Hello marine biology students.
In this video, we're going to talk about the
different types of coral reefs and a bit about
the ecology of coral reefs as well.
[Intro Music]
So there are three basic types of coral reefs
based on their shape and their relationship
to shore.
These reefs are fringing reefs, barrier reefs,
and atolls.
The fringing reefs are the simplest and most
common form of reef.
They develop as narrow strips along the shore.
They consist of an inner reef flat of varying
width and an outer reef slope.
In between these two, we have the reef crest,
with that reef crest being the peak between.
The growth of the coral is typically more
intense on the outer reef slope.
The growth is slower on the reef flat due
to runoff from land, which includes sediments,
fresh water, and pollution.
So here we can see a fringing reef.
Usually it's adjacent to shore, and that will
be some extension of the reef flat.
After the reef crest, we'll see the reef slope,
where there will be more diverse coral growth.
You'll also notice sediment and rubble at
the bottom of the reef slope.
A barrier reef is just slightly different.
They also develop along coasts, yet they're
often farther from shore than the fringing
reef.
The difference with the barrier reef is that
a deep lagoon is found between shore and the
reef crest.
Waves often wash sediment onto the back reef
slope, or reef flat, which causes reduced
coral growth.
Enough sediment may accrue to form small islands
on the reef, and we call these keys, and it
can be spelled in two different ways.
Famous barrier reefs include the Great Barrier
Reef in Australia or also in Belize off the
coast of Central America.
The biggest difference between a barrier reef
and a fringing reef has to do with that lagoon
between the beach and the reef crest.
Here we can see abundant coral growth that
would be on the outer reef slope.
The last category of coral reef are the atolls,
and an atoll is a circular reef surrounding
a central lagoon.
Sand cays and islands may be a part of the
atoll.
The width of the atoll varies from less than
a mile to even over 20 miles.
The inner slope within the lagoon has a more
gradual slope than the outer slope.
The lagoon also normally has a depth of 200
feet or less.
The two largest atolls are in the Maldives
and the Marshall Islands.
When we look at an atoll, we can see the very
steep outer slopes of the fore-reef and then
that more shallow back reef slope making up
the lagoon.
Within the lagoon you may find pinnacles of
coral growth in addition to sand cays along
the edge.
These structures are very mysterious and puzzled
scientists for a while as to how they were
formed because often they are not close to
any other land, and yet the coral reefs are
growing right at the surface.
The person who accurately described how coral
atolls are formed is actually Charles Darwin.
Atolls begin developing when a volcanic island
is formed.
This hypothesis first presented by Charles
Darwin is now supported by a wide body of
evidence.
Around this volcanic island, a fringing reef
begins to form.
An interesting property of volcanic islands
is that they are quite heavy, and they actually
cause the oceanic plate beneath them to begin
to subside locally.
So after this volcanic island is formed, it
very slowly begins to sink.
Conversely, the coral reef grows upwards,
and if the rate of the coral reef’s growth
matches that of the volcanic island’s subsidence,
eventually the island itself will continue
to go down and down, just leaving this ring
shaped reef.
So the island subsides while the coral reef
continues to grow.
The island eventually disappears below the
water surface, leaving only the outer band
of coral surrounding a lagoon.
The round fringing reef becomes an atoll,
and at these atolls, there is no remaining
evidence of that volcanic island except for
the presence of this coral ring.
They did drilling studies, and they actually
had to drill down several hundred meters in
order to find evidence of the volcanic island
that used to be at the surface.
So here we see that formation process.
And again, by the time that atoll is formed,
the original island is buried under a mountain
of calcium carbonate.
Coral reefs are very interesting ecosystems
because they are very diverse and productive,
and yet they are also nutrient poor.
So coral reefs are among the most biodiverse
and productive ecosystems on earth.
The coral reef ecosystem is very complex and
limited by many physical features such as
light, depth, nutrients, wave action, etc.,
and many biological limits as well, including
competition, predation, and grazing.
Warm, tropical waters where reef develop are
low in nutrients, primarily phosphates and
nitrates, due to stratification of the water
column and distance from rivers and other
sources of nutrients.
Due to their nutrient content, the water surrounding
reefs has very low primary production.
They're poor in phytoplankton, and the phytoplankton
isn't excessively abundant.
The food web is basically based around the
primary production of the zooxanthellae within
the coral.
They can also be found in some other invertebrates
as well, such as anemones and even giant clams.
The coral benefits the zooxanthellae by providing
carbon dioxide, nitrogen, and phosphorus in
its excretory products.
The zooxanthellae is then able to provide
organic molecules to the corals and to all
the animals that feed on the corals.
So when we look at what's taking place within
the coral itself, these zooxanthellae, these
endosymbiotic dinoflagellates, they are able
to photosynthesize from sunlight and also
getting carbon dioxide and water from their
hosts and the surrounding area.
They also get nutrients in the form of respiratory
and metabolic wastes from the coral polyp.
As the zooxanthellae produce excessive amounts
of organic molecules, these are exported to
the coral polyp, and that allows the coral
polyp to break them down for energy, but in
the process also release nitrates, phosphates,
and carbon dioxide.
Within the coral reef, there will be a few
other primary producers, such as cyanobacteria,
which are a photosynthetic bacteria present
on the reef, and these cyanobacteria also
function as nitrogen fixers, converting dissolved
nitrogen gas into nitrates.
Seagrasses, mangroves, and seaweeds can also
contribute to primary production in the form
of detritus.
Nitrogen and phosphorus waste products are
efficiently recycled, not only by the corals
and zooxanthellae but also by the other reef
dwellers.
When we look at a typical coral reef food
web, we can see that seaweeds and corals are
the primary producers and specifically the
zooxanthellae within corals.
Now seaweeds and other communities can contribute
to the detritus pool, and the detritus feeders
feed directly on that, including sometimes
the corals.
There are organisms that graze on the corals
themselves, organisms that feed on plankton,
organisms that feed on detritus, and organisms
that feed on the primary producers.
And then we also have the predators which
feed on these primary consumers.
It turns out that a variety of different types
of organisms are able to feed directly on
the coral or the coral products, including
mucus produced by the coral, along with their
reproductive cells.
There is tough competition for space and light
on the reef.
Some corals grow fast and upright, others
grow slow but massive, each in their own way
trying to out-compete with others in their
area.
Corals may feed directly on or sting one another
if they contact each other or are running
up against each other.
Many coral reef inhabitants compete for space
and/or to deter predation by producing toxic
or foul-tasting chemicals.
These include soft corals and sponges.
In this image, we see an intense war zone
going on.
There are three different species of corals
growing into each other.
There is chemical warfare, enzymatic destroying
of tissues, and remarkable amounts of violence
in the competition between these competing
corals.
One is attempting to grow over the other and
killing its polyps along the way, whereas
that central brown coral colony is also being
attacked at the top by another neighboring
coral colony.
In a coral reef, other animals must also compete
for limited resources.
Many species of fish, molluscs, echinoderms,
crustaceans, and members of other groups normally
do this by feeding on slightly different foods,
or feeding in slightly different areas, or
feeding in slightly different ways, or different
times of the day.
Sometimes they reproduce at different times
of the year.
Each species has its own well-defined ecological
niche, and this is the reason that allows
such diversity within a coral reef.
Coral reefs are highlighted by a diverse number
of species living together, surviving together
in a surprisingly nutrient-limited environment.
When we talk about things that shape the reef,
coral predation and grazers play an important
role in structuring 
the coral reef community.
Some examples of coral predators include fish
that directly eat the coral colonies, or one
of the most devastating are the sea stars,
specifically the crown-of-thorns sea star
in the genus Acanthaster.
This crown-of-thorns sea star can actually
extrude its stomach from its mouth and surround
a surface of a coral colony, digesting the
polyps and the tissue, leaving behind nothing
but the white skeleton.
Seaweed grazers also play an important role
of keeping seaweeds from taking over the coral
surfaces in competing with the coral for light.
Here we can see the result of a crown-of-thorns
sea star feeding on a coral colony.
And again, that white skeleton is really the
calcium carbonate without the layer of organic
tissue that otherwise would be providing its
color.
The crown-of-thorns sea star is a natural
predator in the environment, and so, many
marine biologists are torn as to whether humans
should influence these crown-of-thorns sea
stars or not.
Occasionally, they will occur in massive numbers
and can decimate certain portions of the coral
reef.
So how do we deal with these massive waves
of predators on the coral reef, or should
we be intervening at all?
This is an area of debate within the field
of marine biology right now.
One argument is that these predators are natural,
and interfering with the natural balance could
be problematic.
A further argument can be that corals can
sometimes recover, or other corals can make
use of this now-vacated spot within the reef
to grow themselves.
On the other hand, we could argue that corals
are already stressed and dealing with other
challenges, and so these predators might just
be too much.
In the image, you can see a prototype ROV
vehicle that can actually navigate through
a coral reef, identify the crown-of-thorns
sea star, and deliver a lethal injection to
kill the predator.
Is this something that we should be doing
or not?
The debate is still ongoing.
And that completes our discussion of coral
reefs.
Now next week we're going to be talking about
life in the open ocean both near shore and
off shore, and so the question I want you
to think about is: how comfortable is it swimming
when you can't see the bottom?
Alright, see you next week.
