Coral
Islands
A cay is a small, low-elevation, sandy island
formed on the surface of coral reefs. Other types of coral islands
can form when an atoll fills in, but a cay is formed when ocean
currents transport material eroded from the reef, across the
surface of the reef to an area where the current slows and deposits
the sediment. Currents, and wave action builds up the deposited
sediment on the reef surface.
The island resulting from sediment accumulation
is made up almost entirely of the skeletal remains of plants and
animals from the surrounding reef. Cay sediments are largely
composed of calcium carbonate (CaCO3), produced by coral. Small
amounts of silicate sediment are also contributed by sponges and
other creatures. When wave action pushes sediment above the
surface, rainwater can reach the material, which being mostly
calcium carbonate is easily dissolved and re-deposited around the
loose material, cementing it together. The newly formed land is
soon colonized by plants and animals, which also contribute their
own remains to the island, helping soil to develop. Cays can become
large enough to become habitable and can provide arable land to
sustain large populations.
To understand more about cays, we must first
look at coral reefs.
Coral reefs
A coral reef is actually a symbiotic
relationship between an animal (coral) and a plant (zooxanthellae,
a type of algae) that lives in the gut of the coral. The corals
provide a protected place for the zooxanthellae to live and the
metabolic wastes of the coral are used by the plants for
photosynthesis . The plant in turn benefits the coral by producing
oxygen and organic molecules that are used by the coral as
food.
Each polyp produces a calcium carbonate cup that
surrounds the polyp, where it sits retracted for protection,
extending only to feed on the plankton that floats by. Each polyp
is attached to its neighbors, and as polyps aggregate they form
colonies that secrete calcium carbonate, creating a collective
limestone "skeleton". Successive generations of polyps build on top
of previous generations leading to reef formation.
The Barrier Reef
Coral reefs are of three main types: fringing
reefs, barrier reefs and atoll reefs. While atolls are associated
with oceanic islands that subside, barrier reefs occur along the
continental shelf. The largest single coral reef in the world is
the Great Barrier Reef off Australia which measures up to 100 km
across and nearly 2000 km in length. Fringing reefs are considered
immature.
The Belize reef, formally known as the
Mesoamerican Barrier Reef, extends almost 1,000 km from the tip of
the Yucatán Peninsula down to the Bay Islands of Honduras, and is
the world second largest reef system.
Reef Erosion
A coral reef is the result of a balance between
the production of calcium carbonate by algae and corals, and the
erosion of the reef by physical and biological factors. Erosion is
caused by waves, particularly during storms where the limestone is
broken up, then wave action grinds the coral into finer pieces
until it becomes sand. But surprisingly, the erosion by waves is
minor compared to the erosion of coral reefs by biological
factors.
Many organisms contribute to reef erosion,
including fungi, sponges, bivalves, snails, clams, and sea urchins
that burrow in the coral and eat the material as they excavate.
Coral is also eroded from attacks by fish. The Parrot fish has jaws
that are capable of breaking off pieces of coral, and they feed
directly on the polyps. Both the polyps and limestone are chewed
into a fine sand that passes through the digestive tract. By eating
coral a single parrot fish may produce up to 90 kg of sand per
year!
As discussed earlier, it is this eroded sand that
is built up by currents and wave action to form cays.
South Water Caye
Named by local fishermen for the availability of
fresh water to be found in shallow wells on the island, prior to
the 1970’s South Water Caye was home to a few local
fishermen. It is now surrounded by the Belize’s largest
marine reserve - South Water Caye Marine Reserve, and the reef
around the island and was designated a UNESCO World Heritage Site
in 1996.
The posted coordinates takes you to a public
beach on the south end of the island. It is one of the most
beautiful spots in all of the Caribbean. To get there, you can dock
at the International Zoological Expeditions facility and walk a
short path on the western edge of the island. You don’t have
to use the IZE facilities, South Water Caye hosts two resorts: the
Blue Marlin Lodge and the Pelican’s Pouch Resort.
While the beach at the posted coordinates is
open to the public, if you are not staying at the Pelican’s
Pouch, please do not go past the sign; that part of the beach is
reserved for their guests.
Logging Requirements:
1. Stand on the beach at the posted coordinates
and look down. Can you detect any silicates in the sand? If so,
estimate the proportion of sand that is silica as compared to
calcium carbonate?
2. There is a large structure to the south, what is it? Can you
estimate how far away it is? Bonus: Who is the American scientist
who’s name is best associated with the research institution
on that structure?
3. Just before you get to the beach, there is a large tree to the
right, what kind of tree is it? What does its presence tell you
about the geology of the island?
4. Optional. Take a snorkel from the beach out towards the reef.
Have a cold Belikan at the IZE bar. Chill. Please post pictures in
your log, but avoid any spoilers.