Hello marine biology students.
This week we're going to talk about life living
within the water column itself, both near
shore and offshore.
[Intro Music]
So this week, we'll be talking about diversity
of life in the epipelagic, challenges of living
in the epipelagic, then we'll be talking about
the communities in the mesopelagic, or middle
waters, and then the bottom communities as
well, in the deep waters and also on the seafloor.
When we look at the different ways to categorize
regions of the ocean, the pelagic is going
to be all of the open water column away from
the bottom.
So, we call this the pelagic realm.
Now, the pelagic is made up of near shore
and offshore waters.
The epipelagic is going to be the surface
layer of these waters, and we'll see the epipelagic
both near shore, or neritic, and offshore,
or oceanic.
The prefix epi- means “on top of,” or
“surface of,” and so the epipelagic are
the surface waters to a depth of about 200
meters.
They are divided into the neritic waters,
which are the waters over the continental
shelf.
These are the shallow waters which we would
call the subtidal zone.
The oceanic waters are those waters that are
past the shelf break.
They are the waters beyond the continental
shelf.
So, some of the characteristics of the epipelgic,
whether we're talking about near shore or
offshore, these are going to be the warmest
portion of the water column.
This is also the most well-lit portion of
the water column.
We sometimes refer to it as the photic zone,
but light can be limiting both in high latitudes
and also at night.
The epipelagic are vast stretches of water
that support primary production.
This primary production supports organisms
in the epipelagic, as well as organisms in
other communities.
This primary production can go to other communities
by ways of water currents and also especially
sinking.
In the epipelagic, there is no substrate for
attachment.
There's no bottom for burrowing or deposit
feeding.
The places to hide from predators are extremely
limited, but in that same way, predators cannot
easily catch their prey for the same reason.
In the epipelagic, there are the active swimmers
and the drifters.
We’ll talk about the drifters first, the
plankton.
Plankton thrives in the epipelagic.
The plankton includes all organisms that cannot
swim against the prevailing currents.
Many are microscopic.
In fact, plankton is classified by size, by
trophic level, or by the length of time spent
in the plankton.
This diagram shows the designation of plankton
based on size.
The smallest plankton are the femtoplankton.
These are going to be the viruses of the ocean.
Picoplankton are going to be made up of archaea
and bacteria, including cyanobacteria.
Nanoplankton will be very small protists,
and some of these will be photosynthetic,
while others will be consumers.
The next groups of plankton, the microplankton,
mesoplankton, and macroplankton, are considered
to be net plankton because they are the types
of plankton that can be caught in plankton
nets.
Since this was the main means of observing
plankton for a long period of marine biology
history, this group were thought to be the
most important and most abundant for a long
time.
It wasn't until molecular biology studies
were done to show just how much viruses and
bacteria and archaea and ciliate nanoplankton
there are, to realize that the net plankton
is actually just a portion of the picture.
And then there are the large drifting organisms
such as jellyfish, pelagic kelp, and siphonophores.
These aren't necessarily considered net plankton
because they can be too large, but they do
drift with the currents.
Oceanographers have spent a lot of time and
energy studying net plankton
because those were the types that could be
observed with the established techniques.
However, with molecular biology techniques
and genomics techniques, we have discovered
huge parts of the pelagic ecosystems that
were previously unknown to us.
This does not mean that net plankton are not
important, but they are just one part, and
not even necessarily the largest part, of
the plankton community.
So after size, a way to designate plankton
is whether they spend their entire life as
plankton or just part of it.
Holoplankton 
include many types of phytoplankton and pelagic
crustaceans like copepods and ostracods.
They are plankton for their entire life, whereas
meroplankton
spend only a portion of their life as plankton.
This includes the larvae of fishes, molluscs,
crustaceans, a variety of different organisms
that, as adults, will either live on the benthos
or as nekton.
Another way to classify plankton is whether
they are primary producers or not.
Phytoplankton are the primary producers.
They perform photosynthesis, and so they use
light in order to generate organic molecules.
Zooplankton, on the other hand, are heterotrophs,
or consumers, yet the zooplankton can play
a very important role in connecting the production
of nano- and picoplankton
to higher levels of the food web, because
it turns out that many of these plankton that
are smaller than net plankton are also producers
as well.
There are photosynthetic bacteria and archaea,
such as cyanobacteria, and even some of the
smaller ciliates, and those cannot be consumed
by fish.
But they can be consumed by the zooplankton,
and the zooplankton then take their energy
and organic molecules to higher levels of
the trophic web.
We had discussed various types of plankton
in previous chapters, but we'll revisit them
here because they make up such important parts
of the epipelagic communities.
So diatoms—these are considered to be net
plankton.
They are extremely important primary producers,
especially in polar waters.
They're common in all marine waters but tend
to be more important in cold water.
These diatoms may be solitary cells or a colony
of cells, and their outer shell is made out
of silica.
Our next group of phytoplankton are the dinoflagellates.
There are over 1,200 species, and they each
have a unique shape that's reinforced by plates
of cellulose.
They have two flagella in grooves on the body
that produce spinning motion.
Some of these are bioluminescent.
There are some that are toxic, such as Pfiesteria,
and dinoflagellates are some of the species
that cause red tides.
Dinoflagellates are particularly prevalent
in warm waters and can bloom when nutrients
are plentiful.
Our next type of phytoplankton are cyanobacteria.
These are prokaryotes known as blue-green
algae.
They're able to carry out nitrogen fixation
as well as photosynthesis.
They are important primary producers, and
we've come to find out they are far more abundant
in the pelagic than originally thought.
Many grow in filamentous colonies, where others
may be solitary.
Two more groups of phytoplankton are the coccolithophores,
which are members of the nanoplankton
and they can occur both in neritic and oceanic
waters.
They have plates of calcium carbonate which
can be contributed to marine sediments and
can eventually become chalk.
The silicoflagellates 
have a star-shaped skeleton made out of silica
and two flagella of varying lengths.
These two groups are so small that they are
not often caught in plankton nets, yet they
can make up important portions of the waters
that they're found in.
Now, to discuss some of the zooplankton, especially
zooplankton which are also holoplankton.
Copepods are one of the most abundant marine
crustaceans, which make them some of the most
abundant organisms on earth.
They make up perhaps 70% of the zooplankton.
They feed on phytoplankton 
as well as other zooplankton.
They serve, in turn, as a major source of
food for other consumers, and as we can observe
in this diagram, their appendages will be
slightly different based on whether they are
herbivorous or carnivorous.
There are other crustacean zooplankton as
well such as krill and other shrimps and also
ostracods.
Ostracods are interesting crustaceans that
almost appear to be bivalves, because they
have two outer shells.
Yet, they have jointed appendages that stick
out of them for feeding.
You can also almost think of ostracods as
free swimming barnacles.
Another group of zooplankton are the pelagic
tunicates, the salps and the larvaceans.
They use mucus nets to capture food particles.
Salps can be solitary or occur as large floating
colonies and the larvaceans pictured below
will make a house out of mucus to capture
food.
Yet, they will often shed this house and make
a new one shortly after.
The discarded houses end up becoming an important
component of what is known as marine snow,
and that marine snow will carry some of the
energy generated in the epipelagic down to
lower levels of the water column.
Another interesting type of holoplankton zooplankton
are the pteropods, which are molluscs.
In pteropods, the molluscan foot is modified
as wings.
Usually they will have a reduced shell and
they can be found in the epipelagic, but also
in deeper waters.
The pteropods feed on phytoplankton and other
zooplankton.
Some more members of the holoplankton include
the arrow worms, chaetognaths, which are predators
in their environment, often feeding on copepods,
ostracods, and even other arrow worms.
And then we have the jellies.
They can range in size from microscopic, all
the way to hundreds of pounds.
Comb jellies tend not to grow as large, but
can still be much bigger than other members
of the plankton community.
They are considered plankton because they
mostly drift in the currents.
Holoplankton live their entire life as plankton,
but meroplankton will be plankton only during
their larval development.
The meroplankton 
only spend a portion of their life as plankton.
This includes larvae of animals that are part
of the nekton as adults or part of the benthos,
such as molluscs, polychaete worms, crustaceans,
echinoderms and many kinds of invertebrates.
Veliger larvae are common for molluscs, while
the oiphiopluteus larvae would be for a brittle
star and the bipinnaria larvae would be for
other echinoderms.
These distinctive larval shapes are often
tied to different phyla or groups of animals.
The epipelagic is not just home to plankton,
there are nekton as well.
The nekton include organisms that swim against
the currents and purposefully move in any
direction they choose.
These include fishes, penguins, squids, sea
turtles and marine mammals.
That completes our discussion of the diversity
of life in the epipelagic.
Now, before our next video I want you to think
about “What if you played a game of hide-and-seek,
but there was nothing to hide behind?”
We'll talk about that in the next video.
