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.
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