Coral reefs are built from the limestone skeletons of stony corals and cover around 100,000 square miles globally. They are located in shallow, tropical waters and grow gradually as coral polyps multiply and their skeletons accumulate. There are three main types of reefs - fringing reefs adjacent to land, barrier reefs separated from land by lagoons, and atolls which are ring-shaped reefs surrounding a central lagoon. Reefs support tremendous biodiversity but are threatened by human impacts like pollution, coastal development, overfishing, and climate change which can cause coral bleaching or increase ocean acidification.

2. General Characteristics
Coral reefs are built from
the remains of stony (or
hard) corals
Reefs cover about 100,000
square miles of the world’s
shallow marine areas
Located in shallow, tropical
waters
They grow gradually as the
organisms that form their
living surfaces multiply,
spread, and die, adding
their limestone skeletons to
the reef.

3. Types of Reefs
The basic Coral Reef
classification scheme was
first proposed by Charles
Darwin
There are three main types
of reefs:
Fringing
Barrier
Atolls
There are two other
reef types:
Patch reefs
Bank reefs

4. Fringing Reefs
Occur adjacent to the land,
with little or no separation
from the shore
They develop through
upward growth of reef-
forming corals on an area of
continental shelf.
May have a completely
shallow lagoon in some
areas, or no lagoon at all
Most common reef in the
Greater Caribbean and Red
Sea

5. Also surround many South Pacific and Indian
Ocean Islands
Most susceptible to coastal development,
agriculture, pollution, and other human activities
resulting in sedimentation and freshwater runoff

6. Barrier Reefs
Broader and Separated from land
by a lagoon that can be miles wide
and at least a dozen yards deep
Usually parallel the shore
Parts of the reef structure often
protrude above sea level as low-
lying coral islands.
These develop as wave action
deposits coral fragments broken off
from the reef itself
Largest reefs develop on the edges
of continental shelves (called
“shelf barrier reefs”
Great barrier reef, Belize barrier reef

7. Atolls
Large, ring-shaped reefs,
surrounding a central
lagoon
found well away from
large landmasses, such as
in the South Pacific
Usually circular or
horseshoe-shaped
parts of atolls may
protrude above the
surface forming coral
islands as well.

8. Most common in the Indo-Pacific region (over 300)
Rare in Caribbean (only 10-15)
Southern Mexico and Belize
Can exceed 100 miles in diameter
Central lagoons can be several thousand square miles

9. Patch Reefs
Small reefs found within
the lagoons of other reefs
Usually small, isolated
outcrops of coral
surrounded by sand and /
or seagrasses
Provide refuge and food
for may species of
animals and fishes

10. Bank Reefs
Reef structures with no obvious link to a
coastline

11. EvolutionoftheThreeMain ReefTypes
As first proposed by Charles Darwin:
Darwin proposed the three main reef types are
simply different stages in the geological
‘evolution” of Pacific oceanic islands.

12. Darwin’s Theory
Fringing reefs began to
grow near the
shorelines of new
islands when conditions
for growth were ideal

13. As the island began to
gradually subside into the
sea, the coral was able to
grow at the same rate,
and remained in place at
the sea surface, but
farther from shore.
It was now a barrier reef

14. Eventually, the island
disappeared below
the sea surface,
leaving only the ring of
coral surrounding it
It was now an atoll

15. Darwin’s theory of “reef evolution” was
verified for the Indo-Pacific reefs in the
early 1950’s
After analyses of deep core drilling at Bikini
and Eniwetok Atolls
We now know, however, that in some cases,
the three types can be formed by different
processes as well

16. Reef Formation
Millions of Stony coral polyps are the marine
animals responsible for building the hardened
coral reefs you see in the ocean
Reefs are excretions of calcium carbonate
produced by each polyp
New polyps secrete reef on top of older polyps

17. Reef Growth Forms
Hard coral colonies generally exhibit one
of three basic growth forms:
Massive
Branching
Plate-like

18. Massive
These rely on sheer
mass, growing into huge
sheet or boulder
formations that tend to
make up the bulk of a
reef

19. Branching
These require a
small area to anchor
themselves, but
branch out into vast,
tree-like colonies

20. Plate-like
Form flat expanses
that overshadow other
corals and monopolize
the available light

21. Reef Formation
Some corals grow very slowly
Some can grow up to a few inches per year

22. Zooxanthellae
Photosynthetic organisms
that live in the tissue of
each coral polyp
Provide a source of food
to the polyps
Give corals their color
Example of a mutualistic
symbiotic relationship
This is why corals need
sunlight to grow

23. Organisms Contributions
Mollusks and echinoderms add their skeletal
remains
Grazing and boring organisms break down coral
skeletons into sand
Coralline Algae bind the sand

24. Reef Distribution
Coral Reefs are found in all three of Earth’s oceans that
have portions in the tropics. (Pacific, Atlantic, and
Indian). They are scattered randomly, however.

25. Reef Distribution
Grow in clear, sunlit, shallow
water
Average salinity of 36 ppt
Require warmer water
temperatures, as seen in
tropical and sub-tropical areas
Require little or no wave
action
Little or no sedimentation
from river run-off
The two main areas of
development that meet these
criteria are the Indo-Pacific
and Greater Caribbean
Belize – Great Blue Hole

26. Indo-Pacific Region
Central and South Pacific Ocean
Greatest number are in an area from southern Philippines
through western New Guinea and Indonesia
Red Sea are is cut off from large water flow, so has high
number of endemic species

27. Greater Caribbean Region
Tropical, western
Atlantic
Spans the entire
Caribbean Sea,
Bahamas, and south
Florida

28. Parts of a Reef

30. Parts of a Reef
Reef Crest
Narrowest of the reef zones
Massive wall that absorbs the energy of incoming waves
(makes water calmer for back reef zone)
Reef slope

32. Reef Flat and Lagoon Areas “Back Reef”
Contains shallow water habitats
for reef species
Vital role in food web
Experiences large temperature
and salinity changes, reduced
water circulation, and sediment
accumulation
Main Habitats here:
Patch reefs
Sand plains
Seagrass meadows
Mangrove forests

34. Importance of Reefs
Provide protective barrier around islands
and coasts
Slows down strong currents and waves
before it hits the shore
Prevent erosion

35. Importance of Reefs
Create more living biomass than any other marine
ecosystem
Contribute to local economies
Support more species than any other marine
environment
Possibilities in medicines and science
Control carbon dioxide levels in ocean water
The coral polyp turns carbon dioxide in the water into
a limestone shell

36. Coral reefs occupy only 2% of the oceans’ area,
but are home to ¼ of all known marine species.
To date, about 10% of all reefs have been
destroyed.

39. Human Impacts on Reefs
Coastal and ocean
pollution
Oil slicks, pesticides,
chemicals, heavy metals,
garbage, etc.
Poisons coral polyps
Fertilizer and untreated
sewage encourages algae
growth
High concentrations of
algae can cover the coral
and block sunlight if not
kept under control by
herbivores
Massive oil slick as seen
from satellite photos

40. Human Impacts on Reefs
Uncontrolled
development
of coasts
Deforestation
Dredging and
coastal
development
Hotels
Homes
harbors
Soil run-off - Hawaii
Runoff from a pipe
in the Virgin Islands,
directly near a reef

41. Human Impacts on Reefs
Recreation
Boat anchors crush
corals and gouge
the reef
Diving tourism
Divers and snorklers
sit, stand on, or
handle corals

42. Human Impacts on Reefs
Collection of corals and organisms
International seashell and aquarium trades
have put a strain on coral reef ecosystems
Upsets the balance of the ecosystem
Market for coral skeletons, shells, sponges and
reef animals for decorations, jewelry,
sculptures
Tropical fish trade
Coral skeletons are are sold as “live rock”
Popular in aquariums because of the living
bacteria and algae that acts as a natural
biological filter

44. Human Impacts on Reefs
Uncontrolled mining for building materials

45. Human Impacts on Reefs
Destructive fishing practices
Overharvesting of species can
lead to the species’ demise
Fishing with dynamite, cyanide,
or bleach
Careless handling of nets, lines,
and lobster traps can lead to
reef damage
Boating / anchors also leads to
reef damage
Between 1986 – 1991, ½ of the
coral reefs in the Philippines
were destroyed by this
Coca-Cola bottle with
slightly burned fuse
found around the
Capone Islands,
Phillipines

46. Human Impacts on Reefs
Natural disturbances:
Hurricanes and tropical storms can strip
corals from miles of reef habitat

47. Human Impact on Reefs
Increases in atmospheric carbon dioxide
increases the acidity of the ocean
Higher ocean acidity makes it harder for
reefs to construct their calcium carbonate
skeletons
(remember what happened when you added
vinegar to your reef in the lab?)

48. Coral Bleaching
Occurs when corals
become stressed
They expel the
zooxanthellae living in
them
They lose their color and
become whitened
Can be caused with
warmer surface waters
(maybe even by 1-2
degrees)
May have a link to global
warming

49. Pollution can also be a cause
Full recovery takes decades

51. Bleaching can be seen in areas of the Great Barrier Reef

53. Coral Poisoning
Very destructive fishing practice
A diver sprays sodium cyanide to catch a fish
The poison kills all corals in the area

56. Coral Reef Diversity
Healthy reefs can house thousands of
species
Fish
Invertebrates
Sponges
Soft corals
Every part of a reef is used by some animal
Mutualism – relationship between two
different species where they both benefit