Sydney
Startling evidence has emerged from new research to show that when fishes get a mouthful of sand, the days of coral reefs are numbered. 

"We've known for a while that having a lot of sediment in the water is bad for corals and can smother them," said David Bellwood, professor at the ARC Centre of Excellence for Coral Reef Studies (CoECRS) and James Cook University. 

The killer blow for a degraded coral reef is a thick mat of sand and weeds that shrouds the rocky surfaces on which the corals would normally grow, preventing them from re-establishing. This gritty algal 'turf' has shown itself to be remarkably hardy and, once in place, makes it almost impossible for the corals to return. 

If sea levels rise, then the smothered reef 'drowns' and never recovers, Bellwood said. "We know this from geological history, at the time of previous sea level rises. The reason we are doing the work is to see whether or not coral reefs will be able to keep up with rising sea levels under climate change." 

But Bellwood and colleague Chris Fulton from the Australian National University have also uncovered a remarkable link in the chain which explains why the algal turf can win in its 'turf war' with the corals, according to an ARC press release. 

When the water is thick with sediment and it settles on the seaweeds, herbivorous reef fish turn up their noses at the gritty food, much as humans disdain a sandwich that has been dropped on a sandy beach. 

"Remarkably we found that when there is little sediment around and plenty of fish, the fish 'mowed' the weeds very fast, eating two thirds of their length in about four hours. This action by fish in keeping the algal turf down gives the corals a chance to re-establish" said Fulton. 

"But if there is a lot of sediment in the water, the fish go off their feed, the weeds grow, more sand settles - and the murky shroud that smothers the reef becomes more stable, often permanent. Then, when sea levels rise, the reef drowns."