Coral Reef Ecology
I. Importance of Coral Reefs
II. Distribution
III. Reef Structure/Zonation
IV. Key Taxa
V. Coral Biology
VI. Productivity & Diversity
VII. Biotic Interactions
VIII.Abiotic Disturbance
IX. The Future of Coral Reefs
I. Importance of Coral Reefs
• largest biologically formed structures in world
(e.g., Great Barrier Reef is 2000 km long & 150 km wide)
• greatest taxonomic diversity of all marine habitats
(~1 million species)
• remove ~700 billion kg of CO2/yr
• but cover only 0.71% of area of planet
Provide
- food (e.g., feed 1 billion people in Asia)
- income (fisheries & tourism)
- cultural value
- esthetic value aerial photo of the Great
Barrier Reef, Australia
- pharmaceuticals
- protection from waves/erosion
II. Distribution Pattern of Diversity
• worldwide in tropical seas • diversity highest in Indo-Pacific
• lower abundance on western margin continents,
where upwelling occurs
www.noaa.gov
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Conditions for coral reef growth III. Reef Structure/Zonation
• warm water: 18-40 ºC
Reef Formation:
• shallow with light (at least 1% surface intensity <70m depth)
fringing barrier atoll
• salinity: fully marine
(process results from rising
• low sediment sea level + subsidence of
• limited emersion: <1hr underlying land)
• moderate water motion
Reef Zones
• reef flat Barrier reefs have lagoons
• reef crest
• reef slope
IV. Key Taxa Hard corals
• Hard Corals
– stony corals stony corals (order Scleractinia) stony hydrozoans (class Hyrdozoa)
– stony hydrozoans (fire coral)
• Sessile Invertebrates
• Macroalgae
• Mobile Invertebrates
• Fishes
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ascidians
Sessile Invertebrates Algae macroalgae
soft corals
sponges
turf algae coralline algae
gorgonians
hydroids
Mobile Invertebrates Fishes
• echinoderms
• crustaceans
• polychaetes
• molluscs
Functional groups on reefs Primary Producers
• Primary producers
• Herbivores
- bioeroders – remove coral skeletons
- scrapers – remove algae and sediments
- grazers – remove macroalgae
• Planktivores
- fishes & invertebrates, including corals
• Predators
- piscivores, corallivores, invert eaters
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Bioeroders
V. Coral Biology
1. Coral Phylogeny
2. Coral Morphology
3. Life Cycle
4. Sexual Reproduction
5. Asexual Reproduction
6. Coral-Algal Symbiosis
Coral Morphology: hermatypic (reef building) corals
Corals are cnidarians (like sea anemones, jellies, etc.)
Slime covered rocks?
• thin layer of tissue on top of an inorganic skeleton (CaCO3)
hydroids,
fire coral
corals,
box anemones,
jellies etc.
jellies
2mm
Hermatypic corals build CaCO3 skeletons from CO2 and Coral Morphology - corallite structure is variable
calcium in sea water
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corallite structure
Individual polyp within its carbonate calyx removed from
colony, showing relationship of tissue to calcium carbonate No Tissue Tissue Intact
Coral Morphology - colony structure varies Life cycle
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Sexual Reproduction Sexual Reproduction
Asexual Reproduction
Sexual Reproduction
• Fragmentation: portions of a colony break off, then
reattach to the substrate and continue to grow
Molecules move between coral tissue & zooxanthellae
Coral-Algal Symbiosis
• Corals & Zooxanthellae
Zooxanthellae
Zooxanthellae…
- unicellular dinoflagellate algae
- use photosynthesis to fix carbon
- some of the fixed carbon is passed to
the host (coral)
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VI. Productivity & Diversity
VI. Productivity & Diversity High Productivity
• (especially considering surrounding waters are
oligotrophic)
• extremely high productivity
Production (kg Carbon per m2 per year)
• extremely high diversity
Average Oceanic areas 0.1 kg
Rainforest 2 kg
Kelp forest 2 kg
Coral Reef 1.5-5 kg
• high productivity possible because of tight recycling of nutrients,
photosynthetic fixation of carbon (by corals and algae) and nitrogen
(by blue-green algae)
High Diversity Physical structure (cracks and crevices) provides
refuge from predators
• similar species diversity per unit area to tropical rain forests
• highest diversity on earth of higher level taxa (Phyla)
• sustained by high productivity & structural complexity
Facilitation
VII. Biotic Interactions
• common on coral reefs
• e.g., corals provide food and refuge for most coral-reef
• facilitation organisms
• mutualism
• competition
• herbivory
• predation
• disease
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Mutualism
Competition
Limited resources:
• space (e.g., competition among corals)
• light (e.g., competition among corals & algae)
• food (e.g., competition among fishes)
• shelter (e.g., competition among fishes)
Competition for space: corals vs. corals Competition for space: corals vs. macroalgae
• macroalgae have faster growth
rates than corals
• macroalgae respond more rapidly
to nutrient addition
sweeper tentacles
Hughes
1994
Herbivory Predation
• keeps macroalgae from outcompeting corals • on corals
• on other organisms
“wall of mouths”
removes plankton
sea star Acanthaster:
coral predator
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Disease Coral Diseases
• coral diseases black band disease
• occur in hard and soft corals
• Diadema mass mortality
• most pathogens are not known
Diadema before mass mortality:
95-99% died
• temperature may contribute (e.g.,
by increasing bacterial growth)
yellow band
white plague
S. Talmage photo
Storms: hurricanes and cyclones
VIII. Abiotic Disturbance
• storms (water motion)
• water temperature
• sedimentation
Water Temperature & Coral Bleaching Water Temperature & Coral Bleaching
• bleaching occurs when corals expel their symbiotic algae
(zooxanthellae)
• triggered by stress, mainly too warm water
Red dots show areas with
• stress causes mutualism breaks down most severe coral bleaching
(1997-1998 event)
• coral die if the stress is extreme or prolonged (coral starves)
NOAA sea surface temperature
anomalies for January 1998
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Water Temperature & Coral Bleaching Global Air Temperature Anomaly
warm year “normal” year
www.noaa.gov
March 20, 1998 March 21, 2000
Sedimentation IX. The Future of Coral Reefs
Declines in coral abundance (% cover)
• smothers corals
• 80% decline on Caribbean reefs over
• reduces water clarity last few decades
• 50% decline on Great Barrier Reef
GBR
• good news: much less in most of the
Indo-Pacific
Gardner et al. 2003 Science
Caribbean
Bellwood et al. 2004 Nature
Causes of decline of coral reefs Phase shift: corals macroalgae
• climate change (e.g., global warming bleaching) Jamaican reefs – intensively studied since 1950s
• storms
• Overfishing: 1960’s – reduced fish biomass by 80% -
• diseases herbivorous fishes
• Hurricane Allen: 1980 – destroyed lots of large shallow-
• predator outbreaks
water corals
• pollution, runoff, sedimentation (due to coastal • Urchin disease: 1982 – reduced urchin population by 99%
development)
• Bleaching – 1987, 1989, 1990
• changes in community structure (e.g., due to overfishing,
including with dynamite and cyanide)
• coral cover 52% 3%
• ship groundings & anchor damage • macroalgal cover 4% 92%
Result
phase shift (coral macroalgal dominated reefs)
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Phase shift: is it reversible?
Pre 1983 Post 1983
Macroalgal cover down Coral recruitment up
abundant free space for coral little free space for coral
settlement settlement
Trends in Atmospheric CO2 Ocean Acidification: increases in atmospheric CO2 are predicted
to acidify ocean and reduce rate of calcification -- which will
cause coral reefs to degrade
• CO2 lowers pH of ocean
• CaCO3 dissolves or doesn’t form properly
• CaCO3 shells, casings, skeletons affected
Kleypas et al. 1999
Predicted future of coral reefs Summary
• Coral reefs may cease to exist in 20 years
– (Jackson et al. 2001, Pandolfi et al. 2003, Hughes et al. 2003)
• coral reefs are the largest biologically formed structures in
the world
• Coral reefs will exist, but composition will change • highest taxonomic diversity in the world is found on coral
– (Loya et al. 2001, Tamelander 2002, Edmunds 2004) reefs
• very high primary production due to tight recycling
• facilitation common – e.g., coral skeletons provide shelter
• corals contain symbionts (zooxanthellae) which
photosynthesize
• coral reefs are in trouble (e.g., bleaching, diseases,
macroalgal overgrowth)
• future of coral reefs is uncertain
Pandolfi et al. 2005 Science
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