Harsha Nayak

DFK-1204

I PhD, Dept. of AEM

Introduction Coral colonies and reefs form as calcareous (calcium

carbonate) skeletons are deposited and bound by corals.

Living hard coral polyps secrete calcium carbonate

skeletons from their basal plates to form protective cups

in which they then sit.

These skeletons are added to as the coral polyps lift

periodically from their cups to secrete new skeletons in

which they will then rest

As coral polyps ‘bud’ (a form of asexual reproduction), newcoral polyps develop forming additional basal plates fromwhich further calcareous skeletons are secreted.

Also as hard corals die, their skeletons remain within theframework to provide a platform for future coral growth.

These platforms allow new corals to settle and grow overtheir predecessors.

Overtime these processes lead to gradual growth of thecoral reef framework.

Latitude

Temperature

Light

Salinity

Turbidity & sedimentation

Wave action

Aerial exposure

Firm substrata

Growth

Abiotic factors Biotic factors

Species

interactions

(mutualism,

competition,

predation

Latitude

Reef-building corals are restricted in their geographicdistribution.

This is because the algal-cnidarian symbioticmachinery needs a narrow and consistent band ofenvironmental conditions to produce the copiousquantities of limestone necessary for reef formation.

The formation of highly consolidated reefs only occurwhere the temperature does not fall below 18°C forextended periods of time.

High calcification rates are limited to warm waters.Consequently, coral reefs are restricted to tropical seas,generally between 250 N and 250 S latitudes.

Temperature

Reef building corals cannot tolerate water

temperatures of less than 180C (except as in the

Florida Keys of the United States), although some

corals tolerate temperatures of up to 400C , optimum

growth usually occurs between 230C and 290C.

Light

After temperature, light is probably the most important limiting factor to well-developed coral reefs because of the symbiosis between hermatypic scleractinian corals and zooxanthellae

Since light intensity decreases exponentially with depth, active reef building rarely occurs below about 20-30m.

PAR

Salinity Hermatypic corals seem to require open-ocean salinity

Well-developed reefs are not generally found in estuarine orexcessively hypersaline conditions(Persian Gulf reefs,however, develop in salinities of more than 40ppt)

The absence of reefs along much of the tropical coastline ofthe Atlantic coast of South America is due to the influenceof large rivers such as the Amazon and Ornico

High rains, resulting in excessive runoff, cause extensivedamage to corals close to river mouths

Turbidity & Sedimentation

High turbidity and sedimentation rates strongly inhibitreef growth.

Turbidity reduces light penetration whilst, settling siltparticles smother the feeding structures of the delicatecoral polyps.

Rivers carry a large suspended sediment load to thecoastal areas periodically, which is detrimental to corals

Wave action Coral reefs thrive in regions of strong water movement.

Wave action prevents sedimentation and keeps the waterwell oxygenated.

Although the rigid calcareous skeletons of most hermatypiccorals are particularly resistant to wave shock, severe stormsdo occasionally cause extensive damage.

Storm damage can, in fact, be a mechanism of coraldispersal, for pieces of living colonies transported to newsites may survive to cement to the bottom and establish apermanent new colony

Aerial exposure

Corals are unable to withstand prolonged aerial exposure and rarely grow much above the low spring-tide level.

The upward growth of a reef is restricted to the level of lowest tides, as exposure to air for more than several hours kills corals

Firm substrata

New reefs are initially formed by the attachment of meroplanktonic coral larvae (planulae) to a hard substrate, so a firm platform is always necessary for establishment and growth

Mutualism Symbiotic relationships are especially important in

structuring coral-reef communities.

between Zooxanthellae, coral polyp

Carbondioxide, ammonium phosphate

Oxygen, photosynthate

Competition Competition for limited living space on coral reefs is

potentially severe

Fast - growing branching corals are capable of overtopping slow – growing encrusting or massive forms

Coexistence is facilitated however by the combined effects of physical disturbance (storms) and aggressive behaviour(tissue necrosis) of the slower – growing species

PREDATION The major groups of coral predators are star fish, sea

urchins and fish

Acanthaster planci

giant triton (Charonia tritonis)

Fish species that feed directly on coral polyps (puffer fish, file fish, trigger fish)

Surgeon fish, parrot fish digests the algae and endolithicfauna

Natural Sources of Stress on Coral Reefsintense storms

El Niño

disease

volcanic eruptions

predator population explosions

natural stream and river runoff

exceptionally low tides

Storm Wave Damage

Before After

Storm Damage

Crown-of-Thorns Outbreaks

Stream and River Runoff

Low Tides

Acid Rain in Marine Environment

• reduces ability of marine

organisms to utilize calcium

carbonate

• Coral calcification

rate reduced

15-20%

• Skeletal density

decreased,

branches thinner

Nutrients and Algae Growth

Sediments

Bleaching

Bleaching

Some Local Human Impacts

thermal effluents

sewage discharges and agricultural runoff

mechanical damage to reefs

sedimentation

destructive resource extraction practices

introduced species

Global Human Impacts

Global warmingUV radiationOcean acidification

Corals and UV Radiation decreased growth

decreased rates of calcification

transplantation experiments (deep corals brought to the surface) demonstrate corals may be UV-sensitive (exhibit bleaching and increased mortality)

coral sperm appears to be UV-sensitive (note spawning normally takes place at night)

Possible Consequences of Stresses and Impacts on Corals and Coral Reefs

outright mortality of coral tissues

breakage of coral colonies

bleaching

diseases

slower growth

reduced reproduction and recruitment

competitive exclusion by other organisms

increased reef erosion

Modern Uses of Coral Reefs Seafood

Food Additives and Toiletries

Health and Medicine Products

Research and Education

Jewelry and Art

Marine Aquarium Specimens

New Land

Cement & Other Building Supplies

Shoreline Protection

Recreation

THANK YOU