Hello marine biology students.
In this video we're going to talk about how
organisms are distributed within the intertidal
zone.
[Intro Music]
So, in the intertidal zone there are often
very clear bands or zones where you will find
certain organisms and the lines will be so
defined that you can clearly see them as separate
bands on the shore during low tide.
We call this zonation, and the zonation 
is due to varying degrees of exposure, with
regions known as the upper intertidal being
submerged only during high tides and the rest
of the time only being lightly splashed with
water at most.
The middle of the intertidal, which probably
about half the time is exposed and half the
time is underwater, and then the lower intertidal,
which is exposed only during the lowest points
of low tide.
Different organisms will find themselves thriving
in these different regions, or when I say
thriving, at least out competing their competition.
Competition is going to be greater 
in the lower intertidal because it is the
least extreme of the zones.
It is exposed to a lesser degree than the
upper intertidal and so more organisms will
be able to survive in that area and compete
for the limiting resources.
The lower intertidal always has more species
for the same reason, though the species will
vary greatly by location.
So, the lower intertidal is the less extreme
of the regions of the intertidal.
And so, the lower intertidal will have more
and a greater diversity of species.
Some species are better competitors than others
and will exclude others from the community
if left undisturbed.
If a habitat is disturbed, then organisms
move in and are excluded in a predictable
pattern known as ecological succession.
If a region of the intertidal has not been
disturbed in a while, if individuals have
not been removed, if there hasn't been a clearing
of open space, if there hasn't been a new
start to the habitat or a new beginning to
that region of the substrate, well then the
climax community will result.
And the climax community is the makeup of
a community when there has not been a recent
disturbance.
Now, the intertidal is often highlighted by
disturbance.
Whether it's a large piece of driftwood or
log being washed onshore and breaking and
killing certain organisms, cleaning new substrate,
or especially strong waves and tides which
maybe rotate or turn some of the pieces of
substrate.
Mussels are often highly competitive in the
intertidal and so if there is a patch of bare
rock, sometimes the mussels will fill their
space, but sometimes there will be a biofilm
that forms in the area that might then support
seaweeds or might support barnacles or even
limpets could grow on there.
Another question is, what predators are in
the area?
If there are sea stars, sea stars are often
voracious predators on the mussels and so
will allow there to be space for a diversity
of organisms in that intertidal.
However, if there are no sea stars, the muscles
often out-compete everything else and you
end up with a much more homogeneous community.
Sea stars are a keystone predator in that
they maintain and increase the amount of biodiversity
in the intertidal by keeping the mussels,
which are often the strongest competitors,
in check.
Predation by keystone species 
such as sea stars strongly affect the occurrence
and density of animals in the middle intertidal
and experiments can be done either by caging
or removing predators from an environment
to see what the result would be without those
keystone predators.
The result is that they are usually limited
in biodiversity.
Zonation in the intertidal is the result of
competition.
Now, this competition can be for space or
it can also be to compete with the harsh physical
characteristics of the intertidal.
Biological interactions are often important
factors in determining lower limits of the
rocky intertidal, whether it's predation or
competing for resources.
Whereas physical factors such as drying out
and desiccation and temperature tolerance
are usually going to be determining the upper
limit for a particular community.
And this is what causes the banding patterns
we see, again referred to as zonation, in
the intertidal.
This zonation, which is so clear on rocky
intertidal, usually is a little less pronounced
in a soft bottom intertidal, yet the composition
of the sediments for a soft bottom intertidal
can also control which organisms can live
in which areas.
Now, the sediment size can vary between sand
and silt and clay or a mixture of these which
we call mud.
The smaller the sediment size, the less oxygen
will be found in the water-filled space, and
this is due to reduced circulation.
Well-sorted coarse sediments such as send
can drain water very quickly and the water
will be replenished and is more likely to
be oxygen-rich, however as the fine sediments
and especially a mixture of sediments are
together, the water is not going to be drained.
The oxygen that's present is going to be used
by respiration of bacteria and then it becomes
an anoxic or anaerobic environment and only
certain types of bacteria can thrive in that
environment.
So sediment size can be small and mixed so
that all except the top few inches of the
sediment is anoxic.
Infauna, those organisms that are living within
the sediments, must maintain a connection
with the oxygenated zone either via tubes,
siphons, or extended burrows, if they are
located in these anoxic zones of the sediments.
Anaerobic bacteria 
break down organic matter, can thrive in the
anoxic zone.
In soft bottom intertidal, most animals are
infauna, with adaptations to dig or burrow.
Epifauna, those living on the surface, are
usually going to be in low abundance and a
low diversity of species.
So looking at some of the organisms we'll
find in a soft bottom intertidal zone, there
will be burrowing organisms such as clams
and worms of various types, crustaceans, and
even shore organisms and some snails.
In the subtidal you'll see more organisms
like echinoderms, such as sand dollars and
sea cucumbers and variety of crabs and even
fish.
There are some very interesting examples of
species interactions that happen in the intertidal.
So, in a region of the intertidal on the East
coast of the United States, there are tide
pools that are dominated either by Enteromorpha
which is a type of green algae or by Irish
moss, which is a type of brown algae.
And the makeup of these pools that are interesting
in that most tide pools tend to have one type
of macro algae or the other.
The pools that have the green filamentous
Enteromorpha often have populations of crabs
within those pools and those crabs feed on
snails that usually eat the filamentous green
algae.
The filamentous green algae provides the crabs
with protections from their predators, which
are seagulls.
And so, these filamentous green algae pools
are usually going to have crabs and not a
lot of Irish moss.
Conversely, if the crabs are removed from
a pool, well then the snails are going to
thrive and eat the filamentous green algae,
allowing the Irish moss to grow in those pools.
In a tide pool with Irish moss, the crabs
are less likely to survive.
They are more likely to be seen by their predators
and so both types of tide pools will be stable
and self-perpetuating, with the Irish moss
pools usually having populations of snails
keeping the other types of algae down and
the Enteromorpha pools having crabs which
keep the snail population down.
So as you can see, these tide pool interactions
can sometimes be quite complex.
One last concept to discuss as we are closing
out our discussion of the intertidal is how
energy flows through the food web of this
ecosystem.
Our primary producers are going to be photosynthetic
organisms, things like seaweeds, diatoms,
and sea grasses.
Grazers will feed directly on these seaweeds,
but it turns out one of the main food sources
in the intertidal is detritus.
Now, this detritus can come from organisms
living in the intertidal both in the form
of decomposing and defecating, but often detritus
is coming from another source, usually estuary
environments produce large amounts of detritus
that can end up being a nutrition source for
other ecosystems as well.
We'll be talking about estuaries in greater
detail in our next few videos, but they can
often be one of the sources of food driving
the intertidal, but otherwise the primary
production comes from the photosynthetic organisms.
There are a variety of grazers and scavengers
and filter feeders and then the carnivores
usually feed on these organisms as well.
So we see a few steps in the food web, but
not nearly as complex as the food webs for
the Arctic and Antarctic oceans.
And that completes our discussion of the intertidal
zone.
Now, “Have you ever wondered what happens
when freshwater and saltwater meet?”
We'll talk about that in our next video.
See you then.
