Wednesday, 23 April 2014

Life, Death and Rebirth of a Coral Reef: Witnessing ecosystem change over the past 42 years




1972 St. Croix -- I'm getting ready to dive into a career in marine science (but with a clipboard and a spear?)
























HEADING NORTH TO ST. CROIX

At first light on March 18th we departed the Grenadines for St. Croix.  It’s a 370 nautical mile run that took most of three days.  The first 24 hours we had 20-knot winds on our beam and we sailed over 7 knots the entire time…. Terrific!

Approaching St. Croix from the south.
St. Croix and its coral reefs.  Our focus was on the reefs around Buck Island and on St. Croix's northeast shore.
Logistics dictated that St. Croix (U.S. Virgin Islands) would be the last research site of the Antilles Expedition.  Ironically, it happened to also be the place where I was first introduced to coral reefs as an undergraduate student in 1972.

Students at West Indies Laboratory 1972 - I'm reclining shirtlessly

THE WAY THINGS WERE….
A BRIEF HISTORY OF ST. CROIX’S REEFS OVER THE PAST 10,000 YEARS

The coral reefs surrounding the east half of St. Croix including Buck Island are among the best developed of the eastern Caribbean.  The Tague Bay Reef along the northeast shore reef runs nearly 6 kilometers without a break.  It is also one of the best-studied coral reefs in the Caribbean.  My first job after I graduated from college in 1973 was to assist Dr. Walter Adey of the Smithsonian Institution in drilling through St. Croix’s coral reefs and algal ridges to determine how thick they are and how long they took to develop that thickness.  What we learned was that these reefs are 30’ (10 m) thick and they developed mostly over the past 7,000 to 10,000 years.  Reefs were mostly built by the remarkably large branching elkhorn coral (Acropora palmata).  So, this is a wonderful site to study because we know it was a great place for coral reefs to grow for millennia (the same cannot be said for some of the rocky reefs we studied in places like St. Maarten and St. Lucia).
Studying the algal ridges at Isaacs Bay 1973
For younger reef scientists, it is hard to imagine the structure of coral reefs of the 1970s.  The shallow reefs were an expansive tangle of complexly branched elkhorn coral that went on for miles and miles.  The reef fish used the branching habitat for the food and myriad “hidey holes” it provided.

Buck Island elkhorn coral reef in 1973. In the center of the photo lurks a large barracuda
On a few occasions I would get my Nikonos I camera with only enough film to take 36 black and white photos of the coral reef.  I would often photograph large predators such as barracuda, groupers and large snappers. 
Tiger grouper on Tague Bay reef 1973
Everything made sense.  I was taught as a student that highly complex ecosystems such as coral reefs were highly stable.  Scientists of the 1970s published articles speculating that the only thing that could change this coral-dominated state would be another Ice Age over the next 10,000 years.  This was the widely held consensus among most coral reef scientists at the time.  As the famous coral reef scientist, Jeremy Jackson, has frequently pointed out – most of the world’s best coral reef scientists were using the best scientific methods available at the time but none predicted the widespread collapse that we have observed in recent decades.

Speaking for myself, I was studying the St. Croix’s coral reefs for my PhD dissertation (with Jeremy Jackson as my advisor).  I set up experiments on the Tague Bay fore reef in 1979 and continued to follow the dynamics of change there until 1995 (so this included period after I was hired by the University of Maine in 1982).  Over that short period of a decade, I saw the coral reefs of St. Croix collapse into an unrecognizable wasteland.

My 2 and 5 meter sites on Tague Bay forereef collapsed from 1980 to 1990
The 1980s was the disastrous decade for Caribbean coral reefs.  Two diseases hit about the same time but the rate of their impacts were very different.  Chronologically, the first disease was the White Band disease that infected the elkhorn and staghorn corals beginning in the late 1970s.  It was easy to spot.  It attacked the elkhorn and staghorn corals at the base and progressed slowly upward.  The lethal white band spread considerably faster than these corals can grow.  It created a wave of death visible from low flying airplanes. Bill Gladfelter, a scientist working at St. Croix’s West Indies Lab, published a paper in 1982 showing the elkhorn coral mortality and suggesting that if this disease continued, it could eliminate the dominant coral of Caribbean reefs.  His paper was largely ignored until late in the 1990s when it dawned on reef scientists that he was right.  The elkhorn and staghorn corals had died throughout most of the Caribbean.
The black-spined sea urchin Diadema and its well grazed reef.
The second disease attacked the black-spined sea urchin “Diadema” (Diadema antillarum).  This sea urchin had attained remarkable population densities throughout the Caribbean.  In St. Croix on the Tague Bay reef I was studying, we documented over 17 per square meter at my 15’ (5m) site.   So, imagine your desk is about 3’ by 3’ or one square meter.  Now imagine that desk with over 17 large sea urchins with long spines packed with poison.  Until 1983 these were the bane of most coral reef scientist’s existence.  Most scientists and tourists at the time had close encounters of the most unfortunate kind.  Although these urchins hurt, scientists such as Bob Carpenter, Paul Sammarco and John Ogden were conducting experiments to see how they functioned in coral reefs.   All the studies pointed out that Diadema keeps coral reefs and the surrounding areas ‘mowed’ down so no seaweed was present on most Diadema dominated coral reefs.  Studies of experimental removal of all the Diadema showed a very rapid shift to harmful seaweed.

My experiment (coral settlement plates) at 5 m in 1980 with Diadema nestled around the base of the corals.
Suddenly everything changed.  Starting in 1983 and progressing to 1984, Diadema suffered a mass mortality from a disease.  The disease began in Panama so speculation is high that a pathogen from the eastern Pacific caused this outbreak but to this date we simply do not know how the disease started or where it came from.  Nevertheless, what we do know is that the early experiments showing how important this sea urchin is as an herbivore was absolutely correct.  Within a month of the sea urchin disease outbreak, coral reefs throughout the Caribbean were overgrown by seaweeds.  The earlier sea urchin removal experiments were spot-on.  This was an important grazer that kept the coral reefs of the 1970s healthy and free of seaweed.

Because Diadema was so abundant and when the disease hit a reef all of the urchins died in a day or two, there was no mistake that a serious epidemic had struck the Caribbean coral reefs.   The slowly creeping White Band Disease (WBD) was often conflated with the effect of the Diadema mass-mortality (I published a paper in 1994 pointing out the impact of herbivore loss without mentioning that the coral decline was likely from WBD).

The combined effects of herbivore loss and increase in area where seaweed can grow (due to WBD of the dominant coral) effectively diluted the grazing pressure from the remaining herbivorous reef fish such as parrotfish.  With such low levels of herbivory the coral reef “flipped” into becoming a seaweed reef.

By the end of the 1980s none of the reef scientists were saying complex and diverse coral reefs were stable.  What we had all witnessed was one of the World’s most rapid and widespread collapse of a complex ecosystems.

THE DEATH SPIRAL OF CORAL REEFS LOCKED INTO SEAWEED REEF STATE

As I’ve been studying reefs of the eastern Caribbean, I’ve come to recognize the alternative state of coral reefs is one dominated by seaweed.  It is an alternative state from which it is difficult to recover.   Based on all of my surveys at 30’ (10m) depth from this expedition, the abundance of seaweed still exceeds the abundance of coral.  Arguably, these should no longer be called “coral reefs” and since it has been 30 years since coral-dominated reefs became seaweed-dominated reefs, this could now be considered an “alternative stable state”.

Returning to St. Croix for perhaps the last time in my professional career had me thinking that this was going to be just another example of a fully collapsed coral reef ecosystem.  The only reason it was worth studying is that I was revisiting the exact same reef sites using the same methods over the decades before, during and after reef collapse.

REBIRTH OF A CORAL REEF

We revisited the exact area of the Tague Bay reef where I conducted my PhD research starting in 1978.  That was before we had “global positioning systems” (GPS) so I had used distinct houses on the shore to line up my study sites.  Remarkably, they were still there so I was on my old study site!

Swimming up the reef from 30’ depth, the reef hadn’t changed much but then I started to see Diadema and coral.  At 5 and 2 m (15 and 6’) it was clear that the grazing Diadema were back, seaweed was relatively rare, coral was more abundant (so were the baby corals).  To me, it seemed I was witnessing a rebirth of a coral reef.

Trends in abundance of corals, diadema and seaweed from 1980 to 2014 at 2, 5 and 10 m (6, 15 and 30 feet).  Note that the trends are positive at shallow two depths.
Make no mistake, the reef had a long way to go.  While there was some elkhorn coral scattered about, it was nothing like the elkhorn that was here in 1980.  But all of the shallow water trends were positive!

Tague Bay forereef at 5 m (15')  1980.
Tague Bay forereef at 5 m 2014 (same site as the photo above from 1980)

Nevertheless, seeing large proud elkhorn coral at my 2 m site and also seeing lots of baby elkhorn coral indicated to me that the real reef builder could be coming back

Tague Bay forereef at 2 m 1980 - 1988.  No elkhorn coral was alive in 1988.

Tague Bay forereef at 2 m 2014 (same place as above) with numerous live elkhorn coral!
We then travelled to Buck Island National Monument.  This is now a no-take reserve where fishing is illegal (they started enforcing the no-take regulation around 2003).  While some of the coral I had seen in 2003 had died, in the shallow zone, was the most extensive stand of elkhorn coral I’ve seen in the last 30 years.

An elkhorn coral reef on the southern bank barrier reef on St. Croix's Buck Island.   This is the densest stand of elkhorn coral that I've seen anywhere in the Caribbean in over 30 years!

Elkhorn coral is among the fastest growing corals in the Caribbean.  At this Buck Island site, I had placed coral settlement plates for Park scientists to study but that hadn’t happened.  So, the plates were still there.  It was a great opportunity for me to see what had happened to them in the past 12 years.  Some large corals had overgrown the settlement plates.  Baby corals that had attached to them were now adult corals.   It is amazing what you can learn from accidentally leaving an experiment for more than a decade!

Terracotta coral settlement plates I deployed in 2002 (left) and how corals have grown on and over them in the past 12 years

 However, the biggest surprise was coming upon a plate that was completely enveloped by the base of an elkhorn coral.  The coral had grown 60 cm and and across 1 m (24”, 39”, respectively).  That’s an impressive amount of growth.  As I looked around and saw how many baby elkhorn coral I could see, I thought I was in fact seeing the rebirth of a coral reef!

Elhorn coral growing over a terracotta settlement plate (plate is 10 cm or 4").

Elkhorn coral that grew up and over a coral settlement plate (coral is 60 cm high and 100 cm wide or 24" and 39")

THE RETURN OF THE KING!  ..... maybe?

The biggest source of despair among Caribbean coral reef scientists has been the loss of elkhorn coral because it has produced far more coral reef rock than all other species of coral combined.  It grows fastest, it has the capacity to produce coral reefs best able to keep up with rapidly rising sealevel (which is predicted to occur in the next century).  So, the ample evidence that elkhorn corals are proliferating here in St. Croix is great news and offers renewed optimism for the future of Caribbean coral reefs.



I sure hope an assessment of these coral reefs 40 years from now is as positive as this entry.



1 comment:

  1. Really interesting, fascinating, really great informative and interesting to hear how its changed and the potential hope of it making a come back. An amazing life time of work.

    ReplyDelete