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!