienced further periods of high diversity in the Triassic, Cretaceous and the Eocene to recent. Each phase of high diversity is separated by a period of lower diversity that followed high extinction rate (Copper 1994). The extinction episodes corresponded with those of other marine shelf communities and were associated with global climatic cooling and oceanic regression.  The recovery period of corals are much longer than other marine species and further emphases there specificity and suggest that reef ecosystems are especially sensitive to large-scale environmental perturbations (Copper 1994).

The Cenozoic Era.

The Cenezoic era has seen three main extinction highs for coral species (Middle to Late Eocene, Late Oligocene to Early Miocene, and Plio-Pleistocene) (figure 2) each coincided with large scale environmental perturbations. Global drops in temperature due to the Oi-1 glaciation (Southern Hemisphere Glaciation) have been widely cited as the cause of Middle to Late Eocene extinctions in both terrestrial and marine biota (Budd 2000). Increased upwelling, associated turbidity and cooling have been proposed causes of the Early Miocene extinction (Budd 2000) and drops in sea surface temperature associated with the onset of northern hemisphere glaciation affected many different marine organisms during the Late Plio-Pleistocene (Budd 2000). The intensity of the extinction was highest during the Plio-Pleistocene where is occurred over a million years or less.

The creation of coral reefs during the Holocene (the last 10ka) is sea level dependent. At the peak of the Wisconsin glaciation (21ka) sea levels had dropped to about 110-120m below their current levels, as sea levels rose during de-glaciation and approached the critical depths of modern reefs (30m) coral reefs took hold and began to grow in suitable habitats on many island shelves.

.... in my next entry I’ll look specifically at the history of the Caribbean coral reefs.

Pratt et al. (2001) ISBN: 0231106130

Budd (2000) DOI: 10.1007/s003380050222

Copper (1994) DOI: 10.1007/BF00426428

Scrutton (1997) DOI:10.1144/pygs.51.3.177

Wang & Sugiyama (2000) DOI: 10.1080/002411600750053853