Wednesday, 27 November 2013

The Lost City of Antigua

Skeletons of elkhorn coral that once thrived in Antigua is now mostly dead

Coral reefs are cities within the sea.  Corals are the skyscrapers in which myriad fish and other critters live, swim, eat and get eaten.  Reefs have gardens tended by damselfish, they have nurseries and junkyards.  Some animals mow lawns (of seaweed) while others deal with refuse.  However, if the skyscrapers (the major reef corals) die, the city loses some of its capacity to support its vibrant community.  The coral reefs we studied in Antigua now have only a fraction of the corals and associated habitats they once had and now possess only a fraction of the diversity that once thrived in this ecosystem.

Elkhorn coral seen in Bonaire in 1985

Bonaire's coral reefs today.  High coral cover but very little elkhorn or staghorn corals (the "acroporids").  

Diverse Acropora reef or from Bonaire

 In a scientific paper published in 1976 Antigua was reported to have one of the best developed coral reef systems in the Lesser Antilles.  The dominant reef builder was the elkhorn coral, Acropora palmata.    I remember well swimming among miles and miles of live elkhorn coral in Antigua in 1974.  Now, most of what I see are their skeletons.  Why?

A large elkhorn coral in Bonaire.  Some elkhorn coral survives in Antigua as well.

20/20 Hindsight of Paradise Lost

The simple answer is, the reefs were highly dependent on two species of Acropora, the elkhorn and staghorn corals but they died of white-band disease.  This disease was first observed in the 1970s but it gradually spread colony by colony, killing most of those two corals throughout the Caribbean.   Importantly, they were the fastest growing and most abundant reef building corals in the tropical western Atlantic. The white-band disease killed over 90% of those two corals during the 1980s. It was slow enough that most of us in the field at that time didn’t realize the disease was to blame for the changes we were seeing. Today, the US Government officially lists them as Endangered Species.

An elkhorn coral reef monoculture in Antigua photographed in 1974 during our research there

The Caribbean Sea is small and contains relatively few species.  There are only a bit over 60 species of reef corals in the Caribbean compared to over 700 species in the vast tropical Indo-Pacific Ocean.  However only two species, the elkhorn coral and to a lesser degree staghorn corals were the primary reef builders throughout the western tropical Atlantic for the past several thousand years.   The obvious problem of being a one elephant parade is that if your elephant dies, your parade stops …. so to speak.

The Double Whammy and the Resilience of Coral Reefs

I knew about the mass mortality of elkhorn and staghorn corals throughout the Caribbean before going into this project.  My interest over the past decade has been how reefs recover and what drives their resilience.  This topic involves another important player in this ecosystem, the long spined sea urchin known as Diadema and its role as an herbivore (an animal that eats plants).  It was extremely abundant in the 1970s – large areas of reef averaged 15 per square meter.  They created a carpet of long, poison-filled, spines.  They were a pain in the butt both literally and figuratively.  During the decade of decline of elkhorn corals, Diadema suffered a rapid mass mortality.  Between 1983 and 1984 over 90% of all Diadema died and the species suddenly became rare or absent on most coral reefs.  Since this was the dominant scraping herbivore grazing on the seaweed, the ecosystem lost the functional role they played  in keeping  seaweed growth in check.  The loss of both the elkhorn corals and the Diadema created a double whammy allowing seaweed to gain dominance .    While most coral reefs I’ve seen have shifted towards seaweed (plant) dominance, some did not.   Those that did not, such as Bonaire in the southern Caribbean and Roatan Bank in Honduras, maintained relatively little seaweed and high coral cover, but from species other than the elkhorn and staghorn corals. 

A reef in Bonaire - note the general absence of seaweed  (photo taken 2010)
A reef in Antigua note seaweed and a general absence of live coral (photo taken this week)
I'm doing transects in Antigua to record all the reef dwellers (this reef was dominated by seaweed plants)
I’ve seen seaweed dominating all the coral reefs I’ve studied so far in the Antilles (St. Maarten, Anguilla and Antigua).  But is this really a profound change in coral reef ecosystems?  In 1972, Dr. Sylvia Earle who received her PhD on the study of seaweed (algae) wrote:

“Perhaps the most striking aspect of plant life on a coral reef is the general lack of it.  It seems anomalous to even the casual observer that tropical reefs, notable for their dazzling profusion of animal life, are almost devoid of conspicuous plants.”

So, there is more seaweed on coral reefs than there had been in the past… so what?  Recent research has found that seaweed can poison corals, it smothers them, causes them to feed less, increases their susceptibility to disease and can reduce the ability of baby corals to get started on reefs.   Coral reefs that have suffered loss of reef building corals, need baby corals to recover.  

My surveys in Antigua found plants (that’s seaweed) were the most abundant group covering about 25% of the surface.  Only an average of 15% of the reef surface was covered by live coral.   The “good”, coral-facilitating, calcified algae (known as coralline algae) comprised 15%.  Antigua’s coral reefs are far different from the summary assessment made by Sylvia Earle more than 40 years ago.

A big part of my concern is how few juvenile corals I’m finding.  The ones I see are hearty small corals (not usually considered reef building corals).  Nevertheless, they too are growing in a field of seaweed.

A baby coral (Porites) trying to make it with seaweed all around (2 cm scale is about 1 inch)

It is not all hopeless  None of the corals have gone extinct.  We see signs that reefs could recover - especially if seaweed can be controlled.  While in Antigua we came across a shallow thicket of staghorn coral (Acropora cervicornis).  If it stays healthy, who knows - perhaps paradise can be regained.  But I think reef managers may need to redouble their efforts if we are to see coral reef ecosystems function as they had in the recent past.

A thicket of staghorn coral (Acropora cervicornis) in Antigua (photo 20 Nov. 2013)

My journey continues to Barbuda next.  I’m curious to see if the story is different there.

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