Marine Communities: The Coral Reef

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Marine Communities The Coral Reef
Is a coral reef more like a tropical rain forest
or a desert?
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Lecture Outline
I. Types of Coral Reefs
II. Reef Zones
III. Coral Anatomy
IV. The Ecology of a Coral Reef
V. The Distribution of Coral Reefs
VI. Environmental Stresses to Coral Reefs
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Types of Reefs
1. Fringing Reefs
2. Barrier Reefs
3. Atolls
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Fringing Reefs
Fringing reefs grow in shallow waters and closely
border the coast or are separated from it by a
narrow stretch of water.
The reefs of the Florida Keys and most areas of
the Caribbean are fringing reefs.
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Barrier Reefs
A barrier reef is separated from land by a broad,
deep lagoon.
These reefs grow parallel to the coast and are
large and continuous.
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Atolls
An atoll is a ring-shaped coral island and its
surrounding reef, nearly or totally enclosing a
lagoon.
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Charles Darwin
In his book The Voyage of the Beagle (December
1831 to October 1836), Charles Darwin proposed a
theory for the evolution of atolls that includes
stages as fringing reefs and barrier reefs.
Darwin based his theory on the assumption that
the volcanic islands in the Pacific are sinking
into the ocean, something he had no way of
testing in the mid 19th century.
Scientific evidence of the sinking of Pacific
islands was obtained in the 1950s by the Ocean
Drilling Project.
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Atoll Formation
In the earliest stage, a fringing reef forms a
limestone shoreline around an island or along the
continent.
As the island subsides and the coral continues to
grow upward, the expanding lagoon separates the
reef from the shoreline turning it into a barrier
reef.
In the final stage the land vanishes entirely
below the sea and an atoll forms around a shallow
lagoon.
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Reef Zones
Coral reefs are divided into 4 major zones.
These zones are determined by the amount of light
and wave energy .
The Reef Face
The Buttress Zone
The Flat-Top Zone or Algal Ridge
The Rubble Zone or Reef Terrace
The Lagoon Zone
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The Reef Face
The reef face is the very deepest portion of the
reef, extending from the base of the reef to
about 20 m below the sea surface.
This portion of the reef receives low levels of
light.
At these depths, water motion is not energetic,
and where light is sufficient for coral growth
relatively delicate corals, like the staghorn
coral, can survive.
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The Buttress Zone
The buttress zone extends from a depth of 20 m to
the low-tide line.
This reef zone is exposed to the greatest wave
turbulence so that only the most durable forms of
coral, like brain coral and elkhorn coral, can
exist.
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The Flat-top Zone
The flat-top zone extends across the reef crest.
The reef here may be anywhere from a few
centimeters to a few meters below the surface at
low tide.
Light is abundant, and wave energy has been
dissipated by the coral heads in the buttress
zone.
A variety of smaller reef fish inhabit this zone
but few large predators.
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The Rubble Zone
The rubble zone is a shallow (3-5 m) debris field
immediately shoreward of the flat-top zone.
Large blocks of coral are transported to the
rubble zone during storms where they will be
colonized by dense growths of algae.
Few other organisms call this part of the reef
home.
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The Lagoon Zone
The lagoon zone runs from the backside of the
rubble zone all the way to shore.
The lagoon zone averages 3 to 15 meters in depth
and may be as wide as 6 to 8 kilometers.
This zone is scattered with benthic grasses, sand
bottom, and patch reefs.
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Patch Reefs
Scattered within this zone are small patch reefs
that occur with no pattern.
These patch reefs are usually only a few acres in
area, but because they occur in shallow protected
waters, they have many of the thin, delicate
corals that cannot survive the heavy surf
occurring in the buttress and flat-top zones.
Patch reefs are inhabited by populations of small
tropical fish, but few large predators. In many
of these patch reefs the soft coral are as
abundant as the hard corals.
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Reef Zones
With some variations, most coral reefs have all
these zones.
For an atoll, the lagoon zone is in the center,
with the other three zones around the lagoon.
For a barrier reef, the lagoon zone gets deeper,
and then there is 30 to 40 kilometers of coastal
water before the shoreline.
For some of the fringing reefs in the Caribbean,
the rubble zone is the beach, and the lagoon zone
will be on the backside of the island away from
the direction of storm waves.
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The Biology of Coral Reefs
The bulk of a coral reef is composed of loose to
well-cemented organic debris, fragments of shells
and skeletons, made of calcium carbonate (CaCO3).
The living portion of the reef is just a thin
layer at the surface.
The coral animal is a sac-like polyp, a member of
the Cnidaria family.
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The Anatomy of a Coral Polyp
The polyp removes calcium from the sea water and
secretes a calcium carbonate shell, called a
corallite, which forms the foundation of the reef.
The primary soft-tissue structures of the polyp
are the tentacles, a mouth (which also serves as
the anus), and a gut cavity.
Zooxanthallae are dinoflagellates (algae) that
live within the tissues of all reef-building
(hermatypic) corals and can comprise up to 75 of
the polyps body weight.
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Zooxanthellae
A symbiotic relationship that benefits both
participants is called mutualism.
The waste products of the polyp provides carbon
dioxide and nutrients for the zooxanthallae.
The zooxanthallae in turn produce oxygen and
sugars for the coral polyp.
By removing the CO2 from the coral, the
zooxanthallae reduce the acidity in the coral,
enabling the rapid formation of calcium carbonate
(rapid calcification) necessary for reef building.
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Global Distribution
Coral reefs are distributed around the globe, but
are limited to areas of the ocean where the
average monthly temperatures are above 18C (64
F).
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Global Distribution Biodiversity
In general, the coral reef belt is wider and the
diversity of coral organisms is greater on the
western side of the ocean basins.
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Deserts of the Sea
The symbiotic relationship with the zooxanthallae
enable coral to thrive in low-nutrient
(oligotrophic) waters.
In high-nutrient (eutrophic) environments, algae
and other species will compete with coral for the
available nutrients and phytoplankton.
Eventually, the coral dies off.
Coral cannot survive in fresh, brackish water or
highly turbid water.
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Benefits of Coral Reef Ecosystems
Coral reefs are important to the environment and
to people in a number of ways

• They are a home and nursery for almost a million
  fish and other species.

• They protect shores from the impact of waves and
  from storms.

• They provide a lot of benefits to humans in the
  form of food and medicine.

• They provide economic benefits to local
  communities from tourism.

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Environmental Stresses
Major stresses to Florida reefs

• Coastal development has increased turbidity.

• Eutrophication as a result of waste products
  either dumped or leached into coastal waters.

• Over fishing.

• Physical damage from boats and careless divers.

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Artificial Reefs
Man-made reefs provide an alternative to natural
reef systems.

• Relieve the fishing and diving pressure on
  natural reefs.

• Can rival natural reefs in sustaining fish
  populations.

• Located close to marinas and boat ramps.

• Can be built to target certain species.