Coral Reefs and Lagoons

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Conditions necessary for reef development
Large reefs are limited to the warm seawater
areas of the tropics. Calcium carbonate is
easier to precipitate in warm water than in cold
water. Secretion of calcium carbonate is aided
by microscopic cells of algae that live in the
tissues of reef builders (the algae remove carbon
dioxide from the tissues, decreasing the acidity
of the water).
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Conditions necessary for reef development

• Reefs also tend to preferentially form in areas
  where
• Little clastic sediment occurs (such sediment
  particles smother reef builders).
• Nutrient levels are low.
• Water is shallow

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Reef zones
Reef builders are zoned in a reef according to
their form (encrusting forms tend to dominate the
reef crest where wave action is strongest, while
more delicate branching forms are confined to
deeper water zones where water action is more
gentle)
A lagoon can develop behind a reef, where it is
protected from strong waves
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A special kind of reef atoll
An atoll is a special kind of reef that is
ring-shaped and has a central lagoon. It is
likely that Gilligans Island was set in an
partially formed atoll.
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How an Atoll Forms
An atoll is formed first as a reef that fringes a
volcanic island. As the island sinks (after
volcanic activity has ceased and the crust has
cooled, becoming denser), the reef continues to
build upward, eventually ending up as a
ring-shaped structure.
Gilligans Island ?
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Bikini Atoll
Bikini atoll (central Pacific) is a famous
nuclear testing site (the US tested atomic bombs
here in the 1940s and 1950s)
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Darwin-Dana-Daly Theory of Atoll Formation

• First proposed by Darwin during his voyage on HMS
  Beagle (1836)
• Then supported and modified by 2 geologists
  James Daly and Reginald Dana

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3D Theory of Atoll Formation

1. An oceanic volcano, which emerges from the sea
   surface and forms and island, becomes colonized
   by reef building corals
2. This growth of corals begins to form a fringing
   reef around the island. The island begins to
   sink slowly. Coral growth continues

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3D Theory of Atoll Formation (cont)

• 3. The island continues to sink and a barrier
  reef is formed with a lagoon between the reef and
  the island
• 4. Eventually, the small island disappears below
  the sea surface, leaving an atoll consisting of a
  ring of small islands with a relatively shallow
  lagoon in the center

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Evidence For 3D Atoll Formation

• Deep drilling on coral atolls (Bikini Atoll)
• As the depth of the corals increased, the age of
  the corals also increased
• Fossil corals from base of the drilling were
  approximately 50 myo
• Corals were found to have grown on underlying
  volcanic rock
• Fossil corals were found at 1200 meters
• These corals only grow in shallow water!

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Relate the 3D Theory to the Physiology of Coral

• Darwin also produced the first map showing the
  distribution of corals throughout the world,
  identified 3 kinds
• Fringing barrier reefs
• Barrier reefs
• Atolls
• USEFULL SITE!!!!

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Coral Distribution
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• A coral reef is created by carbonate-secreting
  organisms.
• It is a wave-resistant, rock-like structure.
• Most of the reef is composed of loose to
  well-cemented organic debris of carbonate shells
  and skeletons.
• The living part of the reef is just a thin layer
  on the surface.

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Coral reefs consist of several distinct parts
developed in response to their exposure to waves

• The algal ridge occurs on the windward side of
  the reef and endures the pounding waves.
• The buttress zone is the reef slope extending
  down from the algal ridge.
• The reef face extends downward from the buttress
  zone.
• Not many colonial corals live here because
  insufficient light reaches this depth.
• The reef terrace is located landward of the algal
  ridge and lies at mean water level.

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The Morphology of Coral Reefs
FIGURE 12.20a Morphology of a Barrier Reef.
FIGURE 12.20b Reef edge.
Courtesy of Sean Linehan/NGS/NOAA
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• Corals grow continuously upward towards the
  sunlight.
• As sea level rises and/or land subsides, coral
  reefs pass through three stages of development.
• Fringing Reef
• Barrier Reef
• Atoll

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• Fringing reefs form limestone shorelines around
  islands or along continents
• They are the earliest stage of reef development.
• As the land is progressively submerged and the
  coral grows upward, a shallow lagoon expands.
• When the lagoon begins to separate the fringing
  reef from the shoreline, a barrier reef is
  created.
• In the final stage, the land vanishes below the
  sea and the reef forms a ring of islands, called
  an atoll, surrounding a shallow lagoon.

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Evolution of Coral Reefs
FIGURE 12.24a Types of Coral Reefs.
FIGURE 12.24b Evolution of coral reefs.
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Distribution Explained

• Conditions required for growth of coral
• Suitable temperature range
• 16-35C, 23-25C best
• Water clarity
• Clear without silt
• Silt reduces light penetration which inhibits the
  photosynthesis of zooxanthellae
• Depth rapid growth usually occurs within 20m of
  surface
• Salinity
• Suitable rock substrate
• The continuous vertical growth of corals, over
  many thousand of years, occurs if the sea bed
  subsides or there is a rise in sea levels

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Role of Reefs in Dissipating Energy of Waves

• Corals absorb the energy of waves and protect the
  shoreline from erosion
• Protection of ecosystems
• Reduces the cost of providing breakwaters
• Provides safer anchorage for boats
• Significant economic advantages

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Factors that lead from reef growth to reef erosion

• Healthy corals accumulate calcium carbonate
  (CaCO3) at rates between 3-15m every 1000 years
• Many factors affect reef erosion
• Predation
• Storm damage
• Exposure to air

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• Predation Crown of Thorns Starfish (Acanthaster
  planci) found on Indo- Pacific Reefs.
• Exceptionally low tides corals can be exposed to
  air (dry and overheat)
• Storms hurricanes causes physical breakage and
  scouring effect of abrasive sediments
• Hurricane Hattie (61) destroyed 43 km of barrier
  reef of British Honduras, complete recovery will
  take 25-100 years

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Impact of Reef Erosion

• Reef erosion leads to shore and coastal
  properties being more exposed to the damaging
  effect of waves
• Many different materials are used to construct
  artificial reefs
• Concrete
• Stoneblock
• Sacks filled with sand
• Even sunken ships (or old Hathaway Bridges)

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Artificial Reefs

• Become colonized by a wide range of organisms
• Algae, corals, and lots of fishies
• Function as a submerged breakwater
• Dissipate wave energy, reducing coral erosion and
  protecting anchored boats.

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Methods for reef reconstruction

• Geomorphology study of landforms and the
  processes involved in shaping them
• 3 main categories of reefs (fringing, barrier,
  atolls)
• Growth and geological history of a reef can be
  investigated by drilling and carbon dating

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• Deep drilling provides cores of material which
  can be used to id coral and estimate growth
• As coral grow, they produce bands (like tree
  rings)
• Interpretation of bands provides evidence for
  changes in growth rate in response to changes in
  environmental conditions (ambient temperature or
  nutrient availability)

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Carbon dating

• Aka radiocarbon dating
• Can be used to find the age of a sample
• As coral grow, carbon (from the CO2) is deposited
  in their skeleton in the form of CaCO3 (calcium
  carbonate)
• Some of the carbon in the form of 14C slowly
  decays to 12C.
• The proportion of 14C to 12C in a sample is used
  to age corals up to about 50,000 years old

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Investigating the effects of sea level changes on
coral reefs

• Growth of coral is dependent on a number of
  factors (including light)
• For this reason, growth is restricted to the top
  20 m of water
• Fossil coral, found at depths of 1200 m are
  evidence of subsidence
• Over geologic time new corals slowly grow on top
  of the old and the reef gradually sinks

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Investigating the effects of sea level changes on
coral reefs

• Corals are also found above sea level
• In increase in sea level allows growth
• Decrease in sea levels leave them high and dry
• Recently exposed coral in Mexico have correlated
  the growth of corals with changes in sea level
  between ice ages

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Reef Value

• Found in over 100 countries
• Over 500 million people worldwide rely on coral
  reefs
• Major source of food
• Poorer countries
• Fish and invertebrates
• Conch, bivalves, octopus, squid.
• Food for pelagic fish species
• Limestone
• Building materials, breakwaters, cement
• Jewelry
• Aquarium trade
• Tourists
• 6-10billion US tourist industry
• Beach protection
• Wave breaks