Water. Everywhere. Where the crust of our planet falls away, the billion cubic
kilometres of water on our planet find their level. In the interface between land and sea,
this water is shallow enough that light can reach the bottom. And with that solar source of energy,
a myriad of unique lifeforms has evolved to thrive in this salty world. Algae and plankton, corals
and kelp, jellyfish, molluscs, crustaceans, fish and whales… the abundance of life here equals any
found on land. In fact these regions could well be the origination point of life on our planet.
Join me in this series on biomes, as we finally dip our toes in… to the shallow seas of earth.
Earth is often referred to as the “blue planet”, and this is because 71% of its
surface is covered by water. Seas and oceans appear blue because water absorbs more red light
leaving most of the blue to reflect back to the observing eye.
When the earth formed over four billion years ago, a small fraction of the original mass was ice.
The initial violent phase of the planet’s formation resulted in very high temperatures,
well above the boiling point of water, so this initial ice would have been in the form of a steam
atmosphere. But as the surface eventually cooled, this water vapour condensed and settled upon the
crust, pooling in lower lying areas to form the oceans. So the size of the oceans today is
related to that initial mix of ice and rock from the early days of this part of the solar system.
Had it been more, then the earth would have been covered completely in ocean. Had it been less,
and we would have been looking at only isolated salty lakes surrounded by dry land.
The seas and oceans are such a large subject, that it’s impossible to cover all aspects of
it in a single episode. So I have subdivided the topic into two. In the next chapter I’ll discuss
the deep oceans that hold the vast majority of the water on earth, their topography, their
strange life forms, and how these large masses of water influence the climate of the world overall.
In this episode, however, I’m going to discuss the small fraction that
lies close to the land and where light can reach the bottom – the shallow seas.
When you think of continents, most of you will likely imagine the outlines of the
great land masses, The Americas, Eurasia, Africa, Australia and Antarctica. But these continents,
in terms of a strict geological definition actually extend outward some way into the sea,
in the form of shallow continental shelves, before suddenly dropping down into the abyssal plains of
the deep oceans. The width of these shelves varies dramatically from place to place around the world.
For instance, in many parts of the coasts of California, Western Mexico, Peru and Chile there
is virtually no shelf, with the land sloping down into the far depths after only a kilometre or so.
North of Siberia into the Arctic Ocean, however, and the shelf extends as much
as 1,300km (800 miles) before dropping away. The British Isles lie upon the shelf that extends out
from Western Europe, while shelf connects the land of North America and Eurasia at the Bering Strait.
But the Mediterranean and Caribbean Seas are really oceans,
in the sense that they fall away to deep abyssal plains where no light can reach.
The continental shelfs of today have formed over tens of millions of years through the
deposition of sediment from land rivers as well as the build up of dead plant and animal life.
The earth goes through many natural cycles of climate change as a result of slight
perturbations in its orbit around the sun, the sun itself varying in energy output,
and the Earth wobbling on its own axis. Such changes in surface temperature result in a
redistribution of the total water between the oceans and ice caps at the poles.
During colder times, the usual process of evaporation of the oceans goes out of balance,
with the resulting precipitation of this water vapour accumulating as snow on expanding
and deepening ice caps. As a result, the sea level falls. When warmer temperatures come,
the ice melts, and the water is returned to the ocean, leading to sea level rise.
Because continental shelfs are gently sloping, this has led to dramatic changes in the land
area of continents throughout geological time. As anyone who has swum in the sea knows, seawater
is saline, meaning it has a relatively high amount of dissolved salts in it compared to freshwater.
The actual composition of seawater globally is remarkably uniform, with the salinity
varying only between 3.4 and 3.6% by mass when considering large volumes of ocean.
In localised areas, such as near melting icebergs, or by large river estuaries, the salinity is less.
Where high evaporation occurs, such as in tropical seas on a clear day, salinity can increase.
The majority of the salts are in the form of sodium chloride, that is, common table salt.
Any organism that wants to survive in the ocean must evolve its structure and internal organs
so that it can cope with this amount of dissolved salt.
This is important as virtually all organisms have some kind of membranes within their systems
that are designed to pass a certain proportion of salts or water. This explains, for instance,
why species of fish are considered of either freshwater or seawater type, with very few
species able to tolerate both environments. Seawater is very slightly opaque, meaning
that over a long enough distance, light is absorbed and will not reach the far point.
This absorption varies by wavelength, with blue light able to penetrate the furthest.
Since all but a handful of specialised life-forms depend upon sunlight as the base of the food
chain, what depths of water have what amount of light will determine its ability to support life.
In general terms, water down to a depth of about 200m is regarded as “sunlit”,
in that it can support algae and plankton, upon which almost all other sea-creatures depend.
Below 200m in depth and we have a “twilight zone” where only faint blue light reaches,
and below 1000m we have the dark zone where effectively all sunlight is extinguished.
We’ll pick up these two latter zones in the next episode.
But in the sunlit zone, photosynthesis is possible, and microscopic life, believed to
be the oldest type of life on earth, has evolved to capture this light while suspended in seawater.
The collective term for this life is plankton, which in its strictest sense
refers to non-motile organisms – those that just drift in the water and cannot propel themselves.
In this microscopic world, a vast array of different types of organism exist across
all biological kingdoms. These can be grouped as follows:
Phytoplankton – plant-life such as algae and diatoms that photosynthesise sunlight.
Zooplankton – microscopic animals such as protozoa that feed upon phytoplankton.
Mycoplankton – fungi that recycle dead plant and animal material.
Bacterioplankton – bacteria which can be either photosynthetic or feed on other organisms, and
Virioplankton – viruses that outnumber all other types,
but which are much smaller in size, and which regulate the numbers of bacteria within water.
All these types play their role in maintaining balance within the ocean ecosystem, and indeed
the entirety of the earth. Although making up only about 1% of total global biomass, phytoplankton
produce around half of all global oxygen through photosynthesis. This process of converting light
and carbon-dioxide, and building plant matter is the base of the shallow seas food-chain upon which
all other forms are dependent. Occasionally, conditions originate where massive blooms of
this microscopic life are produced, and are so large that they can be seen from space.
But plant life within the sea can extend beyond the microscopic. Seaweed is found throughout the
world and can be either free-floating, such as sargassum, or attached to the sea bottom
as in kelp. Most seaweed is actually a form of multicellular algae, and the
thousands of species are roughly subdivided into the colours of red, green and brown.
The marine animal food-chain is well-known to most of us.
Krill, that is, small shrimp, feeding on plankton,
fish and crustations feeding on krill, bigger fish and sharks feeding on other fish, whales
feeding on krill and fish. Altogether, these food chains are as complex and diverse as any on land.
But one thing that is unique to the shallow seas, and not found either on land or in
the deep oceans, is one of the most remarkable living wonders to be found anywhere on earth.
It can be visible from space, and is the only living thing to create land from the sea. These
are colonies of specialised animals that have a special symbiotic relationship with microscopic
plants within their bodies. It sounds like something from a science-fiction novel,
doesn’t it? And yet they are right here, within the shallow tropical waters of our planet.
Of course, I refer to corals, which, through the excretion of calcium-carbonate rock at their base,
create coral reefs that, over centuries, can grow to the size of islands.
A coral consists of clonal colonies of the same organism,
although sexual reproduction also occurs between corals through the release of spores.
While some corals use lures and tentacles to catch plankton or even small fish,
most corals rely upon the presence of symbiotic algae such as dinoflagellates
within their tissues to photosynthesis light and provide their source of energy in this way.
It is the presence of these organisms within the coral that give it its colour.
Most of this standard type of coral occur within shallow tropical and subtropical
waters where the water is both warm and able to receive adequate sunlight year-round.
Other types of coral that do not have the plant symbiotes, however,
do exist in deeper waters, or colder seas as far from the equator as Scotland, but are rarer.
Humans have made use of the ecosystems of the shallow seas since before recorded history
in the form of fishing. But with the rapid growth of global population over the last couple
of centuries, such natural systems have been put under threat, with over-fishing in Europe,
for instance, leading to the introduction of legal maximum quotas of fish,
in an attempt to restore fish stocks. The difference between fishing and agriculture
is that the former still relies upon sourcing the food in the wild,
as opposed to the domestication of plants and the development of intensive farming in the latter.
So while farming has been able to keep pace with growing population, fishing has not.
Another concern is the bleaching of coral reefs, where the corals expel their plant symbiotes,
losing their colour, and eventually dying. It is still unclear what is causing this bleaching,
but suspects include slight increases in sea temperatures and marine pollution.
The growth of plastics in our daily lives has led to the dramatic rise in marine plastic pollution
that can be found littering beaches and forming gigantic garbage patches in the middle of ocean
gyres. As plastics can survive for centuries before breaking down, they can accumulate and
poison the food-chain. One thing the talking heads in the media never tell you, though,
is that this is a problem that is, for once, not coming from the developed nations of the West,
but rather, that the worst offending countries for this are all in the Far East. This is largely due
awareness leading to historical efforts within Europe and North America to prevent the ingress
of plastics into the Atlantic. So while the issue is still very much in the news in the West, the
message needs to be forwarded to these countries in Asia so that they may also take action.
And that’s the shallow seas. I hope you enjoyed this introduction to this aquatic biome.
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don’t miss future episodes. Thanks again for watching, and I’ll see you in the final video
in the series where we explore the largest of all biomes, the deep oceans of Earth.