There are a number of threats from climate change that are damaging coral reefs. And it is truly complex. The reef and the environment are intricately linked…the chemistry, physics, and biology are all working together as a ‘system’ and should be thought of as a whole. Climate change brings with it several distinct threats:
1. Precipitation, drought, and land runoff changes
2. Storm frequency and intensity
3. Changing ocean circulation
4. Sea Level Rise (SLR)
5. Ocean Temperature
6. Ocean Chemistry
The last three are of particular consequence.
Sea level rise, including king tides, will challenge the growth and energy limits of the corals. It will encourage more sedimentation (and pollution) which is detrimental to the amount of light that reach the zooxanthellae for photosynthesis, which provides the coral 90% of its nutrition. Efforts of the coral to remove the sediments will depend on their energy budget and water motion. Will they be able to adapt to the changes in light attenuation as the water rises rapidly enough? We really hope so.
Ocean temperatures are a big deal. A coral has a threshhold of 1–2 degrees celcius above the mean summer maximum before it will become stressed from heat and expell its zooxanthellae. There are other factors at play that can exacerbate this, including light (clear skies vs cloudy), water motion (calm days vs choppy), and saturation state (related to pH), and pollutants. As I mentioned previoiusly, corals depend on the zoox for 90% of their nutrition, so if the environmental conditions do not improve within a few weeks, the corals will slowly starve and algal competitors for space out on the reef will win. Some corals have fragile skeletons, like plating morphologies, so when the corals die the skeleton crumbles under the physical forces of the ocean and you have now lost the complexity that is the home for other critters, as well as the space for new corals to recruit to. (Recruitment is when larval corals or other critters settle out of the water column on to the reef). However, if the environmental conditions improve, there is a very good chance that the coral can recover, either partially or wholly (this is RESILIENCE). In addition, temperature affects the very nature of the natural bacterial flora of the corals, inciting a Jeckyl and Hyde functional role. A good bacteria turns bad when the heat turns up. Corals that have experienced bleaching have reduced reproductive capabilities and an increased risk of disease, as was so catastrophically witnessed in Florida and the Caribbean. There are a number of different factors that influence coral diseases, including climate change and pollution, and we are still in the learning process. The important thing to know is that diseases are on the rise globally, in both distribution and prevalence (spatial range and frequency). Awareness is the first line of defense.
Ocean Acidification is known as the Osteoporosis of the ocean. The process is actually just a simple chemistry 101 reaction trying to reach equilibrium. The CO2 in the atmosphere is entering the ocean at the air-sea interface. There is a greater concentration of CO2 in the atmosphere, so the water is absorbing it. To date about 48% of anthropogenic CO2 has been taken up by the surface waters of the ocean. How do we know this? Oceanographers sampled long transects of surface waters in the Pacific, Atlantic, and Indian oceans, and using stable isotope analyses were able to confirm anthropogenic sources. CO2 and H2O create carbonic acid, like in soda pop. Due to the ocean’s natural buffering system, carbonate ions are bonded the excess H+ ions to form bicarbonate ion, which keep it from getting too acidic. (This buffering system slows, but does not stop the acidification.) In doing so, the carbonate ions are less available for organisms to use for calcium carbonate (CaCO2) skeletal production and growth. This includes the smallest plankton that is the base of the food web, to the crustose coralline algae that cements the reef together, to the reef building corals themselves. Growth and reproduction are reduced, while fragility increases and complexity (3-dimensionality) decreases. As acidity increases, the ability of the ocean to dissolve calcium carbonate increases, and eventually (about 100 years or so), reefs will be vulnerable to dissolution. (A reef needs a saturation state of 3.2 to grow). A reef that is not growing is eroding. This process of ocean acidification will continue over geologic time frames. There will be winners and losers. But if we can reduce atmospheric CO2, we can slow or even hypothetically reverse this process. Remember, while the planet has seen similar conditions long before man got here, it is the rates of change that are unprecedented.
Someone earlier quoted a website that noted that fish and seastars and worms and snails are eating parts of the reef as a threat…while that is inherently true, it is a bit misleading in the context of the coral reef community, because there are a lot of critters that eat coral, and that is perfectly natural when everything is in its natural balance. When that balance is upset and populations are in excess, then it can be a problem.
While hurricanes pose a natural acute threat, corals are better able to recover from these events than chronic threats such as pollution and overfishing, and hurricanes bring cooler waters up from deeper depths to the reef and can stiffle a bleaching event. I’d like to point out at this point that reefs face multiple threats, and we need to reduce those that we have the ability to change, now.
That’s a simplified version. It is truly complex.
Composing members:
0
Composing members:
0
