What is happening to this coral?

[stonybonytony]

I was diving on Koh Phangan in the Gulf of Thailand, it's been over a year since I last dived this site. It's just off a beach where there a number of basic beach hut resorts. I think the coral has degraded significantly, and there has also been a decrease in the associated fish life, though I don't have any 'before' photos to check against.

It looks to me like large areas of the coral have become covered in grey/green plant material. Is this killing the coral, is the coral dying and the plants just colonising the dead remains, or am I misinterpreting what I am seeing?

I have posted pictures on
http://www.flickr.com/photos/stonybonytony/3291840576/
http://www.flickr.com/photos/stonybonytony/3291024493/
http://www.flickr.com/photos/stonybonytony/3291024993/
http://www.flickr.com/photos/stonybonytony/3291839868/

I would be very interested in what is happening here, and of course is there anything that can be done about it?

[MegapteraNova]

Wow, I have no clue why that's happening.

Maybe an increase in water temperatures or Carbon Dioxide levels?

[Izzy]

My guess here is simply a guess, however I know corals do poorly in high-nutrient areas, because they need ultra-clear waters to grow. I'm going to guess that in this case, there was a high nutrient flux into the area where that coral is, and that caused algae to bloom, and smother the corals. That's just a guess, though.

[Parrotfish]

Yeah I would have to agree about it being a high nutrient area. I read that it happens near golf courses, parks and almost any other place that uses fertilizer

[Izzy]

I'm taking a coral reef course this term, and actually something came up in a lecture a couple of days ago that might be adding to the problem. I definitely think that the reef looks like it has an overgrowth of algae, probably from too much nutrients, and not enough grazing fish to keep the algae population under control. The other possibility is that in addition to this, there might be an increased influx of sediments from nearby land, which can settle down on the reef and kill it, because the corals can not deal with that much sediment coming down onto them. This can have a very similar effect to that of over-algal growth, and one can often lead to the other, so that might be another thing happening to the reef.

[Ghandi]

Depletion of grazing species & or the damaging of the reef itself.

Which is the cause and which is the symptom?

Depletion of grazing fish and invertebrates would make the reef vulnerable to algal matts whilst damage to the reef (through sedimentation or hypernutrification) would cause a decrease in the fish and invertebrate populace and a possible shift in community structure.

Hypernutrification might allow algal matts to grow faster than grazers can control while low numbers of grazing fish will allow algae to overgrow the coral, very difficult to tell at this stage- depends a lot on local activities nd on scene evidence!

Did you observe sedimentation? (fine sediments/sands like a dust coating on the coral or cloudy water) Are there any terrestrial developments nearby or terrestrial run-off? is there deforestation in the local area that may increase the sediment loading of nearby rivers? Is there evidence of agriculture nearby or near any large rivers feeding into local estuaries? What are he local sewage disposal facilities? Is anyone dredging for aggregate in the area or dumping at sea? Are there many sport fishermen? are there many commercial fishermen? Is there a strong trade in tropical marine fish?

all these and many more could contribute to what you hav seen.

[octopiis]

Yeah, I'm going to agree with algaefication. That's what the coral in my fish tank looked like when I had an algae problem. But I don't think partial water changes will fix this. Have the algae eating organisms been driven off?