Algae, Corals and Surgeonfishes
“Coral reefs are widely thought to exhibit two alternate stable states with one being dominated by coral and the other by macroalgae” What does that even mean? Scientists are so good at scaring the general public with big words. They sometimes use this secret language to let everyone know that they think that they are smarter than everyone else! Which is a shame because, behind the jargon often hides a hefty amount of insight into the workings of nature. So, let’s see what we can learn from this statement taken from a paper by Peter Mumby in the Journal ‘Coral Reefs’, published in 2009.
As divers we all know what coral reefs are: large structures, created by the skeletons of thousands to millions of coral polyps. Each such polyp is quite similar to an anemone with a hard (Calcium-carbonate) skeleton. There is a part of the reef which is alive, and, below, a part which constitutes of the dead skeletons of past generations of polyps. A large reef structure like the Great Barrier Reef in Australia is completely built from such previous coral generations. But even a tropical marine habitat like the Evo house reef, which is mainly dominated by sand and seagrass, has a significant amount of corals (in isolated bommies). If you dive said house reef, you will find that most of the fish life happens near these coral blocks; the corals provide spatial complexity, and hiding places for the fish. The coral labyrinths are a perfect escape when a larger, predatory fish such as a groper or shark approaches. Without them, the numbers of smaller fish will inevitably decrease. This is obviously also true for algae-grazing fishes such as the surgeonfishes or parrotfishes.
What are algae? These are relatively primitive oceanic plants. Well, technically they are not plants, because they are missing the reproductive systems (flowers or their simpler predecessors) and the fluid-transporting vessels of true plants; but they generate sugar from CO2 and sunlight, so, they are close enough to plants. Simple filamentous algae are the slimy brownish-red cover of sand or dead corals often seen in some parts of coral reefs. In contrast, macroalgae have real leafs. These macroalgae are the weeds of the ocean. No organism is inherently ‘evil’, but what they do is to take over oceanic real estate on which the corals then can’t grow anymore. A patch of reef in which the corals die will first be colonized over by filamentous algae, and, if it isn’t grazed by algae eating fishes such as surgeonfish for a sufficiently long time (typically for a few months), by macroalgae. Then, newly colonizing coral polyps don’t stand a chance anymore of making this piece of already occupied ocean floor their home.
So, we have a situation where the surgeonfishes and parrotfishes depend on the corals (for hiding places), and the corals depend on the surgeon- and parrotfishes (for algae removal). This mutual dependence creates what’s called a bistability: the whole reef is either in one stable state, or in another. One state has a lot of corals: the algae-grazing fishes have ample hiding places, and will keep the algae in check. If nothing dramatic happens, the reef will stay in this state (the state is stable). Alternatively, if the reef is overgrown by macroalgae, the surgeon- and parrotfish will have a hard time, and will be present in low numbers. They will not be capable of eating enough algae to keep the substrate available for young corals, and the macroalgae will keep winning. This state is also stable, and will not change on its own. We now understand what’s meant by “Coral reefs are widely thought to exhibit two alternate stable states with one being dominated by coral and the other by macroalgae”. Only hefty manipulations, like a typhoon, or human overfishing can push the reef from one state into the other. Sea urchins are similarly important for algae clearing, though less dependent on coral cover.
The studies of this bistability have concentrated on coral reefs (in many cases in the Caribbean). How does the situation play out in a sandy region such as the Evo house reef? Sand is generally a poorer substrate for coral growth. I don’t know how the situation will develop in such a case, and I believe this is generally poorly understood. But we can make some educated guesses based on the studies of reefs. An abundance of filamentous algae, as was observed at the Evo house reef in late 2013, could be a possible sign of insufficient grazing. The surgeon-, parrotfishes and urchins normally taking care of these algae are possibly reduced in numbers due to overfishing. Alternatively, the ocean could be too rich in nutrients, a possible consequence of human populations. It’s important to point out that we can’t overstate the meaning of an observation like the increased occurrence of filamentous algae in the sandy regions just off the coast of Malapascua. But we can intelligently speculate about its origin, and a connection with the well-known relationship between grazing intensity and algal cover seems like a good candidate for an explanation.
I do want to point out that the Evo house reef is still, despite this somewhat unusual growth of filamentous algae, an absolutely incredibly biodiverse dive site with a macro photographer’s dream team of shrimps, nudibranchs and small fishes!
We, as humans, have become the main determinant of ecosystem health on this lovely planet. There are so many of us, and we have become incredibly skilled at taking what we want. With this comes a great responsibility, to which we haven’t risen yet. In the case of coral reefs, we can push them from one stable state (coral dominated) to the other one (macroalgae dominated) by fishing out the algae-eating fishes. This pushes the system into a state from which it can not easily return to the coral-rich condition. Every surgeonfish ending up on the barbecue is a small step towards a sad reef with not that many living corals. Who would have thought? Killing algae-eating fish can be as bad for a coral reef as is breaking off coral fingers.
Interesting? Then join me at the Evolution Photoganza, Sept. 17th -22nd in Malapascua to hear more about life in the ocean, and to learn how to photograph it.
Literature:
Mumby, P.J., 2006. The Impact Of Exploiting Grazers (Scaridae) On The Dynamics Of Caribbean Coral Reefs. Ecological Applications 16, 747–769.
Mumby, P.J., 2009. Phase shifts and the stability of macroalgal communities on Caribbean. Coral Reefs 28, 761–773.
Mumby, P.J., Steneck, R.S., 2008. Coral reef management and conservation in light of rapidly evolving ecological paradigms. Trends in Ecology & Evolution 23, 555–563.
