Fall Asleep While Learning About Coral Reefs

In this episode of the I Can't Sleep Podcast, fall asleep while learning about coral reefs. Did you know that coral reefs are part of the same class as jellyfish? What?! That's so fascinating to me. Thankfully, that information is right at the beginning of the article, so you should learn that much before dozing off.

Happy sleeping!

Transcript

Welcome to the I Can't Sleep podcast,

Where I read random articles from across the web to bore you to sleep with my soothing voice.

I'm your host,

Benjamin Boster.

Today's episode is from a Wikipedia article titled,

Coral Reef.

A coral reef is an underwater ecosystem characterized by reef-building corals.

Reefs are formed of colonies of coral polyps held together by calcium carbonate.

Most coral reefs are built from stony corals,

Whose polyps cluster in groups.

Coral belongs to the class Anthozoa in the animal phylum Notaria,

Which includes sea anemones and jellyfish.

Unlike sea anemones,

Corals secrete hard carbonate exoskeletons that support and protect the coral.

Most reefs grow best in warm,

Shallow,

Clear,

Sunny,

And agitated water.

Coral reefs first appeared 485 million years ago,

At the dawn of the early Ordovician,

Displacing the microbial and sponge reefs of the Cambrian.

Sometimes called rainforests of the sea,

Shallow coral reefs form some of Earth's most diverse ecosystems.

They occupy less than 0.

1% of the world's ocean area,

About half the area of France.

Yet they provide a home for at least 25% of all marine species,

Including fish,

Mollusks,

Worms,

Crustaceans,

Echinoderms,

Sponges,

Tunicates,

And other cnidarians.

Coral reefs flourish in ocean waters that provide few nutrients.

They are most commonly found at shallow depths in tropical waters,

But deep water and cold water coral reefs exist on smaller scales in other areas.

Coral reefs deliver ecosystem services for tourism,

Fisheries,

And shoreline protection.

The annual global economic value of coral reefs has been estimated at anywhere from 30 to 370 billion dollars.

From $25 billion dollars,

1997 and 2003 estimates,

To $2.

7 trillion dollars,

A 2020 estimate,

And $9.

9 trillion dollars,

A 2014 estimate.

Though the shallow water tropical coral reefs are best known,

There are also deeper water reef-forming corals,

Which live in colder water and in temperate seas.

Most coral reefs were formed at the last glacial period,

When melting ice caused sea levels to rise and flood continental shelves.

Most coral reefs are less than 10,

000 years old.

As communities established themselves,

The reefs grew upwards,

Pacing rising sea levels.

Reefs that rose too slowly could become drowned.

Without sufficient light.

Coral reefs are also found in the deep sea,

Away from continental shelves,

Around oceanic islands and atolls.

The majority of these islands are volcanic in origin.

Others have tectonic origins,

Where plate movements lifted the deep ocean floor.

In The Structure and Distribution of Coral Reefs,

Charles Darwin set out his theory of the formation of atoll reefs,

An idea he conceived during the voyage of the Beagle.

He theorized that uplift and subsidence of earth's crust under the oceans formed the atolls.

Darwin set out a sequence of three stages in atoll formation.

A fringing reef forms around an extinct volcanic island,

As the island and ocean floor subside.

As the subsidence continues,

The fringing reef becomes a barrier reef,

And ultimately an atoll leaf.

Darwin predicted that underneath each lagoon would be a bedrock base,

The remains of the original volcano.

Subsequent research supported this hypothesis.

Darwin's theory followed from his understanding that coral polyps thrive in the tropics,

Where the water is agitated,

But can only live within the tropics.

But can only live within the tropics,

Where the water is agitated,

But can only live within a limited depth range,

Starting just below low tide.

Where the level of the underlying earth allows,

The corals grow around the coast to form fringing reefs,

And can eventually grow to become a barrier reef.

Where the bottom is rising,

Fringing reefs can grow around the coast,

But coral raised above sea level dies.

If the land subsides slowly,

The fringing reefs keep pace by growing upwards on a base of older,

Dead coral,

Forming a barrier reef and closing a lagoon between the reef and the land.

A barrier reef can encircle an island,

And once the island sinks below sea level,

A roughly circular atoll of growing coral continues to keep up with the sea level,

Forming a central lagoon.

Barrier reefs and atolls do not usually form complete circles,

But are broken in places by storms.

Like sea level rise,

A rapidly subsiding bottom can overwhelm coral growth,

Killing the coral and the reef,

Due to what is called coral drowning.

Corals that rely on zooxanthellae can die when the water becomes too deep for their symbionts to adequately photosynthesize,

Due to decreased light exposure.

The two main variables determining the geomorphology or shape of coral reefs are the nature of the substrate on which they rest,

And the history of the change in sea level relative to that substrate.

The approximately 20,

000-year-old Great Barrier Reef offers an example of how coral reefs formed on continental shelves.

Sea level was then 120 meters lower than in the 21st century.

As sea level rose,

The water and the corals encroached on what had been hills of the Australian coastal plain.

By 13,

000 years ago,

Sea level had risen by 60 meters,

Lower than at present,

And many hills of the coastal plains had become continental islands.

As sea level rise continued,

Water topped most of the continental islands.

The corals could then overgrow the hills,

Forming cays and reefs.

Sea level on the Great Barrier Reef has not changed significantly in the last 6,

000 years.

The age of living reef structure is estimated to be between 6,

000 and 8,

000 years.

Although the Great Barrier Reef formed along a continental shelf and not around a volcanic island,

Darwin's principles apply.

Development stopped at the Barrier Reef stage since Australia is not about to submerge.

It formed the world's largest barrier reef,

300 to 1,

000 meters from shore,

Stretching for 2,

000 kilometers.

Healthy tropical coral reefs grow horizontally from 1 to 3 centimeters per year,

And grow vertically anywhere from 1 to 25 centimeters per year.

However,

They grow only at depths shallower than 150 meters because of their need for sunlight,

And cannot grow above sea level.

As the name implies,

Coral reefs are made up of coral skeletons from mostly intact coral colonies.

As other chemical elements present in corals become incorporated into the calcium carbonate deposits,

Argonite is formed.

However,

Shell fragments and the remains of coral and algae,

Such as the green-segmented genus Halimeda,

Can add to the reef's ability to withstand damage from storms and other threats.

Such mixtures are visible in structures such as Inuitak Atoll.

Since Darwin's identification of the three classical reef formations,

The fringing reef around a volcanic island becoming a barrier reef and then an atoll,

Scientists have identified further reef types.

While some sources find only three,

Thomas lists four major forms of large-scale coral reefs,

The fringing reef,

Barrier reef,

Atoll,

And table reef based on Stoddart,

D.

R.

1969.

Spalding,

Et al.

List four main reef types that can be clearly illustrated.

The fringing reef,

Barrier reef,

Atoll,

And bank and platform reef.

And notes that many other structures exist which do not conform easily to strict definitions,

Including the patch reef.

A fringing reef,

Also called a shore reef,

Is directly attached to a shore or borders it with an intervening narrow shallow channel or lagoon.

It is the most common type.

Fringing reefs follow coastlines and can extend for many kilometers.

They are usually less than 100 meters wide,

But some are hundreds of meters wide.

Fringing reefs are initially formed on the shore at the low water level and expand seawards as they grow in size.

The final width depends on where the seabed begins to drop steeply.

The surface of the fringe reef generally remains at the same height,

Just below the waterline.

In older fringing reefs,

Whose outer regions pushed far out into the sea,

The inner part is deepened by erosion and eventually forms a lagoon.

Fringing reef lagoons can become over 100 meters wide and several meters deep.

Like the fringing reef itself,

They run parallel to the coast.

The fringing reefs of the Red Sea are some of the best developed in the world and occur along all its shores except off sandy bays.

Barrier reefs are separated from a mainland or island shore by a deep channel or lagoon.

They resemble the later stages of a fringing reef with its lagoon,

But differ from the latter mainly in size and origin.

Their lagoons can be several kilometers wide and 30 to 70 meters deep.

Above all,

The offshore outer reef edge forms in open water rather than next to a shoreline.

Like an atoll,

It is thought that these reefs are formed either as the seabed lowered or sea levels rose.

Formation takes considerably longer than for a fringing reef,

Thus barrier reefs are much rarer.

The best known and largest example of a barrier reef is the Australian Great Barrier Reef.

Other major examples are the Mesoamerican Barrier Reef System and the New Caledonian Barrier Reef.

Barrier reefs are also found on the coasts of Providencia,

Mayotte,

The Gambier Islands,

On the southeast coast of Kalimantan,

On parts of the coast of Sulawesi,

Southeastern New Guinea,

And the south coast of the Louisiana Archipelago.

Platform reefs,

Variously called bank or table reefs,

Can form on the continental shelf as well as in the open ocean.

In fact,

Anywhere where the seabed rises close enough to the surface of the ocean to enable the growth of zooxanthemic reef-forming corals.

Platform reefs are found in the Southern Great Barrier Reef,

The Swain and Capricorn Group on the continental shelf,

About 100 to 200 kilometers from the coast.

Some platform reefs of the Mascarenes are several centimeters deep,

And most of the Mascarenes are several thousand kilometers from the mainland.

Unlike fringing and barrier reefs,

Which extend only seaward,

Platform reefs grow in all directions.

They are variable in size,

Ranging from a few hundred meters to many kilometers across.

Their usual shape is oval to elongated.

Parts of these reefs can reach the surface and form sandbanks and small islands on which many form fringing reefs.

A lagoon may form in the middle of a platform reef.

Platform reefs are typically situated within atolls,

Where they adopt the name patch reefs,

And often span a diameter of just a few dozen meters.

In instances where platform reefs develop along elongated structures,

Such as old and weathered barrier reefs,

They tend to arrange themselves in a linear formation.

This is the case,

For example,

On the east coast of the Red Sea near Jeddah.

In old platform reefs,

The inner part can be so heavily eroded that it forms a pseudo-atoll.

These can be distinguished from real atolls only by detailed investigation,

Possibly including core drilling.

Some platform reefs of the Lackadaise are U-shaped due to wind and water flow.

Atolls,

Or atoll reefs,

Are a more or less circular or continuous barrier reef that extends all the way around a lagoon without a central island.

They are usually formed from fringing reefs around volcanic islands.

Over time,

The island erodes away and sinks below sea level.

Atolls may also be formed by the sinking of the seabed or rising of the sea level.

A ring of reefs results,

Which enclose a lagoon.

Atolls are numerous in the South Pacific,

Where they usually occur in mid-ocean,

For example in the Caroline Islands,

The Cook Islands,

French Polynesia,

The Marshall Islands,

And Micronesia.

Atolls are found in the Indian Ocean,

For example in the Maldives,

The Chagos Islands,

The Seychelles,

And around Cocos Island.

The entire Maldives consists of 26 atolls.

Apron Reef Short reef resembling a fringing reef,

But more sloped.

Extending out and downward from a point of peninsular shore.

Initial stage of a fringing reef.

Bank Reef Isolated,

Flat-topped reef,

Larger than a patch reef,

Usually on mid-shelf regions and linear or semi-circular in shape.

A type of platform reef.

Patch Reef Common,

Isolated,

Comparatively small reef outcrop,

Usually within a lagoon or embayment.

Often circular and surrounded by sand or seagrass.

Can be considered as a type of platform reef,

Or as features of fringing reefs,

Atolls,

And barrier reefs.

The patches may be surrounded by a ring of reduced seagrass cover,

Referred to as a grazing halo.

Ribbon Reef Long,

Narrow,

Possibly winding reef,

Usually associated with an atoll lagoon.

Also called a shelf-edge reef or sill reef.

Drying Reef A part of a reef which is above water,

At low tide,

But submerged at high tide.

Habili Reef specific to the Red Sea.

Does not reach near enough to the surface to cause visible surf.

May be a hazard to ships.

From the Arabic for unborn.

Micro-Atoll Community of species of corals.

Vertical growth limited by average tidal height.

Growth morphologies offer a low resolution record of patterns of sea level change.

Fossilized remains can be dated using radioactive carbon dating,

And have been used to reconstruct Holocene sea levels.

Keys Small,

Low elevation,

Sandy islands,

Formed on the surface of coral reefs from eroded material that piles up.

Forming an area above sea level.

Can be stabilized by plans to become habitable.

Occur in tropical environments throughout the Pacific,

Atlantic,

And Indian Oceans,

Including the Caribbean and on the Great Barrier Reef and Belize Barrier Reef,

Where they provide habitable and agricultural land.

Seamount or guillo Seamounts are formed when a coral reef on a volcanic island subsides.

Tops of seamounts are rounded and guillos are flat.

Flat tops of guillos,

Or table mounts,

Are due to erosion by waves,

Winds,

And atmospheric processes.

Coral reef ecosystems contain distinct zones that host different kinds of habitats.

Usually,

Three major zones are recognized.

The fore-reef,

Reef crest,

And the back-reef,

Frequently referred to as the reef lagoon.

The three zones are physically and ecologically interconnected.

Reef life and oceanic processes create opportunities for the exchange of seawater,

Sediments,

Nutrients,

And marine life.

Most coral reefs exist in waters less than 50 meters deep.

Some inhabit tropical continental shelves where cool,

Nutrient-rich upwelling does not occur,

Such as the Great Barrier Reef.

Others are found in the deep ocean surrounding islands,

Or as atolls,

Such as the Maldives.

The reefs surrounding islands form when islands subside into the ocean,

And atolls form when an island subsides below the surface of the sea.

Alternatively,

Moyle and Chek distinguish six zones,

Though most reefs possess only some of the zones.

The reef surface is the shallowest part of the reef.

It is subject to surge and tides.

When waves pass over shallow areas,

They shoal.

This means the water is often agitated.

These are the precise conditions under which corals flourish.

The light is sufficient for photosynthesis by the symbiotic zooxanthellae,

And agitated water brings plankton to feed the coral.

The off-reef floor is the shallow sea floor surrounding a reef.

This zone occurs next to reefs on continental shelves.

Reefs around tropical islands and atolls drop abruptly to great depths and do not have such a floor.

Usually sandy,

The floor often supports seagrass meadows,

Which are important foraging areas for reef fish.

The reef drop-off is,

For its first 50 meters,

Habitat for reef fish,

Who find shelter on the cliff face and plankton in the water nearby.

The drop-off zone applies mainly to the reefs surrounding oceanic islands and atolls.

The reef face is the zone above the reef floor,

Or the reef drop-off.

This zone is often the reef's most diverse area.

Coral and calcareous algae provide complex habitats and areas that offer protection,

Such as cracks and crevices.

Invertebrates and epiphytic algae provide much of the food for other organisms.

A common feature on this fore-reef zone is spur and groove formations that serve to transport sediment downslopes.

The reef flat is the sandy bottom flat,

Which can be behind the main reef,

Containing chunks of coral.

This zone may border a lagoon and serve as a protective area,

Or it may lie between the reef and the shore,

And in this case is a flat,

Rocky area.

Fish tend to prefer it when it is present.

The reef lagoon is an entirely enclosed region,

Which creates an area less affected by wave action and often contains small reef patches.

However,

The topography of coral reefs is constantly changing.

Each reef is made up of irregular patches of algae,

Sessile invertebrates,

And bare rock and sand.

The size,

Shape,

And relative abundance of these patches change from year to year in response to the various factors that favor one type of patch over another.

Growing coral,

For example,

Produces constant change in the fine structure of reefs.

On a larger scale,

Tropical storms may knock out large sections of reef and cause boulders on sandy areas to move.

Coral reefs are estimated to cover 284,

300 square kilometers,

Just under 0.

1% of the ocean's surface area.

The Indo-Pacific region account for 91.

9% of this total.

Southeast Asia accounts for 32.

3% of that figure,

While the Pacific,

Including Australia,

Accounts for 40.

8%.

Atlantic and Caribbean coral reefs account for 7.

6%.

Although corals exist both in temperate and tropical waters,

Shallow water reefs form only in a zone extending from approximately 30 degrees north to 30 degrees south of the equator.

Tropical corals do not grow at depths of over 50 meters.

The optimum temperature for most coral reefs is 26 to 27 degrees Celsius,

And few reefs exist in waters below 18 degrees Celsius.

When the net production by reef-building corals no longer keeps pace with relative sea level,

And the reef structure permanently drowns,

A Darwin point is reached.

One such point exists at the northwestern end of the Hawaiian archipelago.

However,

Reefs in the Persian Gulf have adapted to temperatures of 13 degrees Celsius in winter and 38 degrees Celsius in summer.

37 species of Sclerctinian corals inhabit such an environment around Larack Island.

Deepwater coral inhabits greater depths and colder temperatures at much higher latitudes,

As far north as Norway.

Although deepwater corals can form reefs,

Little is known about them.

The northernmost coral reef on Earth is located near Eilat,

Israel.

Coral reefs are rare along the west coast of the Americas and Africa,

Due primarily to upwelling and strong cold coastal currents that reduce water temperatures in these areas.

Corals are seldom found along the coastline of South Asia from the eastern tip of India as well as along the coasts of northeastern South America and Bangladesh due to the freshwater release from the Amazon and the Ganges rivers,

Respectively.

Reviews

I wake up from a deep slumber and can't even remember the topic! You're the best, Benjamin.

I can listen to this one again, I think I fell asleep in the first 5 minutes!! Thanks Ben!

I like articles about underwater. I was scuba diving in my dreams.

Sleep has been elusive lately so I’m very thankful for you and the other teachers putting out new content. This was perfect, it at least relaxed me! 🤗🤗