Hello everyone. This screencast is
going to talk about the topics of
nutrient cycling and energy flow
specifically in the epipelagic zone of
the ocean. So if you remember the ocean is
divided up into major zones in the
pelagic zone. The epipelagic, mesopelagic, bathypelagic and  abysslpelagic. In ocean
trenches, we call it the hadalpelagic zone. Each zone has many distinct
characteristics but tend to center on a
few things. Each zone has varying amounts
of light, oxygen, temperature and nutrient
that tend to be present in it. The amount
of each of these variables is going to
determine what kind of animals can live
successfully in each of these
environments. Before we dive into the
details of nutrient cycling and energy
flow, it's helpful to think about how
each of these characteristics varies
across all the oceans zones. Light
tends to vary in a linear fashion with the most
light being in the epipelagic zone and
the least light being in the abysslpelagic or hadalpelagic layers. Oxygen tends to be
slightly more complex, however. There's a
large amount of oxygen in the epipelagic zone
thanks to transference from the air and
the mixing of oxygen in from the waves.
We also have lots of photosynthetic
organisms that live here, such as phytolankton.
In the lower zones, however, we
don't have cross over from the air
into the water and we also don't have
photosynthetic organisms because there
isn't enough light. Furthermore there is
also a large amount of decomposition
that happens in this area right here. The
bacteria use up a lot of the available
oxygen. However we can get some oxygen
mixed in the lower levels thanks to
thermohaline circulation. Temperature
also has an interesting profile. Once we
move out of the epipelagic zone, the
water tends to get extremely cold. There
is some circulation that happens due to
lighter water being less dense and being
warmer, but most of the time it's
extremely cold at the lower depths
unless you happen to be near an ocean vent. Nutrients also tend to follow the
profile of light. There are a lot of
nutrients in the epipelagic zone, but
the deeper you go the less nutrients
tend to be available. Again the exception
to this is when you happen to be near an
ocean vent, which will talk about later
on this term. Ocean vents tend to bring in
new nutrients from inside the earth. As
you can see, it would be extremely
challenging to live in the
abysslpelagic or hadalpelagic zones
because of the lack of light lack of
oxygen,
lower temperatures, and lack of nutrients.
Organisms that manage to live here must
have very specific adaptations that
allow them to survive. In an ecosystem,
the environment tends to influence the
organisms and the organisms also tend to
influence the environment in turn. Two of
the ways that this happens are through energy
flow and nutrient cycling. These
interactions include both living and
non-living factors. For instance the kelp
forest is also home to many different
kinds of organisms. Fish may eat the kelp
for food and may also use it for shelter.
However, the fish may also provide
nutrients for the kelp when it
decomposes its food. The fish also tend
to help keep the kelp forest in check
by eating the weaker damaged leaves. This
allows the kelp forest to grow stronger
and more healthily. The kelp forests also
undergoes photosynthesis and produces
oxygen, which the fish needs to breathe.
The fish takes in the oxygen from the kelp
and exhales carbon dioxide. Carbon
dioxide is then taken in by the kelp
forest so that it can undergo more
photosynthesis. Nutrient cycling involves
many different kinds of organisms; these
include producers consumers and
decomposers. Producers tend to create
their own food which means that they are
photosynthetic. We can also call
producers autotrophs. Consumers eat
producers. There are primary, secondary and
tertiary consumers. Occasionally, we also
have quaternary consumers, but not very
often for reasons that will see. The
decomposers break down dead producers and
consumers. Here's an example of a
stereotypical ocean food chain
starting with phytoplankton, and
zooplankton, then small fish and then
larger fish. The nutrients will flow from
the phytoplankton to the zooplankton when the
zooplankton eats the smaller phytoplankton.
When any of these organisms dies, they are
broken down by decomposers and the
nutrients flow back into the cycle again.
Another important consideration in
ecosystems is energy flow. In the epipelagic zone,
the energy originates in the
sun. The light energy travels to the
earth in the form of photons of light
which then nourish the photosynthetic
organisms that live near the top of the
epipelagic zone. These photosynthetic
producers take light from the sun and
they also take in carbon dioxide. They use
the process of photosynthesis to produce
oxygen and also to produce sugars in the
form of glucose. The energy then flows
down the food chain into the primary
consumers, then the energy goes on to the
secondary consumers, and can then move on
from the secondary consumers to the
tertiary consumers. Now let's look at the
energy flow a little bit more closely.
We're going to do this by looking at the
number of organisms that might
theoretically be at each level of the
food chain or potentially if the number
of calories of food that might be at
each level. The number of creatures at
each level of the food chain decreases
the farther down this list you move.
You'll notice that there are a lot of
photosynthetic producers, but not very
many tertiary consumers. This decrease in
numbers is called the 10% rule. The 10% rule
states that only ten percent of the
energy at one level can then move on to
the next level.
Let's take the example of one million
photosynthetic phytoplankton as an
example. Zooplankton preys upon
phytoplankton, so we're going to need to
have fewer zooplankton than we do
phytoplankton. A population of one
million phytoplankton can only
support a population of 100,000 zooplankton.
Much of this energy differential is
accounted for by heat and also by the
daily activities that phytoplankton
would go through. If we continue to move
up the ladder, a population of 100,000
primary consumers could only support 10%
as many secondary consumers. Again a lot
of the energy is lost as heat and also
in the process of finding a mate and
searching for food. If we continue to go
down the food chain, we will see that this
population can only support one thousand
tertiary consumers. Even though we
started with a very large number of
photosynthetic producers, by the time we
get to tertiary consumers, there's very
little energy left. Not many ecosystems
can support quaternary consumers
and it's probably pretty easy to guess
why. If we follow the 10% rule, a
population of one million producers can
only support one hundred quaternary
consumers and that's if conditions are
ideal. When we look at the structure, it's
not difficult to understand why
populations of sharks and tuna tend
to be much smaller than populations of
zooplankton or phytoplankton, and also
qhy overfishing is so dangerous to these
populations. There really aren't that many of
them to begin with.
Furthermore, these species are further
threatened because they tend to
reproduce very slowly and take many
years to reach sexual maturity. Necause
it takes them so long to reproduce, the
populations grow even more slowly than
they would normally. Unfortunately, it's
very difficult to convince people that
they should make sushi out of
photosynthetic producers or out of
zooplankton rather than that out of tuna.
One final thing to remember about energy
flow is that none of the energy winds up
back in its original source.
unlike nutrient cycling. It will be
important to keep some of these
similarities and differences in your
mind as we go through the rest of this
unit. That's it! See you all in class and don't forget to subscribe!
