Seaweed released chemicals stunting coral growth

SYDNEY: Seaweeds are releasing chemicals that are stunting the growth of corals, according to a study.

Researchers have proved that some seaweeds or algae produce toxic chemical signals that deter coral larvae from settling on reefs devastated by bleaching, storms or other impacts.

The good news is that the clever little coral larvae may also use the algal chemicals to find a good home.

"Seaweeds produce a wide range of chemicals, some of which encourage coral larvae to settle and some of which repel them," said Laurence McCook of ARC Centre of Excellence for Coral Reef Studies (CoECRS) and the Great Barrier Reef Marine Park Authority.

The research team includes Chico Birrell and Lindsay Harrington, of James Cook University, and Bette Willis (James Cook University) and Guillermo Diaz-Pulido (University of Queensland) of the CoECRS.

Chemicals released by the algae into the water can have a significant impact on the success of coral recovery after damage. "We looked at three kinds of seaweeds and found that a green seaweed called Turtle Weed had a powerful deterrent effect on coral larvae, which refused to settle and appeared stressed.

Larvae had difficulty settling with a second seaweed and a third produced chemicals that actually encouraged coral settlement," said Willis.

These chemical mechanisms may have important implications for the long-term survival of coral reefs globally and their ability to regenerate after damage from coral bleaching, which is expected to become more frequent and devastating under climate warming, he said.

McCook said following damage to a reef algae nearly always beat the corals in the race to resettle the devastated area.

A lot then depends on which algae dominate the new system - and whether there are enough fish, turtles and other herbivores around to 'mow' the weeds and give the corals a chance to re-establish.

"On the Great Barrier Reef we have been relatively lucky, but elsewhere we have seen a number of instances where seaweeds simply took over the reef, completely preventing the corals from coming back," said McCook.

The greatest threat seems to be when we get thick mats of algae combined with sediment runoff, which smother the reef and stop corals gaining a foothold - a serious problem for our coastal reefs, he added.

However, he cautions, the picture is not simple. Some weeds repel corals but others, like calcareous red algae, play a vital role in reef building and help the corals to re-establish.

Two papers on the effects of seaweeds on corals have appeared in the journals Marine Ecology Progress Series and Oceanography and Marine Biology.