Food Webs and Challenges of the Marine Environment
Sharon L. Gilman, Ph.D., Coastal Carolina University (2012)
Marine Food Webs
In the last section we said that primary production in the ocean comes primarily
from microscopic one-celled algae and bacteria. Less than 2% of the ocean is
shallow enough with a firm enough bottom to support larger attached plants. So it
is the energy produced by these algae that forms the basis of the food pyramid in
the ocean.
Figure 1. A typical oceanic trophic pyramid. The phytoplankton are the primary producers and
the other layers are the consumers. This image is in the public domain.
The pyramid represents energy in the ocean. The widest part is the microscopic
algae and bacteria. These are at the bottom because they are what converts the
sun's energy into carbohydrates usable by everything else. Organisms that can
make their own food like this are called autotrophs or producers. Since they
have access to the most energy---all the energy from the sun---they have more
biomass (living mass) than the layers above them.
Just above the producers are the organisms that depend on them for food. These
organisms, from microscopic plankton to whales, cannot make their own food
and they are called heterotrophs or consumers. There are several levels of
consumers in an ecosystem. Since there are various "-trophs" involved here,
this is often called a trophic pyramid and the different levels, trophic levels.
Attributed to: [Sharon L. Gilman]
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In the pyramid above, typical of the ocean, there is the phytoplankton on the
bottom. (Perhaps I should add here that plankton are the very small "drifters" in
the ocean. Phytoplankton are the photosynthesizing bacteria and algae, and
zooplankton are the animals.)
Note also that at the bottom there are 1000 "energy units" available. The next
level up is the herbivorous (plant eating) zooplankton. At this level there are
100 "energy units" available. Carnivorous (meat eating) zooplankton eat the
herbivores, these are eaten by carnivorous fish, and in this case, tuna is the top
of the line or the apex predator. Actually, in most ocean systems, and perhaps
most terrestrial systems, people are probably the real apex. Anyway, notice as
you move up that the pyramid gets narrower. This represents the diminishing
energy and is reflected in the diminishing biomass. Certainly there are fewer tuna
in the world than zooplankters. If you look at the energy units, you see that each
layer has only 10% of what the previous layer had. 90% of the energy at each
level is lost. Why is this? Well, phytoplankton don't just produce for the benefit of
those zooplankton. They've got living, growing, and reproducing to do
themselves. In doing that, they use up energy. In fact, they use up 90% of the
energy they take in. You do too. That's why there are fewer sharks than
anchovies in the world. There's simply not enough food to support a lot of big
things.
Here's an interesting thing to consider: what are the biggest animals on land?
Elephants, right? What do elephants eat? Plants! What are the biggest animals in
the ocean? Whales, bigger even than elephants. And what do you think the very
biggest types of whales eat (the baleen whales)? Plankton! Why? That's where
the energy is. The biggest fish in the ocean is the whale shark. What do you think
it eats? Plankton! So if you want to maximize energy efficiency in your eating,
you need to eat low on the food pyramid. It's a good idea even if you aren't a
whale. This figure shows this. Fewer levels in the pyramid means more energy
available at the top.
Attributed to: [Sharon L. Gilman]
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Figure 2. The trophic pyramid for a baleen whale (the kind that eats plankton). This image was
created using the works of Rovag and Uwe Kils at en.wikipedia, and is licensed under the
Creative Commons Attribution-Share Alike 3.0 Unported license.
Okay, we covered the consumers and producers, but there's one more key
category of organisms, without which this system would not work. They are the
decomposers. These are mostly bacteria and fungi, although they get help in
their work from the scavengers or detritivores (things that eat detritus or dead
stuff). Why do they matter? Well, if they didn't exist on land we'd be up to our
ears in dead things, but more important, nutrients that plants need to make food
would be stuck out of reach. The decomposers free up those nutrients so they
can cycle around again.
Figure 3. The paths of nutrient and energy cycling through a food web. (Note that in the ocean
the primary producers would be phytoplankton, not plants.) This image is in the public domain.
This is all neatly arranged in these figures, but in the real world, things are much
more complicated. There's a reason it's called a food web. This figure shows a
typical food web. Note that the arrows to each category nearly all go both ways.
This is true for most organisms. Practically everybody is somebody else’s lunch.
The Marine Environment
It probably seems to you that life on land is a lot easier than life in the ocean, but
not really. Life on earth started in the ocean and only recently, geologically
speaking, moved out onto the land. Land is a nasty place. To live on land you
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have to deal with gravity, dehydration, and large temperature variations. To live
on land, organisms have had to develop specialized systems and appendages to
cope with these things. These include fur, root systems, strong legs, fruits,
skeletons, all sorts of complicated things. In the ocean the temperature is fairly
stable (you remember all about specific heat, right?), water is plentiful, and you
can float and move by friction. In fact, the overwhelming mass of organisms in
the ocean are simple and mostly microscopic things.
Environmental Stability
In the deep sea, things remain unchanged year in and year out. The temperature
is constant. It's constantly dark. The water pressure is constant, although very
high. At the surface things are a little more variable, but not much. Light intensity
changes daily and seasonally and this is especially important in the polar and
temperate regions. Surface temperatures change seasonally, but not by much.
Likewise, rainfall or lack thereof changes surface salinity, but again, not by much.
The organisms that live in coastal water which change the most are much more
tolerant of these changes that organisms that frequent the open ocean.
Regulation of Body Fluids
This is a much bigger problem for a land organism than for a marine organism.
Consider all the adaptations you have to maintain your fluid balance. You have to
drink. You have skin that keeps things both in and out of you. You have very
elaborate kidneys. Since in the ocean an organism is surrounded by fluid, for
most species there's no point in going to great lengths to maintain internal fluid
different from this. Therefore most marine organisms are permeable and
isotonic. In other words, fluids can simply move through their body coating, and
their internal fluids have the same salt concentration as their external fluids. They
spend no energy in trying to maintain fluid balance, but the down side is that they
are intolerant of change. You can't toss an oyster in with your goldfish and expect
it to live---and oysters are among those very tolerant coastal/estuarine
organisms.
For the more complex organisms of the ocean, there are enough advantages to
being able to maintain an internal environment whatever the outside environment
that they do expend energy doing this. Fish, for example tend to travel, so it's
helpful if they have some control over their internal environment. They are
semipermeable. This means that water can pass through their skin, but salt
cannot. A fish has an internal salinity of about 14 ppt (parts per thousand). The
average ocean salinity is 35 ppt. Water can pass through the fishes skin by
osmosis (the diffusion of water) so what tends to happen? Water leaves the
fish for the more salty ocean---it "wants" to balance out those two different
salinities. So the fish tends to dehydrate. But unlike you, who would likewise
dehydrate drinking salt water, the fish has adaptations to take in the salt water
but excrete the salt. It drinks lots of water and secretes the salt out its gills and its
highly salty urine. Sea turtles and crocodiles also have special adaptations to
excrete salt, and this allows them to live in salty water. They both "cry." Surely
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you've heard of "crocodile tears?" The crocodile isn't sad, he's merely getting rid
of salt.
To see if you understand the concepts of osmosis and permeability, see if you
can figure out what freshwater fish have to do to maintain their internal 14ppt
salinity in 0ppt freshwater.
Density Problems
Most marine organisms are more dense than water and so they have a tendency
to sink. This isn't good because the surface is where the sun is, so it's where the
primary production occurs, so it's where the food is. There are several ways of
dealing with this. A lot of marine organisms are mostly water themselves, so they
float. Jellyfish are a good example of this. Mammals and reptiles have lungs
which help. The mammals also have thick fat layers which both insulate and help
them float. Sharks and some other fish and a lot of the very small zooplankton
have high concentrations of oil and you know that oil is lighter than water, so this
helps. These very small creatures, as well as the algae, for whom staying in the
sun is absolutely vital for photosynthesis, have all sorts of interesting shapes and
projections to increase their surface area and make it easier for them to float.
Most fish have swim bladders which are gas-filled sacs that inflate or deflate
according to where in the water column the fish needs to be. The fish adds air if it
wants to go up and releases air if it wants to go down. Some of the faster fish like
tuna, some sharks, and also octopus and squid simply have to keep moving or
they sink. Then there are lots of creatures that simply live on the bottom. Down
there it helps to be quite watery because water is essentially incompressible. No
internal airspaces allowed.
Ocean Environments
For a lot of ocean organisms, floating is key so that they stay near food, but by
no means is this important for everything. In fact, marine scientists divide the
ocean into several distinct environments and each of these presents its own
challenges to the organisms there.
The oceanic or pelagic zone comprises the open ocean away from shore and
away from the bottom. Sort of the opposite of this is the benthic zone or the
bottom. Most organisms are either benthic or pelagic. Within those categories,
are several different zones, however. The neritic or coastal zone is the shallow
water over the continental shelf. The littoral zone is intertidal. The abyss is the
bottom where it's very deep. Most things live where the light reaches. This is
called the photic zone. The aphotic zone is where it's dark. And so it goes. All
these divisions are artificial, of course. It's one big ocean when you get right
down to it. Still, it makes it a little easier to talk about if you can divide things up.
These zones are summarized in the figure below.
Attributed to: [Sharon L. Gilman]
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Figure 4. Ocean zones. This image is attributed to K. Aainsqati at en.wikipedia and is licensed
under the Creative Commons Attribution-Share Alike 3.0 Unported license.
Attributed to: [Sharon L. Gilman]
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