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.

Tuesday, 26 November 2013

Reef Research from a Small Boat: The Challenges and Opportunities

Alaria on station tucked behind a coral reef in Antigua
We need a bigger boat” utters the marine biologist (played by Richard Dryfuss) after he sees the huge great white shark in the movie Jaws.  I’ve heard that phrase dozens of times on this expedition once we saw the huge tasks before us. 

However, bigger boats need deeper water and we have the perfect tool for slipping behind a coral reef, anchoring in the sand and waiting for weather that will allow us to study our target coral reefs.

Alaria's locations around the eastern coral reefs of Antigua
Getting to those perfect anchorages means waiting for good light, no sun glare and a person poised on the bow of the boat with hand signals that tells me where to go and when to stop.

Paul spotting coral heads for us to steer around
Getting to a site to do research involves lots of preparation.  I’ve targeted coral reef sites along eastern shores where coral reefs grow best.  We use a WiFi booster to get on line so I can go over Google Earth images of the reef and the channels that allow us to access them. 

On the foredeck is our dive staging area.  There we have a small portable compressor that fills a scuba tank in 20 minutes. We store the tanks and our dive gear there.  We have a fish tote filled with water to wash down our regulators and buoyancy compensators.

Scuba tanks being filled by our compressor
Foredeck with fish tote for rinsing scuba gear after dives
After we quantify the corals, algae, sea urchins and all reef fishes on our transects, we transcribe them onto our computers and conduct simple analyses.

George doing fish transects.  Note GoPro video camera on his head, writing cylinder on his right arm, dive computer on his left arm

Bob placing a transect line
Returning to the Zodiac after the dive
So, Alaria is functioning as a floating marine lab.  However, our work is not just confined to my group and the partners we work with on the various islands,  We are also collaborating with several other researchers on various topics. 

For example, we have also been collaborating with Liz Madin in Sydney Australia.  She’s using satellite images to find halos around reefs where no seagrass grows.  We are wondering why halos occurs so she sends me images, I check them out.

Halo around a patch reef (note clear area around mark)
Alaria's location to examine the halo site
Another collaboration is with Ove Hoegh-Guldberg and Manuel Rivero (from the University of Queensland, Brisbane Australia) who are using a state of the art camera imaging system to develop three dimensional models of the coral reefs.  On reefs where we both work, we can supply fish and coral community composition.

When I think back to the last time I was in this area of the Caribbean in 1973 and 1974, we had a boat, a compressor and lab but no ability to communicate or network with other scientists in the field.  Our electronic revolution connects us easily.  What is hard, is actually getting out to the field to see what’s going on.  THAT is what tiny Alaria can do very well.

Alaria on station in Antigua

Sunday, 17 November 2013

Anguilla, the northern most island of the Leeward Antilles

Sailing into Anguilla.  Obviously a "low relief" island
View from the Customs and Immigration dock.  Alaria is mast on the right.
We sailed north to Anguilla.  Unlike St. Maarten, Anguilla is a “low island” (no mountains or steeply sloping shores).  This geological feature is ideal for creating a platform on which coral reefs can grow… and grow they did.  Seal Island reef is one of the largest coral reefs in the eastern Caribbean.

The little island with a big reef.  Breaking waves along the Seal Island Reef shows the extent of the coral reef and the extensive lagoon behind it.

Unlike St. Maarten where the Nature Foundation supplied us with scuba tanks for our surveys, now we have to work as an independent floating marine lab.  So we deployed the compressor, charged the scuba tanks and got ready to dive Anguilla’s coral reefs.

The compressor filling tanks on Alaria's bow.  It takes 20 minutes to fill one tank
Anguilla’s best coral reefs are within their Marine Park.  To visit those reefs you must pay $60 per day (expenses go to buoys and park management).  There is a long list of “Marine Park Regulations” that limit activities in the parks.  No harming flora or fauna, no jet skis, no water ski but what interested me most was “Visitors are NOT permitted to fish within Marine Park boundaries.”  That’s terrific because seeing a big reef with the full complement of reef fishes would be very instructive.

Alaria at our Sandy Island reef site

Sandy Island Reef

A small reef surrounds Sandy Island so we made that our first site.  We did three dives to record fish, corals, algae and juvenile corals at several sites.  The coral cover was low and the seaweed was too abundant for this to be a healthy reef.  The fish fauna was more depauperate than we would have expected from an area having no fishing.  However, then we saw why fish are rare.  There was a string of baited fish traps!

Fish trap sitting on the shallow fore reef

Another fish trap on the fore reef.  

So, it was obvious that these reefs are fished.  We suppose that only visitors are not permitted to fish – the local folks can.  However, often it is more important how you fish than whether you fish somewhere.  Fish traps catch fish 24/7.  The very valuable grazing parrotfish have the unfortunate behavior of looking for caves in which to sleep and fish traps often become the parrotfishes death bed.  I’ve seen many parrotfish in fish traps and I’ve seen many dead parrotfish in fish traps. Other fish also get caught and die in fish traps.  So larger fish whether they are eaten or not die as the result of fish traps.  This is why Bermuda and Bonaire have outlawed the use of fish traps.

Georges’ surveys were showing relatively few large bodied fish (neither carnivores such as groupers and snappers, nor herbivores such as parrotfishes were very abundant).   My surveys were quantifying the low coral cover and the high seaweed abundance.  Juvenile coral densities were also low.  These are not very good signs of coral reef health.

Seaweed over growing coral

Quadrat for counting juvenile corals is dominated by a thick coat of seaweed (no juvenile corals here)
Along with the 10 m transects I use to quantify the reef dwelling critters and seaweed, I also take quadrats records of baby coral density.  We know fairly well how fast reef corals can die but we know much less about how coral reefs recover via baby corals.  Usually there are no baby corals with seaweed but often we see them on or near the pink coralline algae.

Juvenile coral on a patch of coralline algae (good for corals)

Seal Island Coral Reef & Algal Ridges

The next day we got up early and sailed over to the big Seal Island coral reef adjacent to Prickley Pear Islands (the reef is also known as Prickley Pear).  We had several priority sites we wanted to study but conditions suggested otherwise.

A large ocean groundswell had developed over night and huge waves were crashing on the reef.  Paul and I went to survey site after site and found the forereefs impossible to work.  We could have conducted our dive but the swell would have shifted us back and forth 6 to10 feet which is unproductive and gets very old after a few minutes.

Paul and me checking conditions where the coral reefs live (white breaker zone between the two islands)
Large groundswell eclipses all of Alaria except for the top of the mast.

The reef is famous for having heavy wave action most of the time.  The best biological indicator of this is the presence of algal ridges.  Algal ridges are solid thick caps of calcareous algae that grows on top of the coral reef.  They only can form in regions having high wave action over most of the year and low tidal amplitude (there is a one foot tide in Anguilla).  Algal ridges are impressive structures.  On rare calm days you can walk around on top of them.  Below there is “room and pillar structure” of large caves (rooms) and vaulted supports.

One of several algal ridges capping Seal Island's long coral reef

We looked for cuts in the reef by climbing up onto the boom and looking over the waves.  We found places we could motor to but they all had too much swell to work.  So our Plan-B is to study backreefs.

Looking for study sites and breaks in the reef (breakers are the reef)
The backreefs had been well developed in the past given the dead coral skeletons evident everywhere. However, there was some cause for optimism.  We found elkhorn coral (the once dominant coral in the Caribbean) juveniles and adults!  George found a collection of juvenile reef fishes so the backreef looks to be a viable nursery habitat. 

Healthy elkhorn coral (which is an endangered species in the US)
Baby elkhorn coral surrounded by sediment-trapping filamentous algal turf.

Wave action stimulates coral growth but also grinds up the produced limestone so the water flowing over the reef often carries lots of sediment.  When it hits the quiet backwater, sediment falls from suspension and drapes the backreef environment.  The fine turf algae traps the sediment and creates a blanket that few other critters can survive.  So the there are unique dynamics in backreef environments that can both promote and suppress reef health.

Even in the stressful environment of lagoonal sand, you can find critters that thrive there.  The heart urchin lives just under the sediment surface feeding on things that live within the sediment.  They then are an "island" habitat for small commensal crabs.  The crabs get a place to live and glean food and possibly parasites off the heart urchins.

A heart urchin just below the sediment surface.

Three hitchhiking commensal crabs (the heart urchin is their habitat)
The swells continue to plague us so rather than waiting for them to subside, we decided to head to Antigua which has by far the most diverse array of coral reefs in the Leeward Antilles.

A porcupine fish of no significance to our study but that it is cute

Anguilla sunset from our anchorage.

On to Antigua!  It's a 125 nautical mile sail which should take us about 31 hours.  Our destination is English Harbor.  It happens to be a harbor I remember very well from 40 years ago.  I wonder if anything has changed?  ;)


Saint Martin/Sint Maarten

Northeast corner of the Caribbean (Anguilla to St. Barts).  St. Maarten is left of center.

When I first starting thinking about which coral reefs should be studied in the Caribbean, I realized the focus should be the reefs of the eastern Caribbean known as the Antilles.   Since I’m starting in the north and working to the south, my initial focus will be the islands of Anguilla and St. Martin.  We sailed to St. Martin because it is a sailor’s paradise with sail lofts, and marine chandleries for buying and fixing what we need for Alaria.  So, we studied the coral reef there first....

Alaria in St. Maarten
Sint Maarten

Sint Maarten and Saint Martin are two names for a small island with an identity crisis.  They reflect the two nationalities (Dutch and French) that lay claim to this tiny 7 mile diameter island.

The island has attractive rolling hills and moderately high relief.  However, the steeply sloping shores leaves little room for coral reefs to develop.  Corals are physiologically plants that need to have good light and water motion to grow. Because of St. Maarten’s steep slopes, there’s just not much space for extensive coral reef development.

Despite modest coral reef development, the Sint Maarten government has worked hard to preserve what they have.  There is no fishing within their marine park.  They have a federally -funded Nature Foundation that patrols the park.  We worked with them to learn about the challenges to the island’s coral reefs.

The Nature Foundation's Marine Park Patrol boat with Chief Marine Park Ranger, Etienne Lake and George Stoyle (our fish expert).

My hosts were Tadzio Bervoets who is the Manager of the Nature Foundation and Etieene Lake who is the Chief Marine Park Ranger.  They took us to their reef sites via their patrol boat.

Tadzio Bervoets, Manager of The Nature Foundation St. Maarten

Coral Reefs of St. Maarten

We asked Tadzio to take us to their most important reefs.  We survey all the reef sites in the same way by quantifying the abundance of all coral species, seaweeds, and other reef-dwellers (this was my task) as well as all reef fishes (this was George’s task).  The fish transects are 30 m long and 4 m wide in which George counts and records the body size of all the important species of reef fishes.  Later this can be changed into biomass (or weight) per species per unit area for the sake of comparison with other reefs and other islands.

The reefs Tadzio and Etieene took us to were coral assembles on rock but not really coral reef ecosystems.  There is almost no coral build up over the rock outcrops.  So, what does that tell us? 

St. Maarten's reefs with low coral cover over rocks with a scattering of seaweeds

Coral reefs are the only ecosystem on planet Earth defined by the rock habitat they create.  Corals calcify limestone so when the corals die, the limestone remains and over time it builds up forming a distinct structure known as coral reefs.  With the exception of bogs that accumulate plant matter, coral reefs are unique by being defined by what they leave behind.  So coral reef ecosystems have a measurable geological growth rate which is essentially how many meters of reef rock they produce per thousand years.  The fastest growing coral reef was “clocked” growing 12 meters per thousand years (or 12 millimeters per year).  That is breakneck speed … geologically speaking.

The St. Maarten reefs we studied mostly had thin veneers of coral over bedrock.  Coral cover ranged between 2 and 20% of the reef surface.  Seaweed is relatively abundant except in areas where the long-spined sea urchin Diadema thrived.  Diadema is a very effective herbivore capable of removing all but the calcareous “coralline” algae in its grazing range

Seaweed carpet makes life hard for reef corals

The long-spined sea urchin Diadema 

Diadema's lair.  They stay in crevices during the day and much seaweed at night
The seaweed covering the rocks is similar to most Caribbean reefs.  Seaweed can kill corals by poisoning them and it can prevent baby corals from getting started on reefs.  So I watch for seaweed on all of the coral reefs I study.  Interestingly, because the Diadema sea urchin can mow algae down, where they are abundant, the corals do well.

Three Diadema with little seaweed except the limestone grazer resistant pink coralline algae (which is a good habitat for corals!)

Reef fish in St. Maarten had been hunted for years by spearfishing, hook and line and trap fishing.  In December of 2011 all fishing was banned from the marine park.  This is a great step in the right direction because reef fish grow very slowly and generally cannot be harvested sustainably.  However, Tadzio and Eiteene told us that every week they find people fishing in the park.  They get warned or arrested but it will take the best part of a decade to see functional reef fish communities.  What I found most encouraging is the “can-do” attitude of the Nature Foundation trying to educate and improve the conditions of the reefs of St. Maarten.

Alaria from the top of the mast (photo: Paul Calder... thankfully)
Now onto Anguilla