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Eutrophication and dead zones

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How eutrophication, caused by extra nutrients in runoff, can lead to dead zones. 

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  • leafers tree style avatar for user Gabriella
    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)
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  • male robot hal style avatar for user 123cvk
    , 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?
    (3 votes)
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  • female robot grace style avatar for user Anna
    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)
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    • male robot hal style avatar for user Greg L
      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.
      (2 votes)
  • leaf red style avatar for user marissaroark2
    How does chemistry apply to eutrophication?
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  • hopper jumping style avatar for user Yago
    Will the dead zones eventually also kill off the algaes and allow other life to enter the dead zones again or are the algaes not dependent on any of the organisms that are harmed by eutrophication?
    (2 votes)
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  • piceratops sapling style avatar for user joja90
    Why isn't eutrophication more widely spread? Why don't the phosphates spread across the water body?
    (1 vote)
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    • winston baby style avatar for user Ivana - Science trainee
      It is a good thing, isn't it?
      You'd need much bigger amounts of phosphates for eutrophication to happen. Just be careful. wait next 100-300 to 100 years. maybe you won't be able to see it but next generations will record negative changes unless we change something now.


      The oceans are really large water bodies, bear it in the mind.
      (1 vote)
  • male robot donald style avatar for user Bis13
    But nitrates also cause eutrophication, right?
    (1 vote)
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  • blobby green style avatar for user walkerz22
    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)
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  • blobby green style avatar for user usamanaseer2014
    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)
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    • female robot grace style avatar for user tyersome
      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)
  • aqualine seed style avatar for user jane_doe
    is it possible to replicate a dead zone on a smaller scale? if so, how would one accommodate for the lack of light, and the other components? Also, how much time would it take?
    (0 votes)
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    • orange juice squid orange style avatar for user Clinton Walsh
      Salt Water aquarists are constantly monitoring and managing the amount of phosphorous/phosphate in their aquariums. Salt water aquariums are notorious for being imbalanced for the first 1-2 years. I'm sure these people could definitely share experiences about their reef tank and how phosphate got out of hand (most folks who worry about phosphate and nitrogen in their tank tend to have coral/reef tanks). There are numerous systems that aquarists use to manage phorphate and nitrogen (I think one thing that might help on a larger scale is setting up a LARGE refugium - basically a giant tub filled with a carefully selected algae that will consume the phosphate. So if the algae dies then you can remove the dead algae so that it doesn't prompt bacterial growth in the waterways). Also of note, there are chemicals for sale that bind to phosphate and then allow it to evaporate out of the fish tank. I have no idea what effect or problems this product might have if it were used outside an aquarium, though.
      (1 vote)

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.