The speed of coral death increases alarmingly if colonies are flooded with polluted rather than "clean" sediments, an international team of researchers writing in a prestigious scientific journal has found.
The work solves one of the big mysteries of coral research: why some corals die suddenly after sedimentation events, while others survive.
The findings, published in the Proceedings of the National Academy of Sciences, show that preventing sediments contaminated with pollutants such as phosphorous and nitrogen from entering coral reef areas is essential for reef health.
In coastal areas with excessive soil erosion, where rivers flush nutrients, organic matter and sediments to the sea, corals can die very quickly. Dr. Katharina Fabricius, a principal research scientist at the Australian Institute of Marine Science (AIMS) in Townsville explains:"Coastal corals are pretty good at coping with naturally-occuring sediment and the microorganisms that it contains – but if that sediment is enriched with even a small amount of organic matter it can cause sudden coral death.
"This study has documented for the first time the mechanisms that cause sediments enriched with nutrients and organic matter to damage corals, while nutrient-poor sediments have little effect on reef health. We were amazed that a mere 1% additional organic matter in the sediments is enough to trigger this process.
Dr Miriam Weber, a post-doctoral scientist at the Max Planck Institute for Marine Microbiology in Bremen and the principal investigator of the study, described the processes that lead to death of coral colonies exposed to organic- and nutrient-rich sediment: "When a thin layer of sediment covers the corals, the corals will stop photosynthesis, as the light is blocked.
"If this sediment layer is enriched with organic matter, then microbial activity is high, depleting oxygen and making the sediments more acidic. These conditions irreversibly damage small areas of coral tissue.
"The dying coral tissue is then digested by other microbes producing hydrogen sulfide, a compound that is highly toxic for the remaining coral. A chain reaction begins and the remainder of the sediment-covered coral surface is killed in less than 24 hours.
Dr Weber went on to say that "First we thought that the toxic hydrogen sulfide is the only killer, but after mathematical modeling and intensive studies in the lab at AIMS we could demonstrate that the organic enrichment is the real cause, as it leads to lack of oxygen and acidification, kicking the corals out of their natural balance. Hydrogen sulfide just speeds up the spreading of the damage. The extreme effect of the combination of oxygen depletion and acidification are important, keeping in mind the increasing acidification of the oceans."
Dr Fabricius added "Better land management practices are needed to minimise the loss of top soil and nutrients from the land, where they are beneficial, so they are not being washed into the coastal sea where they can cause damage to inshore coral reefs."