As if anthropogenic pollution and overfishing isn’t damaging enough for coral reefs worldwide, now certain seaweeds seem determined to see the end of reefs as well. These macroalgae produce chemicals that inhibit the growth of reef-building coral or even kill them.

Normally the growth of seaweed on reefs is kept in check by plant-eating fish, but as overfishing has resulted in a strong decline in these fish, seaweed is gaining the upper hand in the competition between coral and seaweed, at least in Fiji.

Last year the research team from the School of Biology at Georgia Tech already assessed the damaging effect of seaweed on coral when their surfaces touched, but were uncertain about what chemicals might cause this.

Terpenes are responsible

Now a new study in the online edition of the journal Proceedings of the National Academy of Sciences points to four different chemicals as the culprits, all part of the group of terpenes. Although not all species of coral are as susceptible to the substances, they can outright kill certain corals while inhibiting newly-arriving coral larvae to settle in other species.

But while sometimes degraded corals might recover in a relatively short time span, the new findings show that once the terpene-producing algae [Chlorodesmis fastigiata and Galaxaura filamentosa] become established, recovery of reefs is suppressed.

Conservation of just a few fish species might do it

That overfishing has a negative effect on reefs and that fish mass might even be used as an early warning system to assess the health of reefs has been shown before. But in this case it might even come down to the decline of just one or a few species of fish that are responsible for keeping the balance between seaweed and coral.

A next step for the researchers is therefore identifying the species of fish that consume the two species of seaweed. If these allies of the coral reefs in their battle against seaweeds can be identified, Fijian (and other) reef managers can apply focused conservation methods to help at least these fish survive.

Excluding a few species of fish from the Fijians diet seems somewhat easier and faster to implement than reducing the total consumed amount of fish and for the recovery of the reefs, it might just tip the balance.

From there on all we have to do is stop ocean acidification, global warming, pollution and overfishing in general and we may still have reefs by the end of the century.

© Jorn van Dooren | www.bitsofscience.org