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Course: AP®︎/College Environmental science > Unit 7
Lesson 2: Types of aquatic and terrestrial pollutionEutrophication and dead zones
Eutrophication, derived from the Greek for "well-nourished," refers to over-nourishment of aquatic ecosystems due to excessive use of fertilizers and detergents. This leads to algal blooms, which, when they die and decompose, consume oxygen in the water, creating hypoxic or "dead" zones. This imbalance in nutrients can have devastating effects on aquatic life. Created by Sal Khan.
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- 4:43, will the bacteria still survive once all the oxygen is depleted? Since they require oxygen to perform cellular respiration, won't they also die off, meaning EVERYTHING is dead now in a dead zone?(5 votes)
- Yes, the bacteria will die too. But I guess the big deal is, The oxygen is depleted by respiration of the bacteria, which causes aquatic plants and fish to die, which causes even more oxygen to be consumed, and so on. So there is this bad positive feedback going on. Yes. Everything dies. It is literally, "dead zone"(2 votes)
- Are there any ways of removing excess nitrogen and phosphorous contaminating waterways in order to reverse (or at least reduce) the eutrophication process and the environmental damage it does?(2 votes)
- I listened somewhere about the Mangrove forest and its absorption capacity; but I don't know if it's only a theory or consistent and indepth study. In Brazil, they use the Eichhornia, however this plant can become a plague.
Then, for now, the solution is only time and dispersion.
http://www2.uol.com.br/sciam/reportagens/recuperacao_de_zonas_mortas.html(5 votes)
- Isn't eutrophication a natural process that will happen in every lake regardless of whether fertilizers are used or not and only accelerated by these fertilizers?(2 votes)
- Yes and no. The question depends on the scale. The argument could be made that on a technical level there is eutrophication happening in any body of water where there is vegetation growing in it. However, eutrophication tends to refer to reaching a point where the level of oxygen in the water is decreasing enough to stop supporting animal life. Or, the overgrowth of algae and bacteria is choking out other forms of life.
You also run into the problem of what is a lake. Merriam- Webster gives the definition of "a considerable inland body of standing water" (https://www.merriam-webster.com/dictionary/lake) This is a natural process in small bodies of water where is there is no outlet and the water is stagnant. These are usually referred to as ponds, there is however not a hard enough definition to officially differentiate the two. In larger bodies where you have an exchange of water the natural buildup is much less. There are lakes that are thousands of years old. This would indicate that they are not suffering from eutrophication. They do not die out from an overgrowth of vegetation but rather due evolutionary changes in the geography not the chemical content of the water; even though you may find sections where there is dense patches of algae.(5 votes)
- How can we remove excess nitrogen ?(1 vote)
- In fact, what Khan was describing isn't irony. If the fertilizer was a fish food for the fish but it ends up making algae grow and suffocates the fish, that's irony.(1 vote)
- why can we make a large liner or containment thing to stop run of by blocking it, it could also keep more nutrition in the dirt so we can use less fertilizer. plus extra alge can make more oxygen as it dies it will regrow and make more oxygen(1 vote)
- But nitrates also cause eutrophication, right?(1 vote)
- Isn't eutrophication a natural process that will happen in every lake regardless of whether fertilizers are used or not and only accelerated by these fertilizers?(1 vote)
- Yes, eutrophication is a natural process; however, not all natural processes are useful or good, especially when they are happening at a huge scale and accelerated rate. Nitrogen or phosphorus can be high in an area for reasons other than human applied fertilizers, however not at the scale that it is happening in our world today. The vast areas of water ecosystems that are having this problem is because of humans. This is occurring in our oceans! Not just a few ponds or small lakes.
Imbalances can occur due to natural processes being accelerated or decelerated. Nature works in cycles and often balances or "corrects" itself. However, it can get so far out of balance that nature cannot maintain equilibrium and then extinctions occur at high rates.(2 votes)
- Algae do photosynthesis.means it add more oxygen into the water. So why there is depletion of oxygen ?more algae more photosynthesis more oxygen in water!(1 vote)
- The oxygen depletion happens when the algae start to die in large numbers — decomposition uses up dissolved oxygen faster than it can be replenished by algae or by mixing in from the atmosphere.
The algae can't grow indefinitely because they are limited by light availability — only those that are near the surface grow effectively — the algae underneath are shaded and starved. Diseases also tend to happen in high-density populations .(0 votes)
- Why isn't eutrophication more widely spread? Why don't the phosphates spread across the water body?(1 vote)
- Eutrophication is happening at a huge scale that is why it is a problem!
As for why dead zones happen at gulfs and other costal regions rather than the phosphate dissolving across the whole ocean, it is because there is too much coming at once and the life forms that receive the fertilizers first (algae) expand before the years it would take for the fertilizers to spread equally through out the ocean. Algae grow faster than fish or insects hence it out competes everything else.(2 votes)
Video transcript
- We're now going to talk about something called eutrophication. Eutrophication. And it comes from, or it's derived from, the Greek for well nourished. Eu referring to well and
then trophic or trophia, referring to nourished, or nourishment. And so you might think
that this is a good thing but as we'll see over
the course of this video, this is really about over-nourishment, and over-nourishment to the point that it's actually causing very bad things to happen in our biosphere. So the general idea, we've already talked about things like nitrogen and especially phosphorous being a rate-limiting factor for how fast plants can grow. We talked about that in the nitrogen and the phosphorous cycle videos. And because of that, we humans have added a lot of nitrogen and
especially phosphates to plants in order to grow them better, because we need the plants to live, to have more food. So what has happened is, so let's say that this right
over here is some land. And this is, let's say that's a farm, right over there on the land
where we're growing our crops. So these are all the
crops that we're growing. In order to ensure that the crops grow as much as they can, we add fertilizer to it, and a lot of
that fertilizer might be nitrogen, it might be
phosphorous in the form of phosphates, so we add fertilizer. And the biggest culprit
here tends to be phosphates, the phosphorous in them,
and that helps the plants grow more. That helps the plants grow more, which seems like a good thing, but there's another side effect that happens because of this. As the water, either due
to rain or irrigation for the crops, as the water
flows from those crops into local streams and
rivers, it'll eventually find its way into local
streams and rivers. those streams and rivers
and lakes are going to have an excess of those
fertilizers, in particular maybe those phosphates. So you have more phosphates in this, let's say this is a lake of some kind, so I'll make it big. Let's say this is a lake
of it and we already know that things like phosphates
are the rate-limiting factors for plants. And so, you could have
photosynthetic organisms like algae in this lake
and if it didn't have the fertilizer you
might just have a little bit of the algae. But now since they're getting
all of these phosphates, the algae can go crazy. The algae can go crazy and grow all over the surface of the lake. And so this is where that algae is getting over-nourished. It's getting way more
phosphates than would have typically happened just if
there wasn't fertilizer, if there wasn't this runoff happening. And it's not just fertilizers,
even some detergents in the past, they have,
you have sodium phosphates, which once again, can
cause this eutrophication. And you might say okay,
well why is this bad? It could just make the
surface of the water a little bit less clear,
a little bit green, but hey, there's living things here. Well the issue is what
happens once this algae dies. Once this algae dies, and it starts to float down, that can
be food for bacteria. That could be food for bacteria, and as the bacteria consume it, they also use oxygen. We've already talked
about the role of oxygen in respiration. The bacteria will
consume all of the oxygen in the water. Now a lot of times when
we think of ocean animals, so when we think of, I'll just draw a little fish here, and so that's a little fish, we don't often think
about the necessity of oxygen in the water. But ocean animals need
oxygen just like we do, and they get that oxygen, waves can crash and as the
waves crash it brings in oxygen from the surface that ocean animals can
use in order to live, in order to do their respiration. But now all of a sudden,
if this bacteria, because they're able to decompose all this algae that is dying, if they're
consuming all of the oxygen, well it depletes
the oxygen from the water so that the ocean animals
can't live there anymore. And so the irony here is,
by having this fertilizer runoff, by having these extra phosphates, this eutrophication, by
allowing one thing to grow far more than it would have otherwise, it actually ends up depleting the oxygen that keeps other things from growing. And it actually creates
what we call dead zones. So this fish is going
to die and other animals like it aren't going to
live because there's not gonna be enough oxygen in that water. And then we have a Dead Zone. And this is a serious problem. We have some pictures here. This is potential
eutrophication in this picture. Let me show you some more. This is another one, and
you've probably seen this, especially in places that
might have sewage runoff. It's really happening because
there's extra nutrients that are allowing these
algal blooms or aldral algo, I don't know, to go out
of control and when they decompose the bacteria is
sopping up all the oxygen, nothing else can live. And this is happening on a macro scale. So right over here is
kind of a global picture of dead zones that we
have and you see them all through the world. And you see they are
typically concentrated where we have more industrialized nations, where we have either more
agriculture happening, more industrial agriculture, where they might be using more
fertilizer and also where you might have more runoff
from cities that will have nutrients like the
phosphates, that will cause this eutrophication to happen. And if you were to zoom
in, let's say on the Caspian Sea, you can
see this in more detail, you can visually see this. So, right over here is a satellite image of the Caspian Sea and you
can see the eutrophication. You can see the algeal, algal
blooms throughout this area. It's making the water less clear. So this is a serious
ecological thing that we have to think about. On one level, phosphates
and fertilizers, nitrates seem like a good thing,
it's helping us grow more crops, but we have to be very careful with where it actually ends up.