Hello marine biology students.
In this video we're going to talk about energy
flow and nutrient cycles within ecosystems.
[Intro Music]
So energy flows through an ecosystem, and
we call this flow of energy the trophic structure.
It is driven by primary producers.
These are going to be the organisms that generate
food, usually from some external energy source,
whether it's from a light source or a chemical
source, the primary producers are the autotrophs
that make the food and then there will often
be one to many levels of consumers.
These are going to be heterotrophs that feed
on food that was made by the primary producers
and they can either feed directly on the primary
producers or they can feed on animals that
feed on the primary producers, and there can
be many steps in a food web.
On average, only about 10 percent of the energy
is transferred to the next level of the food
chain.
We call this the pyramid of energy.
This means the total amount of energy made
by the primary producers, of that energy,
only about ten percent of it is going to be
represented within the bodies and biomass
of those first level consumers.
And as those first level consumers are consumed
by second level consumers, only ten percent
of that first level consumer is going to be
represented as biomass or energy in that second
level.
The energy pyramid represents each trophic
level by a volume directly proportional
to the energy stored in new tissue per unit
time.
Energy pyramids can never be inverted, meaning
narrow at the base and broad at the top because
of the laws of thermodynamics.
It would be impossible for there to be more
energy in a second level of an energy pyramid
than in the first.
So for example, if phytoplankton generate
10 million calories of primary production
and the krill which feed directly on the phytoplankton
produce 1 million calories, then whales are
only going to have a hundred thousand calories
in their biomass, again, for every ten million
calories made by the phytoplankton.
So this is our energy pyramid.
Now, baleen whales, they are only second level
consumers, but as we can see there are 3rd
4th 5th even 6th level consumers in this Antarctic
food web, meaning that the amount of energy
stored in each killer whale took an extremely
large amount of energy in the previous levels
in order to be developed.
Now we can see energy levels coming from several
different areas, but killer whales are the
apex predator of an Antarctic food web, followed
closely by leopard seals and Weddell seals.
Primary production 
is the organic matter that is left over after
the primary producers have met their own needs,
and this makes up the base of that trophic
pyramid.
So, the diatoms, which are the primary primary
producers in that Antarctic food web.
They are performing photosynthesis, however,
they are also using photosynthetic product
for their own growth and development and reproduction.
They are producing oxygen as a result of their
photosynthesis, but they are also using oxygen
as a result of their own respiration.
So primary production really just is the excess
material after the primary producers meet
their own needs.
Another term to be aware of is the standing
stock.
The standing stock is the total amount of
phytoplankton, the main primary producers
in the water, and so it's the standing stock
which we use to make measurements about photosynthesis
that's happening, but also the consumption
of photosynthetic product necessary for those
primary producers to still be present.
In a marine system, primary productivity is
generally measured as a one meter water column
and all of the photosynthesis happening in
that column, minus the respiration happening
by those primary producers, that is the primary
productivity of that column.
Measures of primary productivity can be done
by taking a water sample and storing it either
in a chamber that is transparent to sunlight
versus a chamber that is opaque or dark.
That dark chamber is going to tell us how
much oxygen is being used by respiration and
how much of that organic matter is being used
and broken down by those diatoms.
So we can compare those two to see what the
net production of oxygen is from the photosynthesis.
In ecosystems we often talk about nutrient
cycles and some of these major nutrients include
things like carbon or nitrogen and phosphorus.
Carbon is used by primary producers to manufacture
organic molecules and they do this through
a process called photosynthesis.
These initial photosynthetic products are
then going to be converted into more complex
carbon compounds, either by the primary producers
themselves or also consumers of these primary
producers.
Carbon dioxide is eventually released from
all organisms through respiration and decomposition.
And so, when we look at the carbon cycle,
the source of carbon for these primary producers
is going to be the carbon dioxide gas that's
dissolved in the sea water.
This carbon dioxide is going to be obtained
both through the atmosphere but also from
other internal sources.
As detritus and dissolved organic matter decompose,
they contribute to the dissolved CO2.
As fish feed on both the primary producers
and on the detritus and dissolved organic
matter, they are going to contribute dissolved
carbon dioxide through their respiration.
Primary producers are mostly a sink for this
carbon dioxide in that they take more in than
they give off from respiration.
However, their death and excretion and consumption
by other organisms will end up taking that
and returning it into the water column.
We'll also see that organic matter in carbon
will be deposited to the sea floor in the
form of sediments, but that those sediments
can also dissolve, resulting in some of that
carbon dioxide being returned back into the
water column.
So as you can see with this carbon cycle,
it can be quite intricate, with many different
sources and sinks for carbon.
Nitrogen and phosphorus will be similar.
Nitrogen and phosphorus are also required
for primary production and must be cycled
through the ecosystem.
Both are important limiting factors 
for primary producers in many marine ecosystems.
The main source of dissolved nitrogen gas
within the water column is from the atmosphere,
yet nitrogen gas is not accessible to most
organisms and so there can be nitrogen-fixing
bacteria and organisms in the water that allow
for that atmospheric nitrogen to go in a form
that can be usable, such as nitrates nitrites
and ammonia.
However, we’ll also see inputs from rivers,
just dust and other lithogenous sediments
coming in from the continents, and then through
the organisms living within this ecosystem
as well.
Again, primary producers will primarily be
absorbing nitrogen and nitrites and ammonia.
They are going to be a sink for this, although
some will be released by excretion.
Conversely, the consumers are mostly going
to be contributing in the form of excretion,
although they might absorb some themselves.
And detritus and dissolved organic matter,
as it decomposes, will also contribute to
the dissolved nitrogen.
The phosphate cycle will be similar in that
most of the phosphate is coming from lithogenous
or atmospheric sources.
So either coming in through rivers or just
simply particulate matter coming down from
the atmosphere and this dissolved phosphate
is essential for primary producers and it
also will be released by excretion and decomposition.
However, precipitation or marine snow, the
falling of organic matter down from the water
column to the ocean floor, can take with it
phosphate and in that way it is removed from
the system.
The last topic for this particular portion
of the lecture has to do with organismal lifestyles
in their ecosystems.
There are three major lifestyles of marine
organisms, there are those of the benthos,
which are going to be associated with the
seafloor.
They might be attached or they might move
around on the bottom, but they are going to
limit their activity to the ocean floor, that
interface.
This is in contrast to the pelagic organisms,
which live within the water column themselves.
Of these pelagic organisms, some of them are
not going to be especially strong swimmers.
They're simply going to move with current,
and we call these the plankton.
This can include both phytoplankton and zooplankton
and there can be microplankton but also macroplankton.
Conversely, the strongly swimming organisms
are known as the nekton.
When looking at the different regions of the
oceans and the different water layers that
we're going to be talking about through these
weeks.
The intertidal is the zone between high tide
and low tide, a region that will be exposed
at least once, but sometimes twice a day in
most marine environments.
Then we have the neritic waters, which are
part of the pelagic system.
They are considered near shore and they are
on top of the continental shelf up until the
shelf break.
From the shelf break onward, we have the oceanic
region of the pelagic zone and there are several
different layers of that oceanic region.
The epipelagic is the surface of the oceanic
region and this is where light can still penetrate
and so photosynthesis will be the primary
form of primary production.
In the mesopelagic, light is reduced and organisms
can still see light, but photosynthesis will
be far less efficient.
In the bathypelagic and abyssopelagic, these
are going to be oceanic regions which have
no light at all except for that that's caused
by the organisms living in those areas.
A variety of very strange and interesting
sea creatures live in the bathypelagic and
abyssopelagic.
We'll be talking about all of these environments
in this next section of the course.
In most of these, we are going to see both
benthic organisms and also pelagic organisms.
So the benthos, so when we're talking about
organisms that are associated with the bottom,
well in the intertidal zone, any permanent
residents would have to be benthic, because
for part of the time, water simply isn't present
this is region that is exposed between high
tide and low tide.
In the subtidal zone, this is going to be
the region below the low tide level all the
way to the edge of the continental shelf,
it will always be submerged and the benthic
organisms here still likely are exposed to
light during the day.
But then in the deep sea, in the bathyl and
abyssal and hadal zones, this is going to
be beyond the shelf break and the benthic
organisms here likely will not include primary
producers or at least not photosynthetic producers.
The primary producers would have to have other
means of generating energy.
For the pelagic organisms, the epipelagic
zone is going to be the surface waters.
This will be from the surface to about 100
to 200 meters.
There will be plenty of sunlight available
to support photosynthesis, as the main means
of primary production.
The mesopelagic 
is the zone that extends from lower limits
of the epipelagic to about 1000 meters.
There is reduced light, but some light can
still be seen, just not enough for photosynthesis.
And then, in the bathypelagic, abyssopelagic
and hadopelagic zones, these are deep sea
zones where light does not penetrate, but
that free swimming organisms can still be
found.
And that completes our discussion of energy
flow and nutrient cycles.
Now, if you ever thought that the annual temperature
fluctuation in Southern Nevada was pretty
extreme, well let me introduce you to the
intertidal.
See you in the next video.
