Ocean Acidification & Thermal Stress

Ocean Acidification

The effects of climate change on land have been extensively studied and chronicled by media. However, an even more worrisome problem is developing in the Earth’s oceans.  
As the amount of atmospheric CO2 grows, the proportion being absorbed by the oceans increases and becomes carbonic acid. The more carbon in the ocean, the lower the pH, meaning more acidic oceans.  

Figure 12 shows a graph compiled from using boron isotopes from seabed pH for over 25 million years. Ocean pH has fluctuated but none to the degree and speed in recent years. This affects marine organisms’ physiological processes like calcification, respiration, photosynthesis, metabolic rate, and more.  

Fig.12: Seabed pH (Turley et al 2005)

How will organisms respond?
Due to the complex life histories of these organisms, the full effects of ocean acidification on various life stages remains to be seen. 
However, most agree that ocean acidification acts as an environmental stressor, pressuring organisms to migrate, tolerate, adapt, or become extinct. This will have far reaching consequences to the food web and interspecific interactions.

For instance, figure 13 shows what possible shifts in energy budgets we can see in organisms. With stress, the energy cost for living increases. It is more expensive to produce shells, move, breath, etc. 

Consequently, there is less energy available for growth and reproduction. This imbalance negatively affects the individual by reducing their survival, size, and reproductive output. Furthermore, populations will be less productive, resilient, abundant, and more likely to go extinct.

Fig.13: Stress brings a negative shift in energy budget (Carrington 2012)

Climate Change’s Effect on Coral Reefs: Thermal Stress

Coral reefs have shown a dramatic decline in prosperity due to climate change. This can be seen through bleaching events which are triggered by symbiotic zooxanthellae expulsion from coral due to environmental stress. 
These zooxanthellae which provided the color and nutrient exchange, leave the corals increasingly vulnerable to the environment. Bleaching has been coorelated with rising sea temperatures. 


Recent past years like 2005 and 1998 have shown large increases of bleaching events all across the globe. If left unchecked, future temperatures will rise to levels intolerable for corals. 

Rising CO2 concentrations and sea temperatures:
Figure 14 shows the amount of fossil fuel CO2 residing in the worlds oceans. The redder areas mean that there is more anthropogenic CO2, while the more bluer areas means that there is less CO2. As you can see, the more concentrated areas (red) are closer to industrial coastlines.The amount of CO2 in the oceans varies based on latitude but it still portrays a worrisome picture of increasingly CO2  concentrated oceans.

Fig.14: Global CO2 absorption nmolkg-1 (Sabine et al, 2004)

Similarly, figure 15 shows how temperatures of the oceans have been rising as well. Different locations across the globe situated in the tropics show how temperatures are radically increasing over decades. 
They are increasing at such a pace, that we have already crosses the safe temperature zone for coral growth(green line) and before 2050, sea temperatures will become too high (red line) for sufficient coral growth.

Fig. 15: Dangerous trend in sea temperatures (Hoegh-Goldberg, 1999)

Consequences = Increased bleaching:
There are numerous negative consequences to increased CO2 and sea temperatures and one of them is bleaching. As mentioned before, bleaching is the expulsion of symbiotic zooxanthellae from the coral colony, which results in a loss of color and nutrition for coral colony. 


Figure 17, shows that bleaching is not an unusual and isolated event. Bleaching events have increased over the years and happen all over the world with differing severity.
Figures 18 and 19 show a time lapse of the effects of bleaching on coral colonies. We see that for some, it took less than a few years to transform a relatively healthy environment to a decaying desolate one. 

Fig.17: Bleaching is global (Nicholas, 1998)

Fig. 18: Bleaching timelapse (Yale, 2010)

Fig.19: Before and after (NOAA, 2012)

Looking at all this data,  one may not be surprised that organisms are struggling. Now, let's look at what's happening to hermatypic corals and how much faster they are succumbing to disease.

Text information compiled from (NOAA, 2012)