Disease and Crown of Thorns

Yesterday when I published my last post, I felt like I was on a roll. Thus, I am going to forget my other work and smash this one out. So today I am going to be introducing you to the impact of disease and a biological enemy on corals and how it will most likely pan out in the future.

Let's get down to it shall we!? Diseases in reef communities are normal. Reefs, like all organisms, are affected by disease but their susceptibility is increased (outbreaks increased) under the influence of other stressors such as bleaching and OA for example. 

A specific disease affected corals is white syndrome in the GBR. A study by Bruno et al. (2007) focused on the interaction between anomalously high ocean temperatures and extent of coral cover, highlighting the role of disease. The mechanism behind disease outbreaks and climate is complicated and if you are interested then please read this paper in detail by Sokolow (2009). Here, however, is a diagram illustrating the complexity from Sokolow (2009).

In a nutshell though, an increase in temperature causes pathogens to reproduce more quickly and as a consequence, disease spreads much faster. This has been seen to be the case in the summer months confirming this theory. In the table below taken from Solokow (2009) shows other climate drivers and their impact on disease. 

Anyway, Bruno et al. (2007) to construct their argument, they evaluated the relationship between occurrence of white syndrome and a) no. of weekly SST anomalies (>1degf against mean records) b) coral cover, and c) interaction between the two. From their statistical analysis, the third variable was deemed to be statistically significant with a strong correlation present. Therefore, temperature anomalies along with high coral cover equal a greater likelihood of white syndrome outbreaks. What of the future? Well, if global warming increases (which it will do), then warm temperature anomalies observed will increase resulting in ever increasing susceptibility and outbreaks, to and of white syndrome. This disease has the ability, if the impact is great enough, to completely transform a vibrant reef into a wasteland. Scary stuff!

It must be noted that in the thorough paper by Solokow (2009), the relationship between coral disease and climate has been difficult to reliably ascertain. The evidence in recent years has been largely poor and uncertainty still lingers. However, research in this area is moving in the right direction. What needs to be achieved next is the understanding of what causes disease emergence at large spatial and temporal scales. But to achieve this, researchers must overcome three hinderances outlined in the Solokow (2009) abstract. Whilst the evidence is in its early stages, it suggests that reducing climate change will reduce disease outbreaks. 

Now then, my focus is going to move towards an coral animal that is becoming a real trouble maker on the GBR. The Crown of Thorns Starfish (COTS) has been found to devastate the coral reefs.  They look like a menace.

Crown of Thorns Starfish (source: pacificislandparks.com)

Watch this from the BBC! The video briefly summarises the state of affairs in relation to the GBR. The study it refers to is this one by De'ath et al. (2012).  The statistics in the video are truly worrying and highlights the impact of both storms and the COTS. (This is also a good paper for the previous post on storms but alas I forgot!). Anyway, from this evidence , one cannot underestimate the COTS and like the video says, more must be done to regulate these biological hazards to ensure the integrity of the reef.  This will be difficult as the ability of the COTS to have a significant impact is also a function other environmental variables that need to be addressed also.

This post is the last one for the series of Climate Crimes stated a while ago. I haven't yet decided on what I am going to do next so I'll leave you with this astonishing and outrageous cliff hanger...lol?

Over and out peeps,

Seb 

Turbulent Times

Today, I am going to be talking about the impact of storms and rainfall on reef communities. To start, I am going to cover storms.  Gardner et al. (2005) states an interesting idea that storms, whether it be a hurricane or tropical storm, are the "most obvious and frequent natural disturbances" affecting coral reef communities. Initially, after reading this I thought that couldn't be true, especially after reviewing a whole load of other impacts that seem to be far more frequent and in my mind more obvious. However, from reading this paper through, I can see where Gardner et al. (2005) is coming from, and if anything, this impact is becoming far more important. The climate is warming and so is the sea. Increased warmth in our oceans causes increased storm events due to a greater rate of latent heat exchange amongst other factors (NOAA). The GBRMPA states that this will cause an increase in frequency and severity of these storms. However we don't really need to someone to state this is happening because we can see this with our very own eyes, especially most recently with Hurricane Sandy. The Wikipedia page for this storm is actually very good; more specifically the 'relation to global warming section'. I agree with the NCAR senior climatologist Kevin Trenberth who is quoted in this section, that global warming does not cause these storms to occur but it increases the likelihood that they can occur. Anyway, that's enough of that... but there is no denying that storm frequency and severity is increasing. Here is a graph to illustrate this...

Bars depict number of named systems (open/yellow),
hurricanes (hatched/green), and category 3 or greater (solid/red), 1886-2004 

Now that we know the frequency and severity of storms is increasing, I can address the impacts they have on reef communities. In a detailed paper by Fabricius et al. (2008), storms are described as the "most severe form of mechanical disturbance" for corals. They are right for saying this. There have been numerous papers detailing how storms act as determinants of reef structure and function through their impacts (references found in Gardner et al. 2005). Simply, the GBRMPA states three impacts: 1. Coral breakage 2. Dislocation 3. Degradation. Furthermore, it is noted that the effects of these storms can have legacies of up to centuries (Connell, 1999). Fabricius et al. (2008) provides are great case study on the impact of tropical cyclone Ingrid that hit the most Northern part of Queensland. This paper looked at the impact on inshore and offshore reefs and highlights the significant perturbations on marine ecosystems. The worst affected inshore reefs amounted to extensive rubble fields whilst the worst affected offshore reefs were stripped and only their solid substratum was left. Their are some statistics presented in this paper that are truly astonishing. For example, on a devastated inshore reef, the coral cover decreased by 800%, diversity 250% and density 30%. Fabricius et al. (2008) clearly illustrates with these figures the role of storms and their ability to break and dislodge corals. Interestingly, from large scale survey analysis, the key factor driving the devastation was the wind speed over a 10 minute average but storm duration was also another significant variable. 

So, thoughts for the future? Well, future projections are somewhat uncertain as they are linked to projections of ocean temperature increase. Sriver and Huber (2006) state that the potential increase in destructiveness could range from 0% to 60%, whilst Knutson and Tuleya (2004) predict an increase of 6-12% in maximum wind speeds by 2090 which equates to half a category on the Saffir Simpson Scale. Relating back to the Great Barrier Reef, a potential increase by half a category in terms of intensity would have dramatic consequences; consequences far greater than ever observed before.

Moving on to rainfall and corals, you may be thinking 'hmmmmmmmm what's going on here?'. Well I thought exactly the same thing, so don't you worry. With storms comes rain and usually these are heavy rainfall events that lead to fresh water inundation in the inshore reefs. The concept is pretty straightforward (and corals are thought to have a low tolerance to changes in salinity). The fresh water reduces the salinity of the coral environment leading to bleaching i.e. the loss of zooxanthellae (please see previous posts for information). A relatively old paper by Nyawira et al. (1987) explored the effects of salinity stress on a specific hermatypic coral. The paper found that this coral species was actually relatively tolerant to salinity change. Normal salinities in the coastal waters that this coral inhabits is no greater than 30% but it is able to acclimate to 42%. However, any change greater than 15% both up or down equalled decreases in respiratory and photosynthetic rates due to a respective decrease in chlorophyll pigments per algal cell and leads to coral demise. Despite this, this evidence (be it slightly old!) has a good point. It shows that corals can adapt and survive with a greater ability to resist prolonged or sudden changes in salinity. But, and this is a big BUT, the combination of other variables makes nothing certain. Therefore we cannot be overly confident and state that corals have a greater threshold than first thought. This would be of no benefit to corals. We as learned individuals have to, dare I say it, prepare for the worst, as modification of our climate is not going to suddenly cease and revert to how it was. 

Over and out,

Seb 

References:

Connell, J. H. 1997. Disturbance and recovery of coral assemblages. Coral Reefs 16 (Suppl): 101-113. Devantier, L., G. De'ath, T. Done, E. Turak, and K. Fabricius. 2006. Species richness and community structure of reef-building corals on the nearshore Great Barrier Reef. Coral Reefs 25: 329-340.

Knutson, T., and R. Tuleya. 2004. Impact of CO2-induced warming on simulated hurricane intensity and precipitation: sensitivity to the choice of climate model and convective parameterization. Journal of Climate 17: 3477-3495.

Sriver, R., and M. Huber. 2006. Low frequency variability in globally integrated tropical cyclone power dissipation. Geophysical Research Letters 33, L11705, doi:10.1029/2006GL026167.

Drowning Coral

To start, watch this... This made me laugh; it will make you laugh too. I am glad Obama won.

So Sea Level Rise (SLR) (or as those fine republican members like to say "recurrent flooding") is another anthropogenic impact that is giving our coral reefs a real test.

This short video below, gives you an idea of the projections for SLR, what has happened so far and how this relates to the projection. It highlights that we are on course for an 80cm rise by 2100. This is concerning and indicates that the projections may be underestimating what will most likely happen.

There are two ways that SLR has and will continue to have an impact on coral reef communities. But first (oh the suspense...), a paper by Nybakken (1993) sets out the basic needs for a coral reef to remain active. Naturally, the most essential requirement is LIGHT. The ability of coral to capture light is dependent on the turbidity of the water column and water depth. If corals cannot harness enough light then their growth ceases with the end result being death. This would be due to reduced photosynthesis by the zooxanthallae, and reduced oxygen production which impedes the coral metabolism thereby limiting calcium carbonate deposition and therefore growth. Nybakken (1993) highlights that corals at a particular water depth need a light intensity that is at least 1-2% of the intensity at the ocean surface. With SLR occurring, the light intensities are being weakened, and as such, reefs cannot and will not be able to keep up with rates of SLR, especially those coral species that exist at the water depth limit of coral growth (70m bsl).

The two ways:

1) SLR in isolation

Hoegh-Guldberg (1999) critically highlights that deep water species under dramatically reduced light intensities at their physiological depth limit will no longer be able to maintain growth and it is projected that they will become extinct. Further, slow growing corals are becoming increasingly vulnerable as they will be out run by SLR, leading to their demise. Grigg et al. (2002) corroborates these former consequences and brings your attention to the fact that coral species are will be significantly impacted under a moderate SLR projection.

However, I have found one source that states that SLR in isolation will have little, if no impact whatsoever. This information is from the Great Barrier Reef Marine Park Authority (GBRMPA). The GBRMPA states that the coral species there can acclimatise to SLR of 3mm per year as coral growth is two fold. For the GBR, this is likely to be true as the majority of reefs are extremely shallow and so the projected increase of +0.68 to 0.9m by 2100 would have little impact on reducing light intensities. Therefore, the impact of SLR in isolation is wholly dependent on the initial depth of the coral species. For species in shallower waters, other factors play a major part.

2) SLR in tandem with other environmental stresses.

Hoegh-Guldberg (1999) states that other stresses such as OA and coral bleaching will exacerbate the impact of SLR, making coral species that were previously unlikely to be effected, effected. The ability of the reefs to keep up with SLR will be drastically reduced. In a paper by Graus (1998), the model simulation illustrated that under a combination of stresses, coral reefs in the Caribbean will be unable to keep pace with projected SLR rates.

It seems that the stress of SLR is more specific than general whereas other stresses are the opposite...

Stay tuned,

Seb

References:

Graus, R.R. and Macintyre I.G., 1998, Global warming and the future of Caribbean coral reefs: Carbonates and Evaporites, vol. 13, no. 1, p. 43-47.

The Acidic Devil

Continuing from my last post, this entry will deal with the impact of Ocean Acidification (OA) on coral reefs. But first, how are OA and climate change related and what is the process behind it?

Here are some worrying facts from Hoegh-Guldberg et al. (2007) that show the link:

• The concentration of CO₂ now exceeds 390 ppm
• 25% of anthropoenic CO₂ emissions currently enter the ocean
• Seawater carbonate concentrations have reduced by ∼30 μmol kg^–1 seawater  
• The ocean has become more acidic by 0.1 pH unit 
• Since 1900 the average ocean temperature has increased by 0.74°C

This image from Hoegh-Guldberg et al. (2007) clearly indicates the relationship of CO₂ and pH (through the carbonate on the left hand side). Therefore as CO₂ concentration in the atmosphere increases, the pH decreases becoming more acidic as indicated by the decrease in carbonate concentration. 

How does this process work? 

Well, from the equation, it is not too difficult to follow. As CO₂ is highly soluble in seawater, it reacts to form an intermediary state of carbonic acid. The carbonic acid then dissociates to bicarbonate (HCO3-), carbonate (CO32-) and hydrogen (H+) ions. With increasing CO₂ concentrations and subsequent increased dissolved CO₂, it results in more H+ ions being buffered by carbonate and consequently, there is a reduced availability of carbonate ions to combine with calcium ions. Crucially, this inhibits the process of calcification and therefore reduces the ability of marine calcifiers, such as coral to build.

The Calcium Carbonate Saturation State (Ω)

This saturation state is highly variable but is greatest in the tropics. Calcium carbonate is deposited by organisms as calcite, aragonite or high magnesium calcite. The crystalline form we are interested in as aragonite as aragonite is incorporated into corals' skeletal growth (Coles, 2008). Therefore the aragonite saturation state is a significant limiting factor in the ability of coral to maintain its structure. 

Coral Response to Reduced Calcification as a Result of Reduced Aragonite

Hoegh-Guldberg et al. (2007) highlights three possible responses:
1. Reduced linear extension and skeletal density of corals.
2. Physical extension or growth rate will be maintained but at the cost of skeletal density. 
3. Maintain their skeletal growth and density by investing more energy in calcification.

These responses do come at a cost as they promote side effects that affect the ability of coral reefs to perform some of their benefits. Please read this paper for more information. 

Future Projections

For ease of viewing I have decided to tabulate the future projections that I have found. Furthermore, it will give you the key elements of information to be able to understand the impacts of OA. Please note that the first three scenarios do not take into account physiological acclimation or evolutionary mechanisms that could alter the arrival of these scenarios. The bottom two scenarios are from a paper by Orr et al. (2005) that focuses on colder environments. 

Projection Limitations

A paper by Pandolfi et al. (2011) highlights that these future projections are complicated by physical acclimation and the role of evolution in terms of phenotypic plasticity. Furthermore, the paper states that there is insufficient knowledge surrounding the impact of OA on other environmental variables. Whilst projections are useful, Pandolfi et al. (2011) argue that a more well rounded approach be adopted that includes all issues. Please have a further read as this truly is interesting!

I hope this has given you an idea of OA and what  the future holds!

Stay tuned, 

Seb

Acidification and Acoustics

Here is something interesting for you to read...

Ocean Acidification Research Suggests Return To Dinosaur-Era Underwater Acoustics

Remarkable eh!?

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.

Climate Crimes

Hey guys,

It has come to my attention that I haven't written a post in a while (I have been a busy man) and so the plan is to catch up and post a few over the next week or so. 

The next series of posts will revolve around the impacts of climate change on coral. The following topics will be discussed:

• Coral biology
• Sea temperature and bleaching
• Ocean acidification
• Sea level rise
• Storms and rainfall
• Coral disease     

Here is a video explaining climate change. This is basic but worth a watch. Simple. Further, I recommend these TED talks as they are, on the whole, informative. You can view a few on this blog on the right hand side of the screen!

Stay tuned,

Seb