Sunday, 27 October 2013

Bermuda's coral reefs

Lessons from a low diversity, isolated coral reef ecosystem

The northern most coral reefs in the world surround Bermuda.  In fact, the ‘footprint’ of  the island’s coral reefs is much larger than the island itself.  These reefs formed, as all coral reefs do, in shallow tropical seas.

Bermuda's coral reefs from space.  The green/brown 'fish hook' is Bermuda, the blue and darker circle are coral reefs.

 Corals are physiologically plants because of the tiny algae (called zooxanthellae) that live in their skin.  Those algae give the coral their color.  The algae feed the coral with sugars for growth and energy.  The coral provides habitat for the plants.  This symbiosis allows corals to grow toward the sun from a depth where they get enough light to grow.  It is actually a only a shallow depth band in which corals can live to get the light they need to grow.

Brain corals showing the color of their zooxanthellae

It is an accident of geology that causes Bermuda have these unusually expansive reefs. The island was formed by a volcano 100 million years ago but has since eroded down to a fraction of its former self.  Over the last million years, sea level has gone up and down with corals building up limestone layers each time conditions were right (underwater with abundant light) for coral reefs.  After the last ice age about 18,000 years ago, sea level was lower because lots of water was frozen at the poles.  When it rapidly melted, sea level rose faster than any coral could grow.  However, with time as the rate of sea level rise slowed the depth of the Bermuda bank was perfect for coral growth.

It is an accident of oceanography that Bermuda sits in warm tropical water capable of supporting coral reef growth.  Those two accidents of geology and oceanography resulted in the impressive coral reefs grew to what we see today.

Because Bermuda is 1000 miles from Caribbean coral reefs and is at the northern limit of coral distribution, there are fewer species of coral and of…. EVERYTHING... compared to typical Caribbean reefs.  For example, only about 20 species of coral live in Bermuda while 64 species are found in the Caribbean.

While coral reefs world wide are composed of a variety of shapes, Bermuda’s reefs are much simpler.   The reefs we saw today were strongly dominated by only a few species of brain corals.  Brain coral is so named because the grooves on the coral look a bit like the structure of a brain.  However, most brain corals are hemispherical.  So rather than seeing tree-like structures or complex branching shapes, we see a seascape of mounds. 
Brain corals mostly of the genus Diploria (for you science nerds)

The curious hemispheres of brain coral making the seascape of this Bermuda reef on the south shore
Grazing queen parrotfish and tang

Queen parrotfish over well grazed (see bite marks) reef surface
Stoplight parrotfish fleeing from me

The reef we saw was healthy.  There were lots of parrotfish grazing the algae to a thin turf.  That’s ideal but I’ll explain this in more detail later in the trip.  There were plenty of baby corals (small button size or golf ball sized corals). 

Bermuda has a low diversity reef as I explained above but that means it tends towards having monocultures.  The brain coral are mostly two species in one genus of reef coral.  It also may make it susceptible to non-native species introductions.  This has happened but one uninvited guest that did make it here is the now ubiquitous Pacific lionfish.  It is a beautiful fish with a nasty habit of eating baby reef fish.  It was introduced to Florida when Hurricane Andrew swept over the state in 1993.  It remained confined to that area for a while but then colonized the Bahamas to very high densities and quickly spread throughout the Caribbean.  No region I know has been spared.

Lionfish - the uninvited beautiful predator of baby fish


  1. Great to see images of healthy reefs in my home country :) One minor correction, however. Lionfish were first reported in Florida waters in 1985, seven years prior to Hurricane Andrew (1992) :) Lionfish were reported from Bermuda about 3-4 years before they were spotted in the Bahamas, but fortunately they have not exploded to the same degree in Bda's more northern waters as they have in the Caribbean and are doing in the Gulf of Mexico...

  2. Thank you Maia. You are right. Turns out Hurricane Andrew was not to blame for the lionfish invasion. As I understand it, genetics indicate it was a single rather than a multiple introduction. I appreciate learning the latest.