Coral reefs health and hazards
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Coral Reefs—health and hazards. Dr. Maia McGuire University of Florida/Sea Grant. Photo by Mike White, FKNMS. What is a Coral?. Animal, vegetable or mineral?. It’s an animal which may live with a plant and makes a mineral-based skeleton.

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Coral reefs health and hazards l.jpg

Coral Reefs—health and hazards

Dr. Maia McGuire

University of Florida/Sea Grant

Photo by Mike White, FKNMS

What is a coral l.jpg
What is a Coral?

Animal, vegetable or mineral?

It’s an animal which may live with a plant and makes a mineral-based skeleton.

Illustration by Geoff Kelley in JEN Veron (2000) Corals of the World, AIMS, Townsville

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Coral structure

  • Individual animals are called polyps

  • Several polyps make up a colony

  • Corals are closely related to jellyfish and sea anemones—they all contain stinging cells called nematocysts.

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Coral reefs

  • Reef-building corals require clear, warm water

    • Shallow areas

    • Most reefs are between 26° N and S latitude

  • There are corals found off Alaska and other cold waters, but they grow very slowly and do not form reefs

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E.C. Peters

  • Many hard and soft corals (and some jellyfish and sea anemones) contain a symbiotic single-celled brown dinoflagellate (algae) called zooxanthellae

  • This is a mutualistic symbiosis

  • Bleaching occurs when corals lose their zooxanthellae

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Coral Bleaching

  • Response to stress

    • Temperature

    • UV

    • Oxygen

    • Darkness

    • Sedimentation

  • Who initiates bleaching?

  • Can corals recover?

J. Hoggesteger

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Patterns of bleaching

  • Seasonal—usually in summer

  • Geographically, often begins in the Caribbean and spreads northwards

  • Physically, there are often bleached patches on the sides of large coral heads (especially Montastraeas)

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Zooxanthellae “clades”

  • Three groups of zooxanthellae have been identified

    • Each group has different environmental tolerances and is found in different locations on the coral head

Photo by Scott R. Santos, SUNY Buffalo

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Where do zooxanthellae go?

  • “Free” zooxanthellae are rarely found in plankton samples

    • May become benthic, sessile

    • May go into dormant stage

  • Presumably free zooxanthellae do exist

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Coral reproduction

  • Asexual reproduction (usually in branching corals; often storm-related)

  • Sexual reproduction

    • Hermaphroditic or gonochoristic

    • Self- or cross-fertilization

    • Internal or external fertilization

    • Brooders or broadcast spawners

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Bette Willis in J.E.N. Veron (1987) Corals of Australia and the Indo-Pacific. Hawaii University Press

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  • Eggs develop into larvae internally and may be brooded for several weeks

  • Larvae are released, often at night, often in response to the lunar cycle

  • Brooders may have up to 12 reproductive cycles per year

R. Hays Cummins

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Coral larvae (planulae)

  • Swim using tiny hairs (cilia)

  • May already contain zooxanthellae

  • Swim for days to weeks before they settle and metamorphose

  • Settlement may be in response to chemical cues and may be gregarious

Bob Richmond


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Status of coral reefs

  • Globally, coral reefs are generally in decline

  • Increasing human population (especially in coastal areas) increases the impacts on coral reefs

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Human impacts on coral reefs

  • Overfishing

  • Sedimentation

  • Nutrient enrichment

  • Chemicals/oil

  • Physical damage (anchors, fishing, groundings)

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  • Changes trophic structure

  • Many large predators are no longer present

  • Grazing fish species are being collected as food fish

  • May allow algal overgrowth of corals


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Nutrient enrichment

  • Nutrients are elements needed for growth

  • If there are not enough of certain types of nutrients, they are said to be limiting nutrients

  • Most common limiting nutrients in the marine environment are N and P

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  • Available in water as nitrate, nitrite, ammonium or organic nitrogen (e.g. urea, plant or animal tissues)


  • Available in water as dissolved inorganic phosphate or organic phosphorus (dissolved or particulate)

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How does nutrient enrichment occur?

  • Septic tanks/sewage

    • Leaks

    • Pumping into the ocean

  • Fertilizer runoff

    • Agricultural

    • Homeowners

    • Golf courses

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Effects of increasing nutrients

  • Cause increase in plant (algae) growth

    • Macroalgae

    • Microalgae (phytoplankton)

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HAB’s/Red tides

  • Blooms of “harmful algae”

    • Pfisteria

    • Cause human health problems

    • Cause fish kills

    • May be killing dolphins, manatees

Mote MarineLab


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Effects of increased nutrients on corals

  • Cause decrease in coral growth

    • Direct chemical interference with skeleton formation

    • Result of overshading by algae

Jennifer M. Smith

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The nutrient-calcification mystery

  • If zooxanthellae help corals calcify, then why do enriched corals, which contain more zooxanthellae, calcify less?

    • Zooxanthellae are N-limited

    • “Excess” photosynthate is given to coral

    • If zooxanthellae grow, there is less photosynthate to give to the corals

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  • There are more zooxanthellae per cm² of coral, so the animal receives the same amount of carbon…


  • Is the type of carbon compound different in enriched and control corals?

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  • Non point-source pollution

    • 51% of the oil entering the oceans is from runoff

    • 5% is from big spills

    • 19% is from routine maintenance

    • 2% is from offshore drilling

    • 13% is from burning fuels (e.g. car exhaust)

    • 10% is from natural seeps

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Physical damage

Thomas Heeger, Philippines

  • Fishing techniques in the South Pacific include dynamiting or poisoning reefs to collect aquarium fish for export

  • Boat anchors and boat/ship groundings cause damage that can take thousands of years to re-grow

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Marine debris

  • Suffocation risk

    • Balloons/bags

  • Entanglement/entrapment

    • Fishing line/ropes

    • Old nets

    • Abandoned traps/pots

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What can you do?

  • Reduce, reuse, recycle

    • Motor oil

    • Fishing line

  • Read and follow instructions on chemicals, including fertilizers

  • Fix automotive leaks

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What does the future hold?

The answer is up to you…