Coral bleachhing Flashcards
What are corals
- most primary reef-building corals are calcifying anthozoans
- Anthozoa is a class of 6000 species (within cnidaria). They only exist as polyps , never medusa
- True “stony” corals are in the order scleritina
What do corals create
Coral reefs - hyper diverse marine environments, they only cover 1% of the sea floor yet hold 1/4 of all marine species
Biology of scleritinian corals
Scleratinian reef corals form an aragonite corallum (colonial skeleton) composed of individual cuplike coralites around and under each polyp
corallum
the entire skeleton of a compound coral
How does coral growth occur
- Growth occurs on top of previous calcification.
- Growth in some acopora corals can be >100mm annually
How large can coral colonies get
- Large coral colonies can reach several M in height and tons in weight.
- The living tissue however is typically <1mm thick on surface
What controls the rate of growth/calcification
Growth is controlled by function of specific calciloblastic tissue (has a direct role in crystal formation)
- Growth is also controlled by photosynthetic contributions of symbiotic zooxanthellae
what is a zooxantehllae
In this case it is a coral containing an algal symbiont.
what is typical of (hermatypic) scleritinian corals
they are typical of shallow seas and almost all are zooxanthellae
What are hermatypic corals
Hermatypic corals are those corals in the order Scleractinia which build reefs by depositing hard calcareous material for their skeletons, forming the stony framework of the reef. Corals that do not contribute to coral reef development are referred to as ahermatypic (non-reef-building) species
What are the most common zooxanthellae relationship
- Most scleritinian corals have a symbiotic relationship with dinoflagellates of the genus symbiodinum
how many types of symbiodinum are there
> 100 genotypes of symbiodinum , some are widely distributed whilst others are specific to species or environments
where are zooxanthellae located
zooxanthellae are inra-cellular and located within host gastrodermal cells
how many zooxanthellae are typically within an individual
- 1-3 or more per cell
- typically 1500000 symbiont cells per cm^2
What are the three functions of zooxanthellae
- influence calcification process
- Provide nutritional material to their hosts
- take up host waste products
How does light affect calcification rates
- light, photosynthesis and calcification are intimately linked
- A cloudy day can reduce carbonate deposition by 50%
- Carbonate production is 3x higher at daytime compared to night
How does the zooxanthellae provide nutrition for the coral
- Carbon fixed by photosynthesis is translocated to the host in the form of amino and fatty acids, glucose and glycerol
What is the ratio of photosynthesis to respiration
In some conditions photosynthesis to respiration ratio (P:R) can be >1. I.E. all metabolic requiremnts met by photosynthesis
- symbiosis is autotrophic (in this case)
- This varies substantially with light, depth ,season ,habitat etc.
Are corals autotrophic
No , most corals also feed heterotrophically, this significantly increases both tissue growth and calcification rates
feed on planktonic prey
How do corals deal with their waste products
- Zooxanthellae take up phospherous and nitrogen from their hosts
- They transform these into organic compounds and transfer them back to host
- Ammonium and nitrate are potentially toxis to corals, zooxanthellae act as “metabollic kidneys”
what is the main benefit of the symbiotic zooxanthellae relationship
Allows reef-building corals to survive in nutrient poor shallow tropical waters
Reef corals are intolerant to the following types of environmental change
- reduced salinity
- prolonged low tides
- disease
- pollution
- temperature extremes (KEY)
- High light levels
What are coral stress responses
At threshold levels
- increased mucous secretion and polyp withdrawal (no longer active)
Beyond physiological thresholds
-zooxanthellae symbionts are expelled (bleaching)
What is coral bleaching
- A complex reaction based on physiology, biochemistry and traits from both partners
- Most obvious forms of bleaching result in host appearing transparent (this is through revealing the aragonite skeleton underneath)
Not all coral bleaching is worrying and can reflect seasonal variation , what is this called
cyclical changes are called physiological bleaching
How does severe bleaching affect corals
- Typically a loss of 60-90% of symbiont cells
- Usually accompanied by a loss of host tissue biomass
- bleaching is significant injury to the coral , not just loss of photosynthesis
How does temperature affect bleaching
- Most reefs are sensitive to small increases in temp
- Just 1-2 degree celcius increase above summer highs can induce bleaching
-El ninos generate wide swathes of such conditions , 30-32 degrees widespread in the tropics
What happens when the corals release the zooxanthellaes
-Without the algae (symbiont) , the coral loses its major source of food , turns white/transparent and becomes more susceptible to disease.
Examples of previous mass bleaching events
- Super el nino (1998) , caused coral mortality across the tropics. Unprecedented in global scope.
- Coral coverage was reduced >90% in many parts of the indian ocean
- similar patterns in pacific + caribbean
- 16% of the worlds shallow water reefs were lost
The role of climate change in coral bleaching
climate change interacts with natural processes and variation to increase the severity and extent of bleaching events
How does coral bleaching effect reef ecosystem services
- Many species entirely dependant on healthy corals for habitat and food
- Coral cover and topgraphic complexity especially important to fish
Example of bleaching effect on reef species composition
Example: Pratchett et al. (2008) Effects of coral bleachng, 1998 on species richness
- Most species declined/died out
- Species which suffered the most were coralivores and coral dwellers
- Herbivores tend to do well as bleached coral gets colonised by macroalgae
When did NOAA declare the 3rd ever global bleaching event
2015 - NOAA declared 3rd ever global bleaching event
- > 1/2 of japans coral reef died
- severe effects predicted globally
- As the year progressed predictions worsened significantly
- covered most of the tropics and pretty much everywhere else
The 2016 bleaching event and the great barrier reef
- 1000km of the reef was affected
- Northern areas hit hardest , 95% of reefs between cairns and papau new guinea showed bleaching
- southern and central regions protected by cooler weather form cyclone winston
What was special about the bleaching of the great barrier reef
first bleaching event that played out on social media
According to Terry hughes what was the extent of damage to the reef
Northern = 26% mortality North = 67% mortality Central = 6% mortality South = 1% mortality
How does the future look for bleaching events
- Projected increase in sea suraface temps (1.2/2 -5/10 degree increase by 2100) will increase bleaching events significantly
- Surface aragonite is going to decrease , this will reduce calcium deposition preventing coral regeneration
- prediction suggest future seas will be unable to support corals
How does aragonite concentration affect corals
There is a critical concentration of aragonite below which calcifying organisms such as corals cannot function
What is the problem with saying the great barrier reef “died”
Russel Bainard , chief scientist at NOAA
there is increasing concern that overstatement of the environmental situations that our planet faces causes people to lose hope and thus reduces efforts to fix the problem.
-only 22% of the reef died , a large proportion of the public think 100% perished
Summary
- Coral reefs are dependant on symbiotic relationship with zooxanthellae
- many stressors can lead to it breaking down
- temperature is primary driver of bleaching
Coral reefs prognosis
- Tropical reef systems are transitioning to a new era in which the interval between bleaching events is too small to allow full coral regeneration
- median return times between events is no 6 years (decreased since 1980s)
- Annual bleaching may become the norm
