With Cruise ships ramming entire reefs these days (recently happened with a UK Cruiseship destroying one of the most pristine Coral Reefs in Indonesia) the question came up what the biggest threats to Coral Reefs are. And of course there is unfortunately more than just one: Ocean acidification, global warming, cruise ships, anchors, Dynamite fishing, bottom trawling, Nutrient input from land, loose sediment being washed in from shore and the list goes on and on.
With so many threats lined up it is a matter of time before reefs get on a declining path and as a matter of fact they have been on it for about 50 years. Ever since the industriel revolution kicked in, Reefs got knocked out. Due to malpractices at sea but also changing climates. One of the biggest issues in the early stages were bottom trawlers, simply wiping out entire reefs in one single pass.
Nowadays we are facing more "sophisticated" problems that are also manmade but more of an indirect impact.
With rising temperatures the Oceans can take up more Carbon Dioxyde and get more acidic, which in return stops coral from binding Calcium to build their skeletons. The same is true for many other marine creatures including shell building plankton. Also with the rising temperatures of the water the Algae, that lifes in a symbiosis with the coral is leaving the coral - anything more than 30 Degrees centegrate becomes too much for them and the coral looses their biggest source of sugar. With the algae gone it only takes few more days to weeks before the coral itself starts to die off.
And of course there is more problems we cause for ourselves: With more and more nutrient input from humans - via big rivers, fertilizers from farmlands or even sewage, that was not properly taken care of in island resorts or (cruise)ships. The extra Nutrients allow algae to grow, overrunning coral reefs around the world. Since coral has an incredibly slow growth rate they cannot compeed with algae of any kind and will simply be burried by fresh algae, loosing their source of light and therefore their livelihoods.
All those and much more in one of our "Simple Marine Biology" lessons right here on youtube.