REEF DYNAMICS OR "MOWING THE LAWN"

What makes for a healthy coral reefs? Delve into this topic and you will quickly find you have opened a veritable "pandora's box". Nevertheless this is exactly the question, which has captured the imagination of University of Maine students, Susie Arnold and Jeannie Brown and figuring out one small part of the puzzle is going to take them three months of field work, sharp eyes, a deft hand with a microscope and a good deal of brain power.

When we talk about coral reef "health" its important to realize that there is no one answer and that "where" and "when" are enormously important parameters. Pacific coral reefs ordinarily have more than ten times the number of corals, fish and other reef creatures compared to the Caribbean. All that diversity makes them robust. The "when" part is just as critical. Just sixty years ago sharks routinely patrolled our reefs. Had you been bold enough to venture into the water with your primitive rebreather set and home made gear you would have seen sharks on practically every dive. The great Austrian underwater pioneer Hans Hass did when he stayed on Klein Bonaire in 1938. Who knows what reef sharks and black tips, bull sharks and tiger sharks used to prowl our waters. You would also have been treated to the spectacle of jewfish flowing over the edge of the drop off on Klein Bonaire like a waterfall. Just imagine.

Clearly some things have changed. Reef scientists have been intensively collecting data about coral reefs for just over thirty years and have amassed such a wealth of data that it's all too easy to believe that we know what is going on down there and that we understand what we see. Yet we lack historical perspective. Scientists are dubbing this "shifting baselines". We know that we have lost the entire top tropic level - the sharks - from our reefs. Many islands have all but lost the second tropic layer too - the predatory grouper, snapper and grunts. But what the repercussions have been we can only guess at.

Answering part of the puzzle "what makes for a healthy coral reef today" is what Susie and Jeannie will be coming to Bonaire from June through August of this year to unravel. A healthy reef in the Caribbean today is one where there is high coral cover, low abundance of macro algae (leafy marine plants) and high numbers of fish, particularly herbivores. Simplistically put the parrotfish eat the macro algae, which leaves plenty of space for coral larvae to colonize and hey presto: healthy reefs with lots of corals.

Of course parrotfish were not always the dominant herbivores on our reefs either. Back in the 1980s it was the black spiny urchin, Diadema antillarum, which did most of the algal munching. Although at the time divers were not entirely unhappy about it, the luckless spiny urchins were all but wiped out in the Caribbean due to an outbreak of disease. One day they were fine and the next they were crawling out onto the sand, shedding their devilish black spines and dying. Luckily for us the parrotfish moved into their niche. Unlucky for places like Jamaica, where the parrotfish were already overfished. Many of their reefs were reduced to unattractive algal wastelands. As Dr Robert Steneck so aptly put it "parrotfish mow the lawn".

What Susie and Jeannie are coming to study is a whole other layer of complexity. One thing they have noticed is that parrotfish don't do much mowing when there are damselfish about. Especially those rather aggressive little three spot damselfish (Stegastes planifrons). Also, using special settling plates, which look for all the world like ordinary terra cotta floor tiles, they have also been able to show that Bonaire has truly astounding levels of coral larval settlement. The plates come out smothered with tiny coral larvae, which have settled on the underside of the tile along the outer edge. What they want to discover now is the role that the damselfish play in coral larval settlement and just how many of those baby corals actually survive. Watch out damselfish. And good luck to Susie and Jeannie as they try to help us understand another whole underworld of coral reef dynamics.