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So, today we're going to talk about, we've talked about
primary productivity on a global scale
last time.
And today, we're going to talk about what regulates that
productivity.
In other words, last
time we just talked about on average the amount of carbon and
biomass that was distributed among the
ecosystems of the globe.
And we talked about deficiencies of transfer of that
biomass through food webs, etc.
By the way, the one universal thing that everybody seems
to like are the DVDs at the end of the class,
which is good.
Unfortunately, in today's class we are not
going to have one.
So you have something to look forward
to on Wednesday.
I'll show you one at the end that is really one of the
cooler ones of the collection.
It has nothing to do with the lecture
but I'm going to show it to you anyway.
OK, so today we are going to talk about what regulates the
productivity.
We've talked about these complex systems in ecology,
and the feedback mechanisms.
So, in the context of productivity we're
going to talk about the factors, the abiotic factors,
to the non-living parts of the Earth,
that regulate this productivity and how the
productivity feeds back on those.
And then Wednesday, we're going to
put it all together in an analysis of global
biogeochemical cycles, how
the elements in the globe cycle, and how that's mediated
by organisms.
And then, that will be the end of the segment.
And when I come back, we're going to move on to
population and community ecology, which will
feel like a totally different subject to you.
So, we'll talk more about organisms.
And some of you said it was so interesting to see
math last time in the last lecture even if they were just
efficiencies.
Well, when you get to population
ecology, you're going to have real math and you'll actually
have differential equations.
So, if you like that you have something to look
forward to.
If you don't like it, well, it's not too bad.
OK, so today the lecture will be in
two halves.
We'll talk about terrestrial productivity,
and then aquatic productivity and what
regulates it.
And, can you see in the back or do I need to turn,
I'm going to have to turn the
lights down, never mind.
So, because we have some colored images,
so let's start with terrestrial productivity.
And look at this map, oops, wrong ones.
I think that might be enough to
see it, which is a satellite image.
You've seen several of these so far.
So just showing the gradients of
productivity on a global scale, where we're just looking
at the land now.
Where it's green, you have high
levels of productivity.
These are grams carbon per meter squared per
year.
Yellow is intermediate, and red is very low levels of
productivity.
So, what determines this distribution of productivity in
terrestrial ecosystems?
Well, got any ideas?
This is not hard.
What's a key factor in regulating
plant growth on land?
Light, absolutely.
That's a given.
But looking at this map,
what's probably more important?
Water, exactly.
If we plot on a global scale, if we go around to all these
ecosystems, and we look at the average
annual rainfall and we plot it against net
primary productivity, you all know what NPP is right,
net primary productivity,
you get a graph that look something like this.
Each one of these dots would be an
ecosystem.
And this one is millimeters rain per year.
So, despite the Sahara Desert, and
this might be a tropical rain forest.
And they scatter, but
there's some sort of general relationship like that,
increasing NPP with increasing rainfall.
Well, what else is probably important?
What else is different between the Sahara Desert and the
northern US?
Temperature.
Exactly, which is not a good example
for comparison.
Let's take tropical rain forests and the northern US.
How's that?
But you get something like this.
In other words, it
doesn't always map directly onto temperature alone.
But on average, you find that places with
higher temperature have higher productivity
if there's adequate water.
So, does a relationship, there's an
interaction between the water and the temperature.
So, those are the key factors for terrestrial
ecosystems.
Now, what we have, so
there's light, nutrients, I mean light,
rain, and temperature, rainfall.
What about nutrients?
Anybody who's had plants and their room,
or nurtured our garden knows that
nutrients are very important.
You have to fertilize in order to get
the most growth.
So, how does that work?
Well, now we are going to do an analysis of a terrestrial
ecosystem.
We're going to use a tree.
Some ecosystems you have, that's a rock in case you
didn't recognize it.
And we have soil.
These are components.
And, we've talked about this over, and over,
and over now.
Photosynthesis is the key, taking up CO2,
evolving oxygen, we are going
to call this biosynthesis.
That's the mass from gas.
So, that's CO2 plus,
and now we're going to add something.
And these are not balanced chemical reactions,
OK?
These are just to give you an idea
of what's going on.
So, CO2 plus, there's all the elements
required for life, OK?
In other words, for a plant to grow,
it doesn't only need CO2 and water.
It needs all of these elements required for life.
And they are converted to organic forms of those
elements.
And then oxygen is evolved.
So, we are in a general sense, just modifying the
equation for photosynthesis to include all of
the elements that are required for life.
So that's the biosynthesis.
And then, the tree, these are leaves
in case you didn't recognize them, that are falling to the
soil.
And the leaves fall down to the
soil and they become, what?
What's that organic manner?
You learn it last time.
It's the whale falling to the bottom of the ocean,
and if that was a carcass, the general term for
dead, organic matter is called detritus.
And there is a detritivore food web, remember?
And we had some discussion with students after
class whether we were detritivores,
because in a sense we are because we eat dead meat,
right?
We don't eat live meat.
It doesn't matter.
You don't have to know that.
Forget that.
But it's interesting to think about.
So, the leaves fall down to the soil.
They are acted upon by the heterotrophic bacteria,
the bacteria that use organic
carbon.
And, what happens is that those bacteria and fungi,
and worms, and everything that chews on organic
matter are responsible for regenerating these elements in
the soil.
So, we are going to call that regeneration.
And that's basically, simply the back reaction of
this, OK?
So, you are starting with organic carbon.
And, organic carbon, organic PNS,
and it's converting it back to the inorganic forms so that
they're available for the tree to take
up again.
So, this is the cycle of biosynthesis regeneration,
and then take it up again.
Some of the feedback I got on the lectures
when people said that they found it
interesting to think about how everything in nature is
recycled and used.
Now, in some ecosystems, there's
another form of available nutrients: calcium here,
you have cations, potassium from rocks,
magnesium.
All of these are also in this equation.
When I go dot, dot, dot, dot,
it's every element that's required for life.
And in some ecosystems, the
dissolution of these elements from rocks is an important
renewal route for nutrients in the
ecosystem, OK?
So these are the two, basically in terrestrial
ecosystems, there is the rock source, and there's the
regeneration source.
And it turns out that different terrestrial
ecosystems have different relative dependence
on these two sources.
And this is an interesting phenomena.
So, tropical rain forests, like in the Amazon,
these are the forests that we're
concerned about losing for many reasons.
And these have, essentially,
no number one up here.
Tropical rain forests have essentially no renewal nutrients
from bedrock.
And temperate forests, however, have a combination
of one and two.
They can have renewal.
The bedrock is exposed, and the water cycle helps
dissolve the rock, and renews the nutrients
to the system.
So, if we look at the soil to biomass ratio of phosphorus and
nitrogen in the temperate forests
versus the tropical, we see something like this.
In the tropical rain forests, all of the nutrients in the
system are basically tied up in the
biomass.
And it's highly dependent, then, on
this regeneration cycle.
The trees fall down, it's regenerated,
it's taken up right away from the
soil whereas in the temperate system, you
see the opposite where there's a much higher proportion of
nutrients in the soil relative to the
tropical system.
And what that means is that if you cut down a tropical rain
forest, which they're doing,
converting to farmland, you will only get a few
years of productivity out of that farmland because once you
shut down the forests and haul away the
trees, you've hauled away most of the
nutrients in that ecosystem that are available to fuel
productivity.
And they can't be renewed from bedrock because they don't have
bedrock there.
So, that's one of the tragedies of cutting down these
forests when they probably would have more economic value
by harvesting some of the natural
products from the forests.
OK, so when we get to aquatic productivity,
we are going to see that we have these same
biosynthesis and regeneration processes.
So, that's what we're going to move
to now.
And let's look at the distribution of aquatic
productivity.
That's too much.
A while.
Can you see that in the back OK, the colors?
All right.
So, now we're just looking at the ocean's ecosystem.
And areas that are blue and green are less
productive than the areas that are
red and yellow in the system.
So you can see all of the coastal
regions; we have coastal upwelling that we're going to
talk about a lot with nutrients fueling that,
that is very important.
And the whole north Atlantic here we'll talk about that.
But before we talk about what regulates
aquatic productivity, now I am going to turn
the lights out, I thought I'd give you a tour.
Since you all know trees look like but you don't
know what primary producers in the ocean
ecosystems look like, I'm going to give you a quick
tour through the phytoplankton,
which as you know, are my favorite organisms.
So, the aquatic productivity is dominated by these microscopic
plants.
There are over 20,000 species, but we really have no idea
how many there are.
Wherever there's water, they exist.
They range from 0.5 to 1,000 µm in
diameter.
And as I told you in the first lecture,
there's as much genetic information in a liter of
seawater that contains these primary producers and all the
bacteria that they live with than there is in
the human genome.
So here's some of my favorites.
These are marine diatoms.
This is a silicon shell.
This is a single cell.
It's about 30 µm in diameter.
And this is made out of amorphous silicon.
It's essentially opal.
And here's another one.
They come in different shapes and sizes.
They're just really incredibly beautiful.
And people are just starting to study what
mechanisms are responsible for laying down
these exquisite architectures.
Here's another one.
To me this always remind me of the Coliseum for some reason.
These are pillbox shaped cells.
And they have two halves like that.
And when they grow, when they divide,
one half lays down another half inside of it.
Now, what's going to happen ultimately if these are rigid?
They get smaller.
One lineage gets smaller, and smaller,
and smaller.
Well, both of them get smaller, and smaller.
And this group of organisms has this really neat
system where when it gets really tiny, they differentiate into
egg and ***.
They mate, and that they make a giant cell again.
And they start the whole thing.
It's cool.
Just to show you some of the things people have studied,
people have wondered why have they evolved
this very heavy armor?
And of course, the first thing you think about
is resistance to predation.
But there was never any evidence for that.
And so, I just found this recent study in
nature that shows where they actually
measured.
I thought MIT students look like this because they actually
measure the force that it takes to crush one of these cells.
Here's the study.
There is a diatom frustule and they're putting a
measured force on it to see what would crush it.
And they were able to show that the amount of force that it
takes is enough to have a selective
advantage against the crunching parts of the
zooplankton that eat them.
I also want to point out, this is from the
website of Dr.
Angela Belcher, who is a professor here at MIT
in material sciences.
And she is studying diatoms.
Here's a diatom.
She's studying them as a material, this amorphous
silicon, looking at the way it's laid down.
And she's also studying coccolithophores,
which is another group of my favorite organisms.
They have these calcium carbonate plates.
Again, this is a single cell.
But its cell wall is made up of calcium carbonate plates.
And they come in all different
shapes and sizes.
Here's a really weird one with these huge,
they're called coccoliths.
And these cells, this
is a satellite image of reflection, of light.
In these cells, the
calcium carbonate, reflects light.
And this is a coccolithophore bloom
somewhere I think in the Bering Sea.
And we can measure these, but we
have no idea what causes a particular species to bloom at a
particular point in time.
It's one of the challenges of oceanography.
Here's another group of organisms.
The cyanobacteria: we've talked
about them a little bit.
These are prokaryotic cells that can fix
nitrogen.
They're one of the few groups of microbes that can take
nitrogen gas from the atmosphere and convert it to
ammonia, which draws it in to the food web.
So you can actually see a bloom here.
That one's called trichodesmium.
It grows out in the open Atlantic and Pacific.
Here's a bloom of trichodesmium sucking
nitrogen into the ecosystem, converting it into ammonia,
making it available to the other
organisms.
Here's the organism we work on, unfortunately very boring
looking, not as exciting looking at the other ones.
And now, this is just under a light
microscope.
These are less than a micron in diameter.
But if you shine blue light on them,
they fluoresce red.
The chlorophyll in them fluoresces red.
And these are the smallest and simplest
photosynthetic cell.
They have 1,700 genes.
And with that, I call them the essence of life
because with 1,700 genes, they can convert CO2,
nitrogen, phosphorus, all inorganic
compounds, basically this rock over here, in sunlight into
life.
And this happens to be life that
dominates the oceans.
They are the most abundant cell in the
oceans.
In some areas, it's about 50% of the
total chlorophyll.
So, they are basically a lean, mean
photosynthesis machine, and we're trying to understand
everything about them.
OK, does a digression, hopefully not
a diversion.
So, what regulates aquatic primary productivity?
Have to turn the lights back on.
Before we get into that totally, I want to
draw a typical, what we call a water column in
an aquatic ecosystem.
And I have seen this little red, sticky thing here.
This is from last year, and it says they
don't understand these axes.
See, I remember from year to year.
So, if you don't understand something I'm
doing, because I can tell when students come up afterwards that
I've completely lost you.
First of all, oceanographers plot things
upside down.
So this is depth.
So, depth goes down, which
makes sense, right?
And then, whatever we are plotting
against depth is on this axis.
So, in this graph I'm going to make, we
are going to plot net primary productivity,
temperature, nutrients, a bunch of different
variables.
OK, and for oceans, this
is about, well, we'll just say 2,000 m,
and for lakes, say, 200
m for a deep lake.
So, I'm drawing here sort of a generic picture of a water
column in either a lake or ocean.
OK, so do we have colored chalk?
Not here, OK.
Sometimes it used to float around.
Well, you're going to have to use
your imagination.
So first of all, let's plot light as a function
of depth.
What does that look like?
Like that, exactly.
It's going to decay exponentially.
In lakes, this is about 10 m, and in oceans this is
about 100 m.
Oh, and this was a question somebody
gave in the instant feedback.
Somebody said, I find it hard to
believe that all life in the oceans disappears where there is
no more light.
And, you're absolutely right.
It's only the photosynthetic life that disappears when
there's no more light.
There's lots of life below there that is using
organic carbon.
So, if you're here, that
the answer to your question.
OK, so then, now we're going to plot
temperature which looks something like this.
And, this is what's called the thermocline.
And we can also think of this as density.
Because colder water is more dense than
warmer water, right, you must have learned
that, did you learn that somewhere?
Did you learn that somewhere?
Where did you learn that?
Fourth grade, great, well-prepared.
So, up on the surface we have biosynthesis here where there's
light, and so it's exactly the same reaction as we have over
here.
And now it gets to the terrestrial
ecosystem.
The production you have in the surface water is you have
phytoplankton photosynthesizing,
making organic matter.
They're being eaten by zooplankton,
by fish that are making feces, etc., that
are being eaten by the detritivores.
But the net effect of this teeming
food web you saw in the DVD last time, is that there's going
to be organic carbon that falls down
below this lit zone.
OK, and that was the whale falling down make,
no, not purposefully, but falling down and making
carbon available to the food web in the
deep water.
So down here, you have regeneration.
So this is light.
And this is in the dark in the system.
But it's directly analogous to the biosynthesis and
regeneration system there.
So the other thing we want to plot on this is nutrients.
They are drawn down to very low
levels in the surface water in the lit layer
because the phytoplankton are sucking them up.
They are nutrient limited.
They're sucking up the nitrogen, phosphorus,
et cetera.
And then, as the carbon and all of that range down to the deep
water, it's regenerated,
and then the nutrients are regenerated.
So that's why you see the gradient
of the low nutrients here and high
nutrients here, because the bacteria in the
deep water are breaking down the carbon and releasing them.
OK, so if we look at this map, we can
see that for aquatic ecosystems, obviously water is
not limiting.
So water is an important regulator.
Light is a very important regulator
of productivity down to about in this region.
And nutrients, it
turns out, are very important.
And it's nutrients that really determine
the tapestry of this map that we're looking at.
And what I'm going to do for the rest of the class is explain
in lakes and oceans how the
physical forces make these nutrients
available in certain regions more than in other regions and
explain this.
OK, so first let's look at lake ecosystems.
So, what we're showing here is a year in the life of a
temperate lake.
So this might be the Mystic Lakes out in Arlington or
something like that.
Well, maybe that doesn't freeze over, I don't know.
But anyway, a lake that freezes
over in the winter.
So let's start during the summer, and here's this
basic graph showing the thermocline, the
nutrient depletion in the surface, and this one indicates
that you actually have oxygen depletion
in the deep water because of all of
this organic matter from productivity raining down and
being consumed by heterotrophic
organisms that consume oxygen.
So, as fall comes, and this is the important part,
in the summertime, this layer is mixed.
So, it's isothermal.
In the fall, you have
the winds and the surface cools.
And as this density gradient here
starts to break down, do the cooling in the winds.
And so, you have this mixing.
It's called fall overturn, which entrains these nutrients
from the deep water into the surface.
So that's the way you get the nutrients for the deep
water back up into the surface, whereas in
the summertime, the gradient is maintained
because of this density barrier, and the mixing can't
bring this down.
And then in the winter, you have the ice cover that
obviously everything then is just isothermal.
There's not much going on, but there is some
photosynthesis.
And then in the spring, the surface waters start to
warm up, the ice melts, you have overturn,
and brings the water up from the deep water.
In lakes, this can mix all the way to the bottom,
OK?
In the oceans, there's no force of
nature that can mix all the way down to 2,000 m.
So you have this thermocline in the oceans,
but it's in a relatively small fraction of
the total water column.
So the scale here is way off.
We're going from 100 m down to 2,000.
So in the ocean, it's just this tiny little,
all this action in the surface.
So we need another mechanism.
We can't mix all the way down to the
deep ocean.
So, we need another mechanism for bringing nutrients to the
surface.
And we're going to talk about, there's four different ways.
There's four different ways that the
deep water nutrients are brought back up where there is
light, because you have to have light
for photosynthesis to use the nutrients.
And one is episodic mixing.
I'm just going to list them, and then
we're going to go through them: coastal upwelling --
-- equatorial upwelling, and on much longer time scales,
what's called the "oceanic conveyor belt" in
quotes, which is basically local ocean circulation.
So let's go through these.
In the oceans, episodic mixing,
let's go back.
Now, just pretend this is an ocean, and that this goes down
to 2,000 m, and there's a thermocline.
What happens in the oceans is that you just have little,
episodic mixing events that erode right
here to get little bursts of nutrients injected into the lit
area.
That's seasonal mixing, but it never mixes all the way
to the bottom.
And we can see, I'll show you where,
this right here is the north Atlantic's bloom.
And in the springtime, you see major bloom
there due to this episodic mixing in the North Atlantic,
which have high winds and a lot of mixing.
OK, so there are also ocean currents
caused by this coastal upwelling phenomenon,
especially along the western coasts of continents.
And I don't have time to go into this.
You need a whole course and physical oceanography to really
understand this because it has to do with
the whole global ocean circulation that
causes this upwelling along the coasts.
But, I'm going to show you how this works in this movie,
or a little movie.
I guess that's as dark as we're going to get.
This is a cross-section of a coastal
ocean.
So, here's the coastline.
Here's the surface of the ocean.
And these little molecules here are CO2.
Can you see blue?
This is probably not going to work because
of this filming.
Well, we'll see what happens.
OK, so I'm going to go through it
in a still, and then I'll show the movie.
But what you're going to see is a blue patch upwelling along the
coast here, and CO2 molecules are
coming up with it.
OK, here's the blue.
That's nutrients, nitrogen, phosphorus,
etc.
The wind is blowing offshore causing the
surface waters to move in that direction.
They have to be replaced by something, so we're bringing the
deep water up to replace that moving surface water.
And as that comes up, the CO2 comes up and is
released.
And then you have a phytoplankton bloom from
the nutrients, and then the CO2 sucked
back in again and you have oxygen going out.
And, here it goes, the
movie.
Upwelling: The Movie.
There comes the CO2.
Here are the nutrients.
CO2 out, CO2 back in.
These are phytoplankton, these green
blobs.
That's a bloom.
So you have now the phytoplankton falling,
big bloom, organic carbon going
down and being regenerated.
So, it's a very dynamic system and that's why
you have a lot of high-intensity fisheries along the coasts,
especially the western coast of continents because of this
upwelling; there's lots of nutrients, lots of
phytoplankton, lots of fish.
A dramatic example of the power of the surface currents,
and how they affect upwelling is this
phenomenon called El Niño.
I think I'll skip the slide.
You don't have it in hand out, anyway.
But here's an animation showing the changes
in the productivity in the Pacific Ocean
along the equator.
Here's an El Niño.
And I'll explain how that works in a minute.
Here's a normal year where you have these
phytoplankton blooms, and let's look at it in this.
You've seen this one before, but let's look at it
more closely.
There's the equatorial bloom caused by upwelling along
the equator in a normal year.
And as we go around, this is like three years in the
life of the globe.
See, there is high productivity at the Amazon where the
Amazon's emptying.
Now, we are going to zoom in here, and this is a
normal year.
And that's an El Niño year.
You see, there's very little productivity,
and is very little upwelling along the coasts here.
And that El Niño in Spanish, what does it mean?
It refers to the Christ Child because this
happens around Christmastime roughly every
seven years.
And what happens in a phenomenon is it turned out that
people have studied enough for years.
It's a global phenomenon in which the prevailing currents in
the whole Pacific Ocean shift from going
in this direction which causes the
upwelling here to going in this direction, which brings warm
water suppressing the upwelling and
reducing the nutrient input into the
system.
OK, so here's an El Niño year, and directly compared to
non-El Niño year.
And that's all due to physical forces changing
the nutrient delivery.
So here's equatorial upwelling, and
then finally on a global scale, over very long periods of time,
you can imagine that these upwelling
events would not be enough to bring, you
have this constant rain of organic matter coming from the
surface waters.
And this nutrient reservoir in the
deep waters, none of these upwelling events
are enough to bring all that back and renew the system.
So you need a bigger force than that on a
global scale over long time periods.
And that's what's called the great
oceanic conveyor belt.
And this is really important, because I think
people think of the oceans as a static, understandably,
you look out there; it looks like a bunch of
water with the surface waters and
with the deep waters.
And if you throw something in the
deep waters it's going to stay there and you don't have to
worry about it anymore.
A lot of people want to bury nuclear waste in the deep
water.
And the point is that that's not true.
The oceans are all interconnected,
and if I'm a water molecule, the average amount of
time, and I'm traveling with the currents,
over a thousand years, I
will make this whole journey where I go along the surface
waters and then I get to the North Atlantic,
and because the waters are cooled and
there's very high winds in the North Atlantic,
you have cold water and the high winds cause high
evaporation.
So you have saltier water, so, cold, saltier water up here
sinks.
And that actually is a force that
drives this whole global circulation.
And so, I'm cruising along here.
I get here, and I sink.
And then I go for this long journey down the
bottom.
And of course, it's much
more complicated.
This is grossly oversimplified.
But I go this long journey to the
bottom of the Atlantic in these deep ocean
currents, and then somewhere along the line,
I get brought up again through zones of upwelling
here, or maybe I would meander up here and
get brought up again.
I'm just one atom on average.
So, through these global ocean currents,
the deep water eventually comes up to
the surface, bringing those nutrients back.
Where it comes in contact with the light and the phytoplankton,
and they photosynthesize and they
take up the nutrients and they make
organic carbonate, and it all settles down again
to the bottom.
So, it's a cycle.
And if you didn't have that, the thing would run down.
If you didn't have the deep water coming up eventually
coming up somewhere, the system would
just run down and you'd have a big anoxic
bottom of the ocean.
And who knows what would happen?
So, this is really important.
OK, so finally, I've talked about
nutrients in general, but what nutrients are the most
important?
And, it turns out that there are some nutrients that are in
much less supply that are required by the
plants.
And this is what's called, so, what nutrients are
important?
Now, of course, they're all
important but some are more important in regulation than
others.
And there's something called the law of the minimum.
And it states that the growth of a plant will
be limited -- -- by that element that is in
least supply relative, this is the
important part, to the requirements of the
plant or the phytoplankton.
When I say plant, it could be phytoplankton or a
tree, or a plant, or whatever.
And this is the important part.
So how we figure out what the requirements for
elements are above plants?
You might grab it,
harvested, grind it up, and measure the ratio of the
elements in that plant.
So, for example, if you do that,
for most plants you get something
on the order of, at least for most
phytoplankton, which are my preferred plant,
you get the ratio of carbon, nitrogen,
and phosphorus, of 106 atoms of carbon per 16
nitrogen per one of phosphorus.
So this tells you in what ratio they
need these elements in order to grow.
So then you look in the environment and you ask,
what are the ratios available?
So, say if the water has a ratio of,
what's going to be the most limiting element
in that system for that plant?
Exactly, nitrogen.
And, alternatively,
you could have something like this.
And what would be limiting there?
Phosphorus limits.
And it turns out that in most aquatic ecosystems,
for now we're going to say that nitrogen and
phosphorus are the important limiting factors.
And just to show you, again, that ecologists do
experiments, here's an experimental lakes area in
Ontario, where there are 22 different lakes
set aside for research.
And in this particular set of lakes, this is a
control lake, and this is the experimental
lake.
They added phosphorus to the lake.
And you can see the phytoplankton
bloom by only adding phosphorus.
They didn't add anything else.
And that means that phosphorus was
in least supply relative to the other
nutrients.
And the interesting thing that happened here was that
when they added the phosphorus, that makes phosphorus in great
abundance relative to nitrogen.
And what that did is make nitrogen the limiting
factor.
And when nitrogen is the limiting factor,
what organisms might have an advantage?
We talked about them.
Yeah, there you go, nitrogen fixing
organisms.
So, if nitrogen is limiting, only
organisms that can take it from the atmosphere can get more
nitrogen than the other organisms.
So they are favored.
And what happens is you fertilize with phosphorus.
You get blooms of nitrogen fixing
organisms.
It's really an interesting phenomenon.
But nitrogen and phosphorus are, one or the other is
limiting in lakes.
And in large areas of the oceans, nitrogen and phosphorus
are also limiting, except we've learned
recently that there are areas of the
oceans where iron is actually a limiting factor.
And this was an experiment that was done by
oceanographers.
And there is Alaska just to get you oriented.
This is the North Pacific where they went
out with a boat and they made it a patch.
They added iron just to a patch of ocean.
And I can tell you about this.
We were involved in some of these
experiments.
It started out with a 10 km x 10 km patch and showed
that if you add iron you get a bloom of
phytoplankton.
And this is a satellite image of that
phytoplankton bloom.
And in the last lecture, I'm going to tell you
all about those iron fertilization experiments,
and the implications for how we are going to use the
oceans in the future.
So, take-home messages: we'll talk about that next
time.
You can take them home, but I don't want to
keep you over.