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(girl narrator) Imagine a world where we weren't surrounded by air. Without oxygen in the air we breathe, we wouldn't survive very long. But what about animals underwater? They need oxygen, too. Where do they get it? The answer is from the water, because water actually also contains dissolved oxygen. Dissolved oxygen is just what it sounds like. It's oxygen that's dissolved in water. What does it mean for something to dissolve? If you've mixed sugar and water, sugar seems to disappear. What happens is that the molecules of water mix with the sugar molecules, breaking the large sugar granules, to make a sugar water mixture. Specifically, since parts of the mixture are spread equally and evenly everywhere, we can call this mixture a solution. So when something dissolves, it means it goes into an evenly distributed mixture called a solution. Now, like those sugar molecules, oxygen molecules also mix evenly with water molecules to form a solution. In other words, oxygen dissolves in water. But how does oxygen get there? There are two main ways oxygen can enter water, like the ocean or a lake. One way is from the atmosphere. Oxygen makes about 1/5 of the atmosphere, and these oxygen molecules bump into and enter water. Even more oxygen enters water through photosynthesis. You probably know photosynthesis mainly as the way that plants feed themselves, but there's a whole slew of other photosynthetic living things aside from plants, from tiny green bacteria called cyanobacteria to little phytoplankton with crazy names like dinoflagellate and even crazier bodies. All these creatures can produce food through photosynthesis. But in doing so, they actually also produce oxygen, which then dissolves in the water. What happens to this dissolved oxygen? Remember that like us, other living things-- the animals, plants, phytoplankton, and bacteria living underwater-- also take up oxygen to live. Furthermore, oxygen molecules are continually shuttling back and forth between the atmosphere and the water. But there's always some amount of oxygen that stays dissolved. This amount depends on the saltiness and temperature of water and how much dissolved oxygen there is compared to the oxygen levels in the air. Why does the amount of oxygen in the water and air matter? It's like walking into a crowded room from an empty hallway. You don't want in, and people inside want out. If there's more oxygen in the water than the air, the oxygen moves to the roomier atmosphere. If more oxygen is in the air, the oxygen moves to the water. The principle's the same for the saltiness of water. You know how salty seawater is. That's because there's salt dissolved in seawater. Those salts take up space in the water. It's like adding more people to your crowded room, making less room for you. The saltier it is, the less space there is for dissolved oxygen. What about temperature? When things heat up, they move around. Imagine the crowded room again. As everyone starts jostling around, it's easier to leave the room. Again, it's the same for dissolved oxygen. The oxygen molecules warm up and start vibrating, making it easier to escape the water. Let's sum that up. Dissolved oxygen enters water from the atmosphere, from photosynthesis, then gets taken by living things or exits into the atmosphere, depending on water temperature, saltiness, and the amount of already dissolved oxygen. Who cares what the amount of dissolved oxygen is? Well, living things need oxygen to live. The more oxygen there is, the more animals can live there. Scientists measure dissolved oxygen to find out how healthy a lake or a river is. Animals also adapt to the amount of dissolved oxygen. Salmon need lots of oxygen and live in colder places. Catfish live in warmer waters because they need less oxygen. If you've ever wondered why fish are different, this is one reason why. Unfortunately, these fish die off at very low oxygen levels, and humans have been decreasing freshwater oxygen levels in a process called eutrophication. Eutrophication just means adding nutrients good for growing, like nitrogen and phosphorus, to a lake or a river, usually by pollution. That causes a lot of plants and cyanobacteria to bloom. Aren't plants good, though? Don't they produce dissolved oxygen? Yes, but when they die, they sink and decay. This decay uses up oxygen in the lake. With no oxygen to breathe, the fish die, and the lake becomes unhealthy. So throw your trash properly and recycle. Together we can preserve marine and freshwater ecosystems.

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