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Human Run-off Biggest Threat to Reefs

Study reveals that nurtients from pollution such as sewage and fertilizers increases the damage caused to coral reefs by ocean acidification by as much as 10 fold

Scientists at Woods Hole Oceanographic Institution (WHOI) were looking the multiple threats to coral reef ecosystems and were stunned to discover that the run-off from septic tanks, sewers, roads, farm fertilizers, and other sources of nutrient pollution increases the damage done to reefs by such a large extent.

Healthy reefs are a balance between new growth and the steady bioerosion caused by animals burrowing into the reef skeleton. Ocean acidification caused by global warming has tipped the balance against the reefs - the process removes carbonate ions which inhibits coral from building reef structures and benefits the bioeroders such as mollusks, worms, and sponges.

However, the scientists looking at reefs right across the Pacific from Pamama to Palau, were shocked to discover that the erosion of the reefs is increased by as much as 10 fold when the coral is also exposed to high levels of nutrients from run-off.

 

150107123138-largeA floating makeshift lab in the South China Sea. Photo by Kathryn Pietro, Woods Hole Oceanographic Institution

 

'A healthy coral reef ecosystem exists in a constant and often overlooked tug-of-war. As corals build their skeletons up toward the sea surface, other organisms – mollusks, worms, and sponges – bore into and erode the skeletons to create shelters,' said Thomas DeCarlo, a graduate student in the WHOI-MIT who is lead author of the report into the study published in this month's Geology.

The scientists used underwater drills to collect cores of coral skeletons. They put the cores through a CAT scanner at WHOI to get 3-D images of tunnels and borings made by bioeroders with a resolution of about the width of a human hair. That allowed them to calculate precisely how much skeleton the bioeroders had removed.

The researchers found that relatively acidic reefs were more heavily bio-eroded than their higher-pH counterparts. But their most striking finding was that in waters with a combination of high nutrient levels and lower-pH, bio-erosion was ten times higher than in lower-pH waters without high nutrient levels.

'The ocean will certainly absorb more CO2 over the next century, and ocean acidification is a global phenomenon that reefs cannot escape,' DeCarlo said. 'But the encouraging news in our findings is that people can take action to protect their local reefs. If people can limit runoff from septic tanks, sewers, roads, farm fertilizers, and others sources of nutrient pollution to the coastal ocean, the bioeroders will not have such an upper hand, and the balance will tip much more slowly toward erosion and dissolution of coral reefs.'

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