All language subtitles for BBC.Natures.Microworlds.08of13.Great.Barrier.Reef.720p.HDTV.x264.AAC.MVGroup.org Eng

Our planet is the greatest
living puzzle in the universe.

A collection of worlds
within worlds.

Each one a self-contained ecosystem
bursting with life.

But how do they work?

The intricate web of relationships

and the influence of natural forces

makes each microworld
complex and unique.

So, to discover their secrets,

we need to explore them one by one,

untangle their interlocking pieces

and, ultimately,
reveal the vital piece,

the key to life itself

hidden deep within
each of nature's microworlds.

And we may think it's brimming
with life from shore to shore,

but, actually,
that's not quite true.

In fact, in most of this ocean,
life can be hard to come by.

Instead, most marine species
are condensed into intense pockets,

crammed along the coasts.

And, in tropical seas,
these colourful coastlines

are the most diverse ecosystems
in the ocean -

the coral reef.

These coral reefs are found
across the world,

but one of them
is the master of them all.

It is simply the largest

living structure on Earth.

The Great Barrier Reef.

Of all the reefs on the planet,

how has this one managed to achieve
this incredible size?

But scale isn't the only success
of this microworld.

The water that this reef basks in

lacks many of the nutrients
vital for life.

So such a vibrant ecosystem
should have struggled

to get a foothold here.

So how has this enormous place
managed to survive?

To understand this,

we first need to find out

what it was that allowed the
Great Barrier Reef to get so big.

The reef stretches along
the north-eastern coast of Australia

for over 2,000 kilometres -

a distance equivalent to that
between Britain and North Africa.

Underwater,
it's a marine metropolis

and home to a hustling, bustling
whirlwind of life.

But this microworld encompasses
more than just a reef.

All along its lengths are lagoons,

islands and seagrass meadows.

On the western boundary
is the Australian mainland.

And to the east,

the open Pacific Ocean,

where the outer wall
of the Great Barrier Reef

meets the deep blue sea.

In our search for the key factors
that make this place so big,

we need to look at some
of these habitats in more detail.

And, to begin,
the one that dominates all others...

..the underwater world of coral,

home to the majority of species
on the Great Barrier Reef.

The fish are perhaps the most
conspicuous of all the animals here

and there are some 1,500 species,

each with their own way
of making a living.

Clownfish live amongst anemones.

There are more than seven species
of these fish

that live along this reef

and each species depends
on the presence

of their host anemone to survive.

Spiny damselfish
are another reef resident.

And they face the challenges
of parenthood in a watery world.

They keep their school of miniature
offspring in sight at all times,

only venturing away from the safety
of the reef to grab a quick snack

before returning back to the cover
of their coral cave.

This diversity of fish
is only the beginning

of the Great Barrier Reef story.

This microworld is jam-packed
with thousands of other species,

like mantis shrimp

and feather stars.

All these lives play out
against a backdrop

of hundreds of coral species,

which can stretch
as far as the eye can see.

This huge array of life
may be impressive,

but it doesn't set the Great Barrier
Reef apart from other reefs.

Amongst all of these players,
it isn't the fish,

the invertebrates or even the corals

that explain why
this reef is so big.

We need to look further.

We need to visit
the other environments

that make up the Great Barrier Reef.

And one of these is seagrass.

Remarkably, seagrass
is actually a flowering plant -

the only one adapted
to live in the ocean.

And like huge grasslands,
such as the Serengeti,

seagrass provide important
feeding grounds for herbivores.

More related to the elephant
than to any dolphins or whales,

these are herbivores
that can hold their breath.

This is a dugong.

Meadows of seagrass stretch
right along the Great Barrier Reef

and cover up to 40,000 square
kilometres of the sandy bottom.

This huge area allows
impressive numbers of dugongs

to congregate in these southern
stretches of the coast.

But what part do these vast
areas of seagrass play

in the Great Barrier Reef's size?

Well, the seagrass supports
massive creatures

and provides additional habitat...

..but this seagrass is just a
product of the reef, not its cause.

In fact, the seagrass actually
depends on the reef

for the material in which it grows.

Sand.

And some of that sand comes
from animal life.

The aptly-named parrotfish
is a conspicuous sight

to come across on any reef.

And, by following these fish,

we soon see where some of this sand
comes from.

The fish's teeth
are fused into a parrot-like beak

that crunches its way
through the calcium structure

of dead and living coral alike.

But what seems to be pure
destruction has a surprising twist.

The digested bits of coral skeleton

are ground down
inside the humphead parrotfish

into a fine calcium powder

and excreted as sand...

..sand that builds up
with other bits of calcium rubble

and, with wind and waves, becomes
mounds that push up above the water.

The beginnings of islands

that allow other creatures
to prosper on the reef.

There are some 600 islands
dotted all along this coast.

Oases of life that have been
colonised by plants

and become vibrant ecosystems
of their own.

Some of the islands in the southern
reaches of the reef

have become central to generation
after generation of green turtles.

Every year, 38,000 female turtles

come to the sandy beaches
of the Great Barrier Reef

to lay their eggs.

This female was born on this
same beach over 45 years ago

and she's now returned to lay
her own clutch of around 100 eggs.

It might sound like a lot
but of each 1,000 eggs laid,

only one turtle might survive
to adulthood.

So the reef's protected
sandy beaches

provide vital nesting habitat
for an ancient and iconic reptile.

But the islands have brought
other life to the reef too.

They have become important
breeding grounds

for the 20 or so species
of seabirds.

And white-capped noddies
are here in their thousands.

They produce a single egg

and build their nests
among the trees and shrubs

of coral sand islands.

Without the sand
that built these islands,

these birds would never be able
to nest here in such great numbers.

So sand plays a vital role

in creating the islands and seagrass

that allow a rich diversity of
animals to make the reef their home.

But sand is just one of the reasons
it's so vast.

The Great Barrier Reef
has 2,900 reef systems,

600 tropical islands

and more than 270,000
square kilometres of coral.

To find the ultimate reason
for this immense size,

we need to look wider still.

And the answer is right under
its feet.

Seen from space,

the reef sticks out from the coast.

It's sitting up on a huge platform.

This is an extension
of the continental shelf -

over 2,000 kilometres long

and up to 200 kilometres wide -

that provides the reef

with the perfect shallow-water stage
on which to grow.

But there's another reason
why it's so massive.

In the northern section of the reef,

a gap in the coral
allows a huge current

to pour in from the Coral Sea.

It's the East Australian Current

and it's far from just a trickle.

This movement of water
can be up to 100 kilometres wide

and 500 metres deep.

It carries over 30 million
cubic metres of water per second.

And the current is a vehicle
that carries life.

Combined with wind-driven currents,

planktonic animals are transported
for hundreds of kilometres

up and down the reef.

It's a great highway
that has spread life

to all its individual parts

and it's joined up
this massive ecosystem

to make the longest reef
on the planet.

So the Great Barrier Reef's
immense size

is a product of the huge
shallow-water platform

created by Australia's
continental shelf,

combined with the effects
of a great current

that spreads colonising life
right along the coast.

And it's this unique
set of circumstances

that has led to such success.

But survival on this coast
is far from guaranteed.

The Great Barrier Reef is an
ecosystem in an impoverished ocean.

So how does it actually work?

It's almost unbelievable

but all of this is created
by a tiny creature.

An animal
just one millimetre across,

which barely looks alive at all.

The coral polyp.

This is a reef-building coral

and it literally manufactured
the Great Barrier Reef

from the ground up.

It's made by a creature you can
barely see with the naked eye,

and it's created enough coral reef

to cover the entire land area
of the United Kingdom.

270 billion square metres.

How does it do it?

Reef-building corals
are made up of tiny polyps

and they each live in a little
box of calcium carbonate.

This stony structure
is called their skeleton

and they deposit it beneath them
in order to grow.

Each colourful structure here
is a colony

and each has slowly built up
their fortified-calcium castle

of all shapes and sizes.

They look like sturdy rock giants

but, remarkably, the living tissue
forms a thin veneer on the outside

of sometimes metres
of inanimate coral rock.

This tiny layer of life
makes corals incredibly fragile

and vulnerable to attack.

The dainty mouths of angelfish
might appear harmless

but they actually eat coral.

And, for the coral under attack,

the daytime is best spent
hiding indoors.

But, at night,
when the reef goes quiet,

the coral polyps emerge,

and, finally, we get to see
the animals within.

All across the tropics,
stone coral statues come to life.

The coral animals
are relatives of the anemone

and are equipped
with stinging tentacles

that they use to gather
the plankton floating by.

These corals are clearly
far from lifeless rocks.

And, on some of these crammed reefs,

these tiny animals will even
wage war on their neighbours.

The growing corals
send out digesting tentacles

that, over hours,
can eat away at their neighbours

in a quest for dominance.

And their thin layer of living
tissue is soon stripped back

to their limestone skeleton.

But, for the most part,
these corals can grow peacefully.

And, in good conditions,

will deposit new layers
of their calcium skeleton

at up to five centimetres per year.

This remarkable process
is what lays down

the limestone building blocks
of the reef.

Between them, the polyps
of the Great Barrier Reef

lay down one billion tonnes
of calcium carbonate every year,

creating the foundations

for this awe-inspiring
shallow-water ecosystem.

But building their skeletons
is demanding

of both energy and nutrients

and the coral animals
can't manage such a feat alone.

The crystal-clear water
of the Great Barrier Reef

might seem like the stuff
of postcards and dreams...

..but this bright blue sea is
missing something vital for life.

It lacks the nutrients

that microscopic plants
and animals need to grow -

the nitrates and phosphates

that fuel plankton blooms,

which are the base
of the food chain.

So, despite feeding every night,

the coral simply can't get
enough energy to survive here

from plankton alone.

But deep in its evolutionary
history,

coral developed a clever trick
to get around this.

The key to this
is a remarkable association

between the coral
and a single-celled algae,

called zooxanthellae.

These tiny single-celled algae are
incorporated into the coral tissues,

at up to five million
per square centimetre.

The coral's waste products feed the
algae with the nutrients they need.

And, in return,
the algae develops inside the coral,

photosynthesising, using sunlight
to create oxygen and sugars.

These much-needed sugars
are the vital energy

that the coral needs to grow.

So this unique partnership
allows coral to thrive

in the nutrient-poor waters
along this coast.

But this relationship also sets
the rules for this entire ecosystem.

By having an algae
within its tissue,

the coral effectively becomes
half plant and half animal.

And so, like plants,
coral has become a slave to the sun.

It needs the sunlit waters
of the tropics to grow.

But these shallow tropical waters

leave the coral exposed

because each Australian summer,

destructive forces
move towards the coast.

150kmph winds.

Destructive waves.

Tropical cyclones develop
over the warm seas

of the South-Western Pacific

and hit parts of the
Great Barrier Reef every year.

Huge waves smash the fragile
coral skeletons,

breaking them up into rubble

which tumbles down the reef edge...

..leaving behind a wasteland
of coral skeletons.

This coral takes the brunt
of the storm

and, in doing so,
protects other parts of the reef.

The seagrass is sheltered
in protected lagoons

and the rich island ecosystems
are kept intact.

The sturdy outer wall of coral
protects this coastline

from the wild forces of the Pacific
Ocean that roll in every year.

But if life in these shallow waters

is threatened
on such a regular basis,

how has this coral reef
managed to survive?

The secret to this reef's success

is that it's bigger than any storm.

And to see how this saves the reef
from ruin,

we need to take a look at what
happens when the dust settles.

This coral graveyard is not an
untimely death but an opportunity.

An opportunity for new life.

Not all of the reef
was damaged in the storm.

Even in the worst cyclones,

85% of it can escape damage

and new life comes from these
undamaged parts each November

when the reef experiences
an incredible reproductive event.

In the moonlit night,

the corals of the Great Barrier Reef

start to release eggs and sperm
into the water

and it becomes alive.

Tiny coral nomads take shape.

But they don't just settle on
the reef where they were released.

This is where the ocean current
comes into play.

It carries the larvae of coral,
fish and invertebrates

all along the reef.

This allows bits of damaged reef
to be re-colonised with life.

A single coral polyp.

The beginning of a new colony.

And a renewed stretch of reef.

The new life created by
the annual coral spawn

helps rebuild damaged parts
of the reef...

..allowing them to flourish
once more.

This process repairs the ramparts

and can help replenish the reef edge

before the next cyclone arrives.

It might seem like a pointless
reconstruction

but the re-growth of coral on
a bedrock of dead calcium skeleton

is vitally important
to the continuation of life here.

And, to see just why this is,

we need to look back in time.

Just some 10,000 years ago,

this whole stretch of coastline
was completely different.

Since the end of the last ice age,

sea levels have been rising

and as they rise,

flooded land gives way
to shallow seas.

The sea surface was about
100 metres lower than it is today

and this shallow-water ecosystem
used to be on dry land.

And the sea hasn't stopped there.

It continues to rise

and threatens to flood the reef too,

leaving it in the dark depths
of the seabed.

This could have spelled the end
for the Great Barrier Reef

but not yet.

In the same way
that it recovers from cyclones,

the reef builds up on itself,

layer by layer.

The dead coral skeletons of the past
are cemented together

by colourful coralline algae.

A living mortar that binds
the calcium reef together.

From here, new life is brought
on the currents to colonise

and the reef spreads ever upwards.

Keeping up pace with the rising seas

and creating the shallow-water
ecosystem that we see today.

This exceptional process
not only ensures

that the coral stays
near the surface

so that its algae
can photosynthesise

but, in doing so,

it allows this whole shallow-water
ecosystem to exist.

The Great Barrier Reef
is more than just coral and fish.

It's a network of thousands of
islands, reefs and other habitats

that allow animals
both large and small to prosper.

The reef is remarkably resilient.

Not only can it withstand some of
the most ferocious forces of nature

but it has also managed
to exist in nutrient-poor waters

that could otherwise be lifeless.

But the reason this reef has got
bigger than any other on Earth

is the massive
shallow-water platform

of the Australian continental shelf,

the great current
that runs right along the coast

and the simple partnership
between a one-millimetre coral

and a single-celled algae.

These are the secrets
of the Great Barrier Reef.