The Blue Planet

There are some seas where fish swarm in millions...

..and plankton blossoms in vast clouds.

The sheer quantity of life here is unmatched anywhere in the oceans.

These are the most productive seas on Earth.

They are the Seasonal Seas.

The Seasonal Seas border the temperate zones,

where conditions change through the year.

The seasons also affect the underwater world.

The power of the sun is constantly changing.

in the far north, during the summer,

there are long hours of sunlight,

but in winter, that dwindles, and there can be weeks of darkness.

The summers are warm and gentle,

the winters racked by savage storms.

in conditions like this, life of any sort

has to struggle to survive.

january on Sable island off the coast of Nova Scotia.

Grey seals have managed to get ashore

through the crashing breakers.

Gales here can blow for days on end.

Sable island has the world's largest colony of grey seals.

100,000 come here to breed each year,

just when the weather is at its worst.

To add insult to injury,

the pups, having suckled for only 18 days, are then abandoned.

Their mothers must return to the sea to find food for themselves.

As yet unable to dive, the pups are marooned,

sustained only by their fatty blubber.

it will be five weeks before they are strong enough to swim,

but then it will be spring and the ocean will be teeming with food.

By May, spring has reached the coasts of Scotland.

Underwater, it arrived rather earlier.

in March, the seas had warmed enough

to trigger a magical transformation.

These are phytoplankton -

tiny floating algae, each much smaller than a pinhead.

They multiply with amazing speed

to produce more annual growth than all the plants on land together -

six billion tonnes of it.

This immense bloom spreads across the face of the ocean

and, within a couple of months,

turns vast areas of it a dense green.

Animal life reacts to the blooming sea.

These look like small anemones, but they're about to change.

They separate, revealing themselves to be tiny common jellyfish.

They are less than three millimetres across.

Within a few months, they will have assembled into vast swarms.

Minute copepods are part of their staple diet.

These appear every spring in vast numbers

and graze on the phytoplankton bloom.

Their beating legs create currents that sweep the little algae

into the filters round their mouths.

0n this microscopic scale, water is so viscous

that the phytoplankton can't swim against the current.

Whilst feeding, sensors on the copepod's antennae

give warning of dangers ahead.

Lighting by lasers

reveals that feeding copepods leave wakes behind them,

like jet trails in the sky.

Slightly larger floating predators

are able to use these trails to find their prey.

A close call, but some predators are simply too large to avoid.

jellyfish.

They may appear to be delicate as well as beautiful,

but they are deadly hunters.

With every pulse of the delicate bells,

plankton-rich water is drawn into their lacy throats

and pushed out again, leaving copepods stuck to the membranes.

A single sea-nettle jellyfish, only a few centimetres across,

can collect thousands of copepods in a day.

So, the killing power of giants like these is hard to estimate.

Each of these jellyfish weighs up to 30 kilos

and has tentacles stretching over eight metres.

But there are even greater dangers awaiting the copepods.

By late spring, the baby common jellyfish are fully grown.

They gather in millions,

forming swarms hundreds of metres long,

which filter out all the small planktonic animals in their path.

Nevertheless, there are such astronomic numbers of copepods

that enough will survive to form swarms of their own.

A basking shark.

This huge eight-tonne fish feeds on plankton and nothing else.

Every hour it filters 1,000 tonnes of seawater through its gills.

Numbers of these shark appear when the bloom is at its peak

and travel along currents where the plankton is thickest.

Even so, the supply of plankton is so abundant

that another group of animals can feast -

animals that live on the sea floor.

Every centimetre of rock is covered with invertebrates of some kind -

anemones, sea squirts, gorgonians -

all trapping plankton from the passing water.

Not all life in the Seasonal Seas depends upon plankton.

it is early spring in British Columbia.

Seaweed has started to grow slowly in the cold water.

As the hours of sunlight increase and the water warms,

these small plants turn into great beds of bull kelp.

The immense 30-metre long strands

have small gas-filled floats which keep them high in the water,

within reach of the energy-giving sunlight.

Further south, the sunshine is more powerful,

so here, on the coast of California,

the biggest kelp of all can grow.

This is giant kelp, and by midsummer,

each plant is growing in length by nearly a metre a day.

The main stem is fixed to the bottom 35 metres below

by a small but immensely strong holdfast.

Fully grown,

these gigantic plants are 100 metres long

and they form vast marine forests.

Great schools of fish find sanctuary here.

These are blacksmith fish.

They are plankton feeders and, whenever it seems safe,

they head into open water to feed where the plankton is thickest.

But at the first hint of danger...

..they rush back to the safety of the forest.

Fish are not the only creatures that shelter here.

Californian sea otters gather in the kelp forest

to rest and snooze in safety.

To prevent themselves being carried away

into dangerous open water where the big predators cruise,

they anchor themselves by winding straps of kelp around their body.

Sooner or later, they have to find food,

and that lies on the seabed a long way below them.

Sea otters, however, can stay underwater for up to ten minutes,

ample time to find the shellfish they need.

Many of the smaller creatures that live in these forests,

such as urchins, graze on the kelp

and can seriously damage it if their numbers are unchecked.

The sea otters feed on some of these grazers

and prevent them getting too numerous.

So, in effect, the sea otters are the guardians of the forest.

Garibaldi fish do not, in fact, damage the kelp.

if anything, they help it

by picking off tiny animals that encrust the leaves.

They graze on bryozoans, tiny colonial animals

which build their colonies

like a patchwork of white skins on the leaves.

When night falls, there are fewer predatory fish around,

and the bryozoans emerge from their white shelters.

Now, just like coral polyps,

they start filtering out the plankton under cover of darkness.

They're not the only animals to venture out at this time.

This is an amphipod, just two centimetres long,

and it does eat kelp.

in turn,

it is excellent food for many predators.

To protect itself, it produces silk, just like a spider,

and uses it to sew together two sides of a kelp frond

and so form a shelter.

This one is in particular need of a secure home.

She's a mother.

There are over 50 youngsters clustered on her abdomen,

so her home is becoming somewhat cramped.

They will soon be old enough to leave,

and, whenever she can, she kicks them out

to get a taste of the outside world.

Dawn.

Beds of eel grass grow in the shallows

between the kelp forest and the shore.

A harbour seal has found sanctuary here,

and is sleeping after a hard night's foraging.

But not for long.

A male seal gives a wake-up call.

More seals are attracted from all directions.

it's june, the time when young male harbour seals

start their strange mating displays.

0ne listens attentively to the grunting noises made by the other.

These calls are almost certainly a way of establishing

which of the two will be dominant.

But if the animals are closely matched in size and experience,

grunts won't settle the issue.

The rivals will have to come to blows.

And seals can be surprisingly violent.

in a month's time,

the breeding season will start and then fights will be in earnest,

but now, in midsummer, these exchanges are harmless.

They're just playing.

Many of the creatures that shelter in the kelp

have to venture out in order to feed.

The bat ray for one.

Unlikely though it seems,

the sandy floor of the open sea

is, for the bat ray, a rich feeding ground.

There is food hidden within the sand,

and the bat ray has a special technique for finding it.

it uses jets of water

to blow the sand aside and expose small invertebrates.

A kelp bass hangs about alongside waiting for scraps.

0ther hunters are also on the prowl.

The fan-tailed sole.

There are mantid shrimps here, living in tunnels.

But, once again, hunger compels them to venture into the open.

That, of course, is a gamble.

They will either eat...

..or be eaten.

A sea slug called janolus.

its colours suggest that it is poisonous,

and so it is, to everything except another kind of sea slug...

..the predatory Navanax.

Navanax pulls itself along the trail of slime