Hello, marine biology students.
In this video, we're going to talk about the
ecosystem of the subtidal.
[Intro Music]
So this week, we're going to learn about the
subtidal, kelp forests, coral reefs, and the
different categories of coral reefs along
with their ecology.
The subtidal zone is below low tide.
We're going to talk mostly about the benthic
communities right now, as the pelagic communities
will wait for next week.
The continental shelf is the submerged edge
of a continental plate (is the subtidal
zone for the benthos).
It extends from low tide to the shelf break
or the edge of the continental shelf.
The slope of the continental shelf is normally
more gradual than the shelf break, which is
typically steeper.
The slope of the shelf, the depth of the water
at the shelf break, and the distance of the
shelf break from shore varies greatly by location
(narrower in active than in passive margins).
The depth at the shelf break varies greatly,
with an average of around 150 meters (or just
less than 500 feet) in depth.
The continental shelf is biologically the
richest part of the ocean, including a major
portion of commercial fisheries.
It is also an area of potential natural resources
such as oil and natural gas; for this reason,
nations want to protect these resources from
exploitation by other nations.
In the 1950s, the United Nations agreed that
each nation should have control over the marine
resources out to 200 nautical miles off of
their coasts.
This includes the continental shelf and its
resources.
On the continental shelf, the temperature
of the water varies much more than out in
the deep sea, but less than we would see in
an estuary or on rocky shores.
The water is shallower than in the deep sea
and is mixed by waves and currents: warmer
surface waters mix readily with the rest of
the water column, and that's because even
though it seems quite deep to us it is shallow
enough that the current and the winds are
able to mix the waters.
Water turbulence 
from waves and surface currents keep the water
column well mixed with generally no stratification.
This affects not only water temperature but
also the available nutrients as well.
Nutrients never have a chance to sink to the
bottom and become unavailable, as is the case
in the deep sea, and so phytoplankton and
other photosynthesizers often have access
to resources that are lost in those deep sea
environments.
There are several sources for nutrients on
the continental shelf.
These include: decaying organic matter, nutrients
brought in from estuaries and terrestrial
sources by way of rivers, and nutrients from
detritus originating in other communities,
such as seagrass meadows and kelp forests.
Because of all of these inputs, water can
also have decreased transparency
due to suspended sediments.
So there are four types of subtidal communities
that we'll talk about on the continental shelf.
These include: soft-bottom subtidal (where
the bottoms are made of mud or sand), seagrass
meadows, hard-bottom subtidal, and kelp forests.
In the soft-bottom subtidal, the three major
categories of animals are the epifauna, which
live on the surface, the infauna, which live
within the sediments, and the meiofauna, which
are so small they actually live between the
grains in the sediment.
With the soft bottom intertidal, there will
be very few primary producers, and that's
because there aren't a lot of attachment sites
for those primary producers.
There may be some benthic diatoms, however,
on the sediments.
Sessile, or non-motile, species will also
be rare due to the lack of hard substrates.
While there are more species present in this
area than in the adjoining intertidal, the
distribution is often patchy.
And here we see a diagram of different types
of distribution from random to regular to
patchy.
A patchy distribution tells us that there
are some physical or biological factors that
will collect organisms together—whether
it's larval settlement or chemical cues—for
some reason the organisms are going to group
together.
As I had mentioned, larval settlement plays
a big role, and there can be chemical signals
that tell a larva where it should undergo
metamorphosis and become a member of the benthos.
So the larvae select particular bottom habitats
to settle in.
Several particular mechanisms are known: being
abiotic, meaning physical and chemical factors,
or biotic, meaning factors determined by other
living organisms.
On unvegetated soft bottom communities, there
will be many infauna, being organisms that
live within the sediments themselves.
These can include: polychaetes, molluscs,
and others, and there will be some epifauna
as well, such as sea stars, brittle stars,
and, as seen in this picture, a sea pen, which
is a colonial cnidarian that's a filter feeder.
There will also be some demersal, or bottom-dwelling,
fish.
Here we see a community of sand dollars living
embedded within the sediment.
These are relatives of urchins, and they position
themselves at an angle in the sediment to
filter feed the current using their tube feet.
Here we see a collection of different bottom
dwellers like molluscs, echinoderms, polychaetes,
crustaceans, and others.
The meiofauna are also a diverse group.
The meiofauna are organisms that live in the
spaces between the sediment particles.
Many of these species are unique and found
nowhere else.
On soft bottoms, there are many deposit feeders,
meaning the food that is consumed by many
of these organisms have already settled on
the bottom, and it's by going through the
sediments that they find their food.
If we're looking at an environment that has
a coarser substrate, such as sand instead
of mud, then we'll find more of the feeders
will be suspension feeders, where they collect
their food particles from the water itself.
The difference between these two types of
environments usually has to do with the amount
of turbulence, or water movement.
Organisms that dig in the sediments are known
as bioturbators.
These are burrowing or digging animals that
disturb bottom sediments.
This helps to oxygenate the sediments, and
these can include things such as polychaete
worms, molluscs, certain fish, and even whales.
A unique soft-bottom community that I would
like to talk about are the soft-bottom oceanic
communities near the poles, both North Pole
and South Pole.
These marine organisms face unique challenges
and also have unique diversity.
One of the challenges of living in this environment
is the formation of sea ice.
The ice grows from the surface down, and it
can reach all the way to the sediments, freezing
and capturing marine organisms along the way.
The sea ice can also move, which can actually
scour the surface of sediments, removing and
killing marine organisms that had been present.
So there can be a large amount of disruption
to the soft bottom sediments in these marine
areas.
Not only sea ice but also gray whales and
walruses can function as bioturbators in these
environments.
In this image, you can see the wall of sea
ice, and you can also see marine organisms
embedded within that wall.
Many of these organisms move so slowly that
they would not be able to escape the encroaching
ice.
In some of these soft-bottom communities,
we get some of our few marine plants growing
in dense mats called a seagrass meadow.
Seagrasses are flowering plants, and there
are over 50 different species of seagrasses.
A seagrass meadow will usually develop in
sheltered, shallow waters with good water
clarity and mostly soft sediments.
In tropical areas, the most common is turtle
grass from the genus Thalassia and then in
temperate waters, eelgrass from the genus
Zostera.
There is a high diversity of species associated
with seagrasses, including organisms that
live on, around, and within the seagrass beds
themselves.
The density of the individual plants tends
to be high like you would assume with terrestrial
grasses as well, and they generate a very
high biomass.
These seagrasses have high levels of primary
production, and unlike seaweeds, they can
actually get nutrients using their roots,
often acquiring things that had otherwise
been lost in the sediments.
Some herbivores do feed on the leaves, such
as turtles and sea urchins, but most biomass
becomes available to consumers as detritus.
Not only are the seagrass able to get nutrients
from the sediments, they also help to stabilize
the sediments, and stability is usually a
rare commodity within the soft-bottom communities.
When we look at the food web of these seagrass
meadows, we see most of the primary production
comes from the seagrasses, with some organisms
feeding directly on them, but more of the
organisms feeding off of the detritus.
The carnivores feed on the primary consumers.
There are many interactions at many levels
of the food web in a seagrass meadow.
If there is a lack of a primary or apex predator,
that can cause other populations to grow.
As those populations grow, their prey become
limited, and that can in turn affect the levels
below them.
The change in the amount of predators 
can cause a drastic change in the ecosystem,
and this is known as a trophic cascade.
In this particular example, in the absence
of sea otters, crabs develop.
Those crabs feed on the organisms that typically
feed on the epiphytes that grow on the seagrass.
If there are fewer epiphyte grazers, the seagrass
actually becomes encrusted with other organisms
that can limit the seagrasses’ ability to
photosynthesize and to be effective primary
producers.
With the introduction of the sea otters as
a predator on the crabs, that means that there
are more grazers to eat the things that are
growing on top of the seagrasses, meaning
the seagrass is healthier and able to produce
more biomass.
It's interesting how these keystone predators
can balance so many different aspects of an
ecosystem.
And that completes this video.
Now before our next video, I would like you
to think about: if you could live in the forest,
what part of the forest would you live in?
We'll talk about a different type of forest
in our next video.
